ABSTRACT. Potato, Solanum tuberosum L., is one of the most economically important specialty

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1 ABSTRACT LANGDON, KEVIN WILLIAM. Identification of Risk Factors that Influence Wireworm Damage in North Carolina Potato for the Development of an Integrated Management Program. (Under the direction of Mark R. Abney). Potato, Solanum tuberosum L., is one of the most economically important specialty crops in North Carolina, and the incidence of wireworm damage to the crop has increased significantly in recent years. A series of studies was conducted over three years to evaluate potential risk factors that influence wireworm damage in potato. A two year commercial field study was conducted to determine the wireworm species composition and the risk factors associated with wireworm damage in potato. Potato fields were selected for the study based on the following potential wireworm risk factors: potato cultivar, harvest date, insecticide use patterns, soil organic matter, and crop rotation schedule. Fields were baited using steam crimped oat baits and potato fields were harvested at 14d intervals post tuber initiation. The wireworm species Melanotus communis (Gyllenhal) and Glyphonyx spp. comprised over 70% of all wireworm species collected. Three insecticide efficacy trials were also conducted to determine the most efficacious insecticides for wireworm control in potato and determine the effect of manipulating harvest date on wireworm damage in potato. Insecticides such as fipronil or imidacloprid + bifenthrin consistently reduced wireworm damage incidence and severity from the untreated check. Over a 14d period of tuber harvest, wireworm damage levels increased from 25% to nearly 45% averaged across all insecticide treatments. A laboratory bioassay was conducted to determine the relative susceptibility of six cultivars to wireworm feeding by two species under two soil moisture conditions. Relative susceptibility was measured by: 1) percent of tubers with feeding injury, 2) number of feeding holes per tuber, and 3) volume of tuber consumed. Averaged over all cultivars, M. communis created

2 more feeding holes and consumed more tuber tissue than G. bimarginatus. Glyphonyx bimarginatus feeding was negligible in all cultivars indicating that this species may not be an economically important pest of potato. Dark Red Norland and Yukon Gold cultivars sustained the most wireworm feeding damage and are considered more susceptible relative to other cultivars tested. A field study was conducted at two locations over two years to evaluate the efficacy of insecticides applied to field corn for the reduction of wireworm damage to potato planted in rotation the following year. Field corn was planted in 2011 and potato was planted in 2012 at each location. Fipronil and clothianidin treatments provided a significant reduction in wireworm damage incidence and severity as compared to the untreated check. A combination of baiting and absolute sampling was conducted to evaluate the effect of insecticide on the relative abundance of wireworms in the year after treatment. The number of wireworm larvae collected through both baiting and absolute sampling did not reflect levels of tuber damage observed therefore we cannot rely on baiting to effectively determine whether insecticides applied to corn in 2011 reduced wireworm populations of economically important wireworm species in Wireworms live in the soil and are difficult to study both spatially and temporally without destructively sampling their natural habitat and potentially changing their behavior. An X-ray machine was used to evaluate the behavioral response to insecticides applied to the soil. Significantly fewer M. communis exposed to bifenthrin permeated the treated zone compared to all other treatments and damage incidence and severity was reduced. Mortality of wireworms exposed to bifenthrin was not different than the untreated check indicating that bifenthrin is not effective at killing M. communis and therefore reducing M. communis populations. More wireworms exposed to fipronil were found dead and a reduction in damage incidence and severity was observed.

3 Results indicate that fipronil is a desirable compound to use for the control of M. communis in potato production systems in North Carolina. Learning how wireworms respond to insecticides applied to the soil is important for the development of management strategies that optimize insecticide use. The overarching goal of this research is to develop management strategies for wireworm in potato based on implementation of reduced risk practices.

4 Copyright 2012 by Kevin William Langdon All Rights Reserved

5 Identification of Risk Factors that Influence Wireworm Damage in North Carolina Potato for the Development of an Integrated Management Program by Kevin William Langdon A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Master of Science Entomology Raleigh, North Carolina 2012 APPROVED BY: Mark R. Abney Committee Chair George G. Kennedy Dominic Reisig Thomas P. Kuhar

6 BIOGRAPHY Kevin William Langdon was born on June 3, He is the youngest of three children of Mr. and Mrs. Andy William Langdon. Kevin was raised in Knightdale, North Carolina located in the once rural outskirts of eastern Wake County where his lifelong affinity for agriculture was begun through his work on a local tobacco farm. This interest in agriculture complemented his already cultivated passion for the outdoors through countless trips with his father to the family farm in Benson, NC in pursuit of fish and game. Kevin was graduated from East Wake High School in He attended North Carolina State University where he earned a Bachelor of Science degree in Agricultural and Environmental Technology with a minor in Soil Science in In the summer of 2009, Kevin moved to Vero Beach, Florida to fill an internship position with Syngenta Crop Protection working in the Insect Control Lab at the Vero Beach Research Center. It was in Vero Beach where Kevin decided to pursue a graduate degree in entomology and where he met his future wife, an intern in the Weed Control Lab, Miss Barbara Lee Adams. Kevin was accepted into the graduate program of the Department of Entomology at North Carolina State University in 2010 where he began his Masters research under the direction of Dr. Mark R. Abney. His research aimed to determine the risk factors associated with wireworm damage to potato in northeastern North Carolina. On October 6, 2012, Kevin and Barbara were married and have since lived together with their two English Pointers, Junior and Bailey. Kevin remains passionate about hunting and fishing and looks forward to one day, passing on the obsession. ii

7 ACKNOWLEDGMENTS I would like to express my heartfelt appreciation to those with whom I worked through the duration of completing my graduate degree at North Carolina State University. First and foremost, I would like to thank my major professor, Dr. Mark R. Abney for providing me the opportunity to work in his lab. He not only acted as my academic mentor from whom I was able to learn, develop, and grow professionally, but as a friend for which I am forever grateful. I would also like to give thanks to Dr. George G. Kennedy, Dr. Dominic Reisig, and Dr. Thomas P. Kuhar for their support, assistance, and constructive criticism. I greatly appreciate support from Rocio Davila, Matt Stallsworth, Katherine Gleason, Justin Jones, Melissa Somody, Tommy Batts, and Rebekah Ray through their assistance in the laboratory and field. I would also like to express a sincere thanks to the potato growers of North Carolina that allowed me to conduct my research in their fields. I would like to bestow the greatest thanks to my parents for raising me with the highest moral values from Day One. Their continuous and unconditional support has given me the internal drive and determination to be the best person that I can be. I would like to thank my Uncle, Winky Barber and his wife, Rebecca, for support, great southern cooking, and for allowing Barbara and I to occupy their guest home in Pittsboro, NC through the duration of my graduate degree. Lastly, I would like to thank my wife, Barbara Lee for whom I will ever be indebted. Your support, constant words of encouragement, and understanding of long hours in the field and lab pushed me to achieve this accomplishment, and for that, I thank you. iii

8 TABLE OF CONTENTS LIST OF TABLES... LIST OF FIGURES... v vii CHAPTER I. IDENTIFICATION OF RISK FACTORS THAT INFLUENCE WIREWORM DAMAGE IN POTATO... 1 Introduction... 3 Materials and Methods Results Discussion Acknowledgements Literature Cited CHAPTER II. RELATIVE SUSCEPTIBILITY OF SELECTED POTATO CULTIVARS TO FEEDING BY TWO WIREWORM SPECIES AT TWO SOIL MOISTURE LEVELS Introduction Materials and Methods Results Discussion Acknowledgements Literature Cited CHAPTER III. UTILIZING INSECTICIDE SEED AND IN-FURROW TREATMENTS IN ROTATIONAL FIELD CORN TO REDUCE WIREWORM (COLEOPTERA: ELATERIDAE) DAMAGE TO POTATO Introduction Materials and Methods Results Discussion Acknowledgements Literature Cited CHAPTER IV. USING RADIOGRAPHY TO STUDY WIREWORM BEHAVIOR IN RESPONSE TO SELECTED INSECTICIDES Introduction Materials and Methods Results Discussion Acknowledgements Literature Cited iv

9 LIST OF TABLES Table 1.1 Table 1.2 Table 1.3 Table 1.4 Table 1.5 Table 1.6 Table 3.1 Table 3.2 Table 3.3 Table 3.4 Total number of wireworms collected in oat baits in potato, corn, and soybean during the 2010 and 2011 potato growing seasons Insecticide application rate and results from an insecticide efficacy trial conducted at NCSU Tidewater Research Station in Plymouth, NC during the 2010 potato growing season Insecticide application rates and results from an insecticide efficacy trial conducted at NCSU Cunningham Research Station in Kinston, NC during the 2011 potato growing season Insecticide application rates and results from an insecticide efficacy trial conducted at NCSU Tidewater Research Station in Plymouth, NC during the 2011 potato growing season Total number of adult click beetles collected using yellow sticky traps in commercial fields during the 2011 growing season Climatic characteristics summary for the 2010 and 2011 growing season summarized by CRONOS at NCSU Tidewater Research Station in Plymouth, NC In-furrow and seed treatment insecticides applied to field corn in 2011 for the control of wireworms in potato planted to the same field in Agronomic practices applied to field corn in 2011 at NCSU Cunningham Research Station in Kinston, NC and at NCSU Tidewater Research Station in Plymouth, NC Agronomic practices applied to potato in 2012 at NCSU Cunningham Research Station in Kinston, NC and at NCSU Tidewater Research Station in Plymouth, NC Mean±SEM number of wireworm larvae collected per bait in potato in 2012 at NCSU Cunningham Research Station in Kinston, NC and at NCSU Tidewater Research Station in Plymouth, NC v

10 Table 3.5 Table 4.1 Mean±SEM number of wireworm larvae collected per absolute sample in potato in 2012 at NCSU Cunningham Research Station in Kinston, NC and at NCSU Tidewater Research Station in Plymouth, NC Application rate of insecticide active ingredient applied to the soil in a laboratory bioassay in order to study M. communis behavior and ultimate health after exposure Table 4.2 Total number of wireworm larvae lively, moribund, or dead at 26d, 46d, 83d, 138d after exposure to selected insecticides vi

11 LIST OF FIGURES Figure 1.1 Figure 1.2 Species composition for wireworms collected in oat baits from potato fields during the 2010 and 2011 potato growing seasons Abundance of total wireworms, M. communis, and Glyphonyx spp. captured in oat baits in potato fields by insecticide active ingredient during the 2010 and 2011 potato growing seasons Figure 1.3 Abundance of total wireworms, M. communis, and Glyphonyx spp. captured in oat baits in potato fields by proximity to drainage ditch during the 2010 and 2011 potato growing seasons Figure 1.4 Percentage of wireworm damaged tubers for each potato cultivar grown during the 2010 and 2011 potato growing seasons Figure 1.5 Percentage of wireworm damaged tubers by insecticide active ingredient for all potato cultivars grown during the 2010 and 2011 potato growing seasons Figure 1.6 Percentage of wireworm damaged tubers by previous crop for all potato cultivars grown during the 2010 and 2011 potato growing seasons Figure 1.7 Percentage of wireworm damaged tubers by individual tuber size for all potato cultivars grown during the 2010 and 2011 potato growing seasons Figure 1.8 Percentage of wireworm damaged tubers by harvest date in a small plot study conducted at Tidewater Research Station in Plymouth, NC in Figure 2.1 Comparison of M. communis and G. bimarginatus feeding on potato in laboratory bioassays Figure 2.2 Effect of soil moisture on wireworm feeding on potato in laboratory Bioassays Figure 2.3 Effect of head capsule width on wireworm damage incidence to potato in laboratory bioassays vii

12 Figure 2.4 Figure 2.5 Figure 2.6 Figure 2.7 Figure 2.8 Figure 3.1 Figure 3.2 Figure 3.3 Figure 3.4 Figure 4.1 Effect of head capsule width on total tissue consumed in laboratory Bioassays Effect of head capsule width on wireworm damage severity to potato in laboratory bioassays Susceptibility of selected potato cultivars to wireworm feeding damage incidence in laboratory bioassays Susceptibility of selected potato cultivars to volume of tissue consumed in laboratory bioassays Susceptibility of selected potato cultivars to wireworm feeding severity in laboratory bioassays Effect of in-furrow and seed treatment insecticides applied to field corn in 2011 on wireworm damage to potato at the NCSU Cunningham Research Station in Kinston, NC and the NCSU Tidewater Research Station in Plymouth, NC Effect of in-furrow and seed treatment insecticides applied to field corn in 2011 on wireworm abundance in potato in Effect of soil depth on the abundance of total (all species) wireworms collected in absolute samples in Composition of wireworm species collected using oat baits in potato in 2012 following in-furrow and seed treatment insecticides applied to field corn in Percentage of wireworm larvae that breached the zone of insecticide treated soil Figure 4.2 Effect of insecticide applied to the soil on wireworm feeding behavior Figure 4.3 Percentage of wireworm larvae lively, moribund, or dead following exposure to insecticides viii

13 CHAPTER I Identification of Risk Factors that Influence Wireworm Damage in Potato 1

14 Abstract Potato, Solanum tuberosum L., is one of the most economically important specialty crops in North Carolina, and the incidence of wireworm damage to the crop has increased significantly in recent years. A low market tolerance for insect damage to potato tubers and a lack of thresholds and effective monitoring tools result in the prophylactic use of broad spectrum insecticides applied to the soil for wireworm control. A two year commercial field study was conducted to determine the wireworm species composition and the risk factors associated with wireworm damage in potato in North Carolina. Commercial potato fields were selected for the study based on the following potential wireworm risk factors: potato cultivar, harvest date, insecticide use patterns, soil organic matter, and crop rotation schedule. Commercial potato, soybean, and corn fields were baited using steam crimped oat baits and potato fields were harvested at 14d intervals post tuber initiation. The wireworm species Melanotus communis (Gyllenhal) and Glyphonyx spp. comprised over 70% of all wireworm species collected. Three insecticide efficacy trials were also conducted to determine the most efficacious insecticides for wireworm control in potato and to determine the effect of manipulating harvest date on wireworm damage in potato. Insecticides such as fipronil or imidacloprid + bifenthrin consistently reduced wireworm damage incidence and severity from the untreated check. Over a 14d period, wireworm damage levels increased from 25% to nearly 45% averaged across all insecticide treatments. Wireworm pressure was high with 64% of harvested tubers from the untreated check having wireworm damage. The overarching goal of this research is to develop management strategies for wireworm in potato based on implementation of reduced risk practices. 2

15 Introduction Potato, Solanum tuberosum L., is one of the most economically important specialty crops in North Carolina, and the incidence of wireworm damage to the crop has increased significantly in recent years. More than 6 thousand hectares are planted to potato annually in North Carolina, generating over 30 million dollars of revenue each year, ranking it as the second most valuable vegetable crop in the state (NCDA, 2011). A low market tolerance for insect damage to potato tubers and a lack of thresholds and effective monitoring tools result in the prophylactic use of broad spectrum insecticides applied to the soil for wireworm control. Under the US Environmental Protection Agency s Food Quality Protection Act of 1996, many of the most efficacious insecticides used to target wireworm in potato production have already lost or will inevitably loose registration (Kuhar and Alvarez, 2008; Willis et al. 2010a). Alternative approaches to wireworm control are needed to reduce incidence of wireworm damage to potato in the state. The purpose of this study is to identify risk factors that influence wireworm damage in potato; understanding factors that contribute to wireworm damage could facilitate the development of improved integrated management strategies for the pest in North Carolina potato. There are three economically important insect pests that attack potato, in North Carolina: Colorado Potato Beetle, Leptinotarsa decemlineata (Say); European Corn Borer, Ostrinia nubilalis (Hübner), and; wireworm (IPM Centers, 1999). Of the three, wireworms are the most difficult to manage, and North Carolina potato growers suffer economic loss each year due to wireworm damage. Wireworms are the subterranean polyphagous larval 3

16 stage of click beetles (Coleoptera: Elateridae), and they are pests of many of crops worldwide. Wireworms are attracted to CO 2 emitted by germinating seeds and developing roots of most cultivated plants (Doane et al. 1975). Wireworms utilize this cue to direct movement toward and to attack the signal emitter. Economic loss by wireworm can occur in three ways: 1) reduced crop stand, 2) plant injury resulting in yield reduction (Belcher, 1989), or 3) direct damage to the harvestable portion of the plant (i.e. potato tuber). A recent increase in wireworm damage to many crops has been observed (Kuhar and Alvarez, 2008; Parker and Howard, 2001). Root and tuber crops such as sweetpotato, Ipomoea batatas (L.) Lam. and potato are especially at risk for economic injury because the harvested portion of the plant is below the soil surface where it can be damaged directly for most of the growing season. At least 39 wireworm species from 12 genera attack potato throughout the world (Jansson and Seal, 1994; Parker and Howard, 2001). Geographic region, climatic conditions, soil characteristics, and crop rotation sequence have been found to influence wireworm species diversity (Fulton, 1928; Kuhar and Alvarez, 2008; Kuhar et al. 2003; Lefko et al. 1998; Parker and Howard, 2001; Seal and Chalfant, 1994). Identification of the wireworm species complex in a given agricultural system and understanding the biology of each species is necessary to develop strategic pest management plans for that system. Wireworm biology varies considerably among genera and among species. Length of time required to complete one life cycle, peak larval activity, timing of adult emergence/egg deposition, host preference, and susceptibility to insecticides are biological features that can differ among 4

17 species and potentially be exploited for control. Management strategies such as insecticide selection and timing of application, manipulation of plant and harvest date, crop rotation, and cultivar selection can be employed based on the biology of the most economically important wireworm species in a region for the control of that/those species. Many wireworm species require multiple years to complete a life cycle, and they overwinter in the soil as larvae. Potatoes are planted in late February and March in North Carolina into fields with existing wireworm populations where the seed pieces and growing roots and tubers are subject to immediate wireworm attack as soil temperatures increase in the spring (Thomas, 1940). Previous studies conducted in North Carolina and Virginia identified the presence of wireworms in the genus Aeolus, Conoderus, Glyphonyx, and Melanotus in agricultural systems (Briggs 1980; Deen and Cuthbert, 1955; Herbert et al. 1992; Kuhar et al. 2003; Kulash and Monroe, 1955; Willis et al. 2010b; Youngman et al. 1993). The wireworm complex occurring in Virginia potato fields has been identified (Kuhar and Alvarez, 2008; Kuhar et al. 2003), but the number and relative abundance of species in the North Carolina potato production system is unknown. Identification of the most economically important wireworm species in the potato producing areas of North Carolina could allow management strategies to be directed towards those species. In North Carolina, potatoes are produced in silt loam soil with high (15-30%) organic matter or low (0-15%) organic matter. Table stock cultivars are typically produced in soils with low organic matter because soils with high organic matter can discolor the potato skin, often rendering them unmarketable. Soil type and organic matter content could influence 5

18 incidence and severity of wireworm damage in North Carolina potato (Gui, 1935). Organic matter content can influence physical properties of the soil such as the ability to absorb and hold water; previous studies indicate wireworms prefer high soil moisture conditions as opposed to low soil moisture conditions (Fulton, 1928; Lefko et al. 1998; Parker and Howard, 2001; Seal and Chalfant, 1994). Insecticides are also affected by soil organic matter; Campbell et al. (1971) found that the efficacy of insecticides aldrin and diazinon was significantly reduced in soils with high organic matter and that the insecticide phorate was not affected by organic matter content. Crop rotation is known to affect wireworm population diversity and abundance (Belcher, 1989; Bryson, 1930; Nash and Rawlins, 1941; Seal et al. 1992; Shirck and Lanchester, 1936). Shirck (1945) found that planting alfalfa in rotation with potato or corn may result in increased wireworm populations in potato and corn, leading to increased wireworm damage. It is well documented that planting wireworm susceptible crops behind sod will also result in increased wireworm damage (Belcher, 1989; Nash and Rawlins, 1941). Because wireworms overwinter as larvae and many species require multiple years to complete development, planting a highly wireworm susceptible crop such as potato in the season following a preferred host may increase the risk of wireworm damage in the second year. The number of M. communis collected in oat baits in sweetpotato fields was shown to be higher in years following corn and soybean than any other rotation evaluated (Willis et al. 2010a). Any crop planted into a field with high wireworm population density, regardless of whether it is a preferred host, is at increased risk of wireworm damage (Willis, 2008). 6

19 There are two commonly used crop rotations in potato production systems in North Carolina: a two year rotation and a three year rotation. The two year rotation consists of potato with early maturing soybeans planted in the same field after potatoes are harvested in year one and corn planted in year two. The three year rotation consists of potato with early maturing soybeans planted in the same field after potatoes are harvested in year one, corn planted in year two, and late maturing soybeans planted in year three. In the past, planting corn in rotation with high value, high-risk crops was a commonly recommended management strategy for reducing wireworm populations (Bryson, 1930; Seal et al. 1992; Shirck, 1945). In the 1930 s, corn planted in the same field in consecutive years was shown to reduce wireworm populations (Bryson, 1930). Seal et al. (1992) studied the effect of crop rotation on wireworm abundance and determined that sweetpotato should be planted behind corn and soybean for reduced wireworm damage incidence. Until recently, broad spectrum carbamate and organophosphate insecticides such as aldicarb and terbufos were applied at plant in corn production. Although corn may have been a suitable host, the insecticides applied to corn were extremely effective at reducing wireworm populations. Most corn seed planted today are treated with neonicotinoids such as imidacloprid or clothianidin for the control of soil insect pests. Recent studies have shown that neonicotinoids such as imidacloprid and clothianidin do not effectively kill wireworms, but instead cause them to enter a moribund state (Vernon et al. 2009). Moribund wireworms eventually recover; and though the seedling corn is protected from damage, wireworm numbers are not reduced (Vernon et al. 2009). It is possible that wireworm populations actually increase over time as a result of this practice, 7

20 leading to an increase in economic damage to potato. Northeastern North Carolina potato fields are low in elevation (<2.4m), and drainage ditches are abundant (usually less than 100m apart). Drainage ditches often harbor a wide variety of non-cultivated flora that could potentially serve as hosts for wireworm larvae when a non-preferred crop is planted into a field. Wireworms of economic importance are known to inhabit grasses (Belcher, 1989; Nash and Rawlins, 1941), and potatoes in close proximity to weedy drainage ditches may be at an increased risk of wireworm damage. It is well known that susceptibility to wireworms varies by potato cultivar (Johnson et al. 2008; Kwon et al. 1999; Langdon, 2012; Strickland et al. 1962). Factors such as market demand, yield potential, and adaptation to local growing conditions influence the selection of potato cultivars grown in a particular region. The following cultivars are the most widely grown in North Carolina: Dark Red Norland, Yukon Gold, Superior, Snowden, Atlantic, and Frito Lay Studies conducted in North Carolina showed that among the commonly grown cultivars, Dark Red Norland and Yukon Gold are the most susceptible to feeding by M. communis, and Atlantic is the least susceptible (Langdon, 2012). Both Dark Red Norland and Yukon Gold cultivars are grown for table stock and are subject to lower damage thresholds than processing potatoes. Planting the least susceptible potato cultivar into fields with a known high population of economically important wireworm species may reduce damage incidence and severity. As mentioned previously, wireworm populations in a field may be influenced by insecticides used in any of the rotation crops planted there. Insecticide use patterns have 8

21 changed considerably in recent years in the potato production system in North Carolina. The implications of these changes to wireworm abundance and pest status in potato have not been studied. For example, corn planted in rotation with potato in North Carolina is often treated with clothianidin, a neonicotinoid that does not cause wireworm mortality, and can lead to increased wireworm abundance (Vernon et al. 2009). Recent changes to insecticide use in potato have also occurred as the most efficacious insecticides for wireworm control in potato such as chlorpyrifos, fonofos, diazinon, carbofuran, and aldicarb have already lost or are at risk of losing registration in the United States (Kuhar and Alvarez, 2008; Willis et al. 2010a). Kuhar and Alvarez (2008) summarized the efficacy of the most common commercially used insecticides over a seven year period; each insecticide reduced wireworm damage incidence in potato by at least 50 percent on average with a combination of ethoprop and imidacloprid providing an 80 percent average reduction. Insecticide efficacy trials provide only relative comparisons of damage to harvested tubers at a single location and year and are often inconsistent between locations and years (Table 1.2, Table 1.3, and Table 1.4). Differences in species composition between locations and differences in climatic conditions between years may contribute to inconsistency of wireworm insecticide efficacy trials. Studies are needed that evaluate how insecticide management strategies currently implemented in North Carolina potato production systems influence overall wireworm species composition, species abundance, and wireworm damage. Time of planting and time of harvest have been shown to influence wireworm damage in potato (Rawlins, 1939; Kuhar and Alvarez, 2008). In New York, early (May) 9

22 planted potatoes have been shown to acquire increased levels of wireworm injury compared to late (July) planted potatoes (Rawlins, 1939). Rawlins (1939) speculated that early planted potatoes coincided with peak wireworm activity. North Carolina potato growers begin planting in mid-february and continue planting through March. In some years, high winter rainfall or low temperatures delay planting. Wireworms become active as soil temperatures increase in the spring (>13ºC) (Fulton, 1928) which in most years would be after potatoes are planted in North Carolina. In North Carolina, potato harvest typically commences in June and continues through July. Kuhar and Alaverez (2008) found that the percent of wireworm damaged tubers increased over time in Virginia where Melanotus communis (Gyllenhal) was the most abundant wireworm species. A better understanding of the relationship between tuber damage and the number of days the crop is in the field could lead to a harvest date selection that balances yield potential and risk of insect damage to maximize profit. The overarching goal of this research is to develop management strategies for wireworm in potato based on implementation of reduced risk practices. Insecticide intensive management programs for insect pests often lead to resistance problems, failure of biological control, and outbreaks of secondary pests (Seal et al. 1992). Alternative approaches to soil insect management that account for potential environmental impact on both a local and global scale while mitigating the risk of resistance to synthetic insecticides in potato are needed (Kwon et al. 1999). Little is known about the wireworm complex in potato in North Carolina, and there is currently no way to predict the risk of wireworm damage in an individual field. 10

23 A two year commercial field study was conducted to determine the wireworm species composition and the risk factors associated with wireworm damage in potato in North Carolina. Commercial potato fields were selected for the study based on the following potential wireworm risk factors: potato cultivar, harvest date, insecticide use patterns, soil organic matter, and crop rotation schedule. Three insecticide efficacy trials were also conducted to determine the most efficacious insecticides for wireworm control in potato and to determine the effect of manipulating harvest date on wireworm damage in potato. Materials and Methods Thirty commercial potato fields were selected in 2010 based on potential risk factors; six soybean fields and 13 corn fields that would be followed in rotation by potato in 2011 were also included in the study. In 2011, 30 potato fields were selected based on potential risk factors; five soybean fields and 10 corn fields were also included in the study. Selected fields were located in Washington, Tyrrell, Hyde, and Pasquotank Counties in NC. Selected fields were rectangular in shape and ranged in size from 2.5 to 8 ha. Evaluating risk factors that influence species composition and abundance Wireworm larvae were collected from selected fields using oat baits in Spring 2010 and Baits consisted of approximately 78g dry steam crimped oats, Avena sativa L. (Purina Mills, St. Louis, MO) soaked in water for 24h. A handheld bulb planter 7.5cm in diameter and 10cm in depth was used to extract a soil core directly in the row to a depth of 11

24 approximately 12cm. Oats were placed into the void and covered with 2cm of soil. One wire flag was inserted into the center of each bait for identification. Six baits were installed in pairs within each field at defined distances from the drainage ditch. Baits remained in the field for 14d after which time a golf course cup cutter 10.5cm in diameter and 15cm in depth was used to extract the bait and adjacent soil. Each 1300cm 3 bait/soil matrix was placed into a resealable plastic bag labeled with site information and the date of collection. Bagged samples were transported to the lab at North Carolina State University, Raleigh, NC and stored at 4ºC until they could be processed. The contents of each plastic bag were spread over a cafeteria tray, and all wireworms were removed from each sample. Wireworms were placed into an 80% ethanol solution and identified to species using the keys of Rabb (1963), Riley and Keaster (1979), and Seal (1990). There is no morphological way to accurately identify the larvae of different species of Glyphonyx; ten Glyphonyx spp. wireworm larvae were reared to the adult stage and, sent to an expert taxonomist for identification. One hundred percent of the specimens were determined to be Glyphonyx bimarginatus (Shaeffer), which gives us reason to believe all Glyphonyx specimens collected in this study were G. bimarginatus. In 2011 only, two yellow sticky traps were installed in each baited field. Each yellow sticky trap consisted of one sticky card fixed to the top of a 3m piece of bamboo using clothes pins. Yellow sticky traps remained in the field for 14d intervals. Click beetle adults were collected from sticky traps and placed into an 80% ethanol solution pending identification to species (Table 1.5). 12

25 Evaluating risk factors influencing wireworm damage in potato A total of 50 potato tubers were harvested from each field beginning 14 days posttuber initiation and every 14 days thereafter until commercial harvest. To evaluate the effect of proximity to drainage ditch on wireworm damage, each field was separated into two sections: 1) edge (a distance within 6 rows of the drainage ditch/field edge), and 2) middle (a distance of greater than 6 rows from the drainage ditch/field edge). Five plants were randomly selected from each field section, and 5 tubers were randomly selected from each plant. Harvested potato tubers from each field section were placed into a large paper bag and transported to the lab at North Carolina State University, Raleigh, NC where they were washed with tap water to remove soil and organic material. Tubers were air dried and evaluated to determine the incidence of wireworm damage to tubers. Potato tubers with at least one wireworm feeding hole were considered damaged in this study. The size of each potato tuber was also measured and recorded. Small plot study evaluating insecticide efficacy and harvest date Three total studies were conducted at 2 locations over 2 growing seasons to evaluate the efficacy of selected insecticides for control of wireworm larvae in potato. potato was planted on 12 April, 2010 and 25 March, 2011 at North Carolina State University (NCSU) Tidewater Research Station (TRS) in Plymouth, North Carolina (TRS 2010 and TRS 2011, respectively); and planted on 9 March, 2011 at NCSU Cunningham Research Station (CRS) in Kinston, North Carolina (CRS 2011). All studies were arranged in a randomized complete 13

26 block design. Tidewater Research Station 2010 contained 10 treatments and 4 replicates, TRS 2011 contained 14 treatments and 4 replicates, and CRS 2011 contained 9 treatments and 4 replicates. Plots at TRS 2010 and TRS 2011 were 1 row by 9m long planted on 91cm centers with unplanted guard rows and CRS 2011 was 1 row by 9m long planted on 107cm centers with unplanted guard rows. Seed pieces were planted by hand using 23cm within row spacing and plants were managed according to local standard practices. In-furrow soil insecticides were applied immediately after seed pieces were placed in the furrow and before furrows were closed on day of planting. Drag-off soil insecticide was applied to the soil surface directly over the row and incorporated with hilling discs. Phorate was applied as a granular formulation using a handheld granule shaker. All other insecticide applications were applied using a CO 2 powered backpack sprayer with a single nozzle boom equipped with a DC25 core and a D4 disc delivering liters per hectare at 2.72 atm. Plots were harvested on 16 July at TRS 2010 and on 16 June at CRS 2011 using a one row tractor-drawn chain digger. To determine the effect of time of harvest on wireworm damage incidence, 3 linear row meters of each plot were harvested by hand at TRS 2011 on 16 June, 23 June, and 29 June. At each harvest, tubers (n=50) were randomly collected from each plot and transported to the lab at NC State in Raleigh, NC. Tubers were washed and evaluated individually for wireworm damage as previously described. Data Analysis Ninety four fields (60 potato, 23 corn, and 11 soybean) were sampled for wireworms 14

27 between 2010 and Selected fields represented production practices of 10 commercial potato growers in northeastern North Carolina. Potential landscape level risk factors included in the study were 10 growers, 2 soil organic matter levels, and 2 crop rotations. Field level risk factors included 6 potato cultivars, 9 insecticide treatments, 3 levels of proximity to field edge in baiting and 2 levels in evaluating wireworm damage, tuber size, plant date, harvest date, and length of time plants remained in the field. Confounding of variables limited inclusion of all risk factors into the statistical model. For example, 5 growers included in the study produce potato only in soils of low organic matter and 5 growers produce potato only in soils of high organic matter. The cultivars Dark Red Norland and Yukon Gold were only grown in soils of low organic matter and were treated with a wide range of insecticides for wireworm control as compared to other cultivars studied. A maximum of 3 cultivars were planted by a single grower and insecticide use varied greatly among growers. Data Analysis - Evaluating risk factors that influence species composition and abundance Mean number of wireworms per bait data were subjected to square root transformation to homogenize variance prior to analysis. Transformed data were analyzed using a general linear mixed model in SAS v9.2 (Proc Glimmix, SAS Institute, 2008). In 2010, fixed effects were insecticide active ingredient and proximity to drainage ditch; random effects were field nested within insecticide active ingredient. In 2011, fixed effects were insecticide active ingredient, proximity to drainage ditch, and cultivar; random effects 15

28 were field nested within insecticide active ingredient. Untransformed means are presented in all figures and tables. Mean separations were based on Tukey-Kramer where means were significantly different at α Data Analysis - Evaluating risk factors influencing wireworm damage to potato All percent data were subjected to arcsine square root transformation and damage severity (holes per tuber) data were subjected to square root transformation to achieve homogeneity of variance prior to analysis. Transformed data were analyzed using a mixed model analysis of variance in SAS v9.2 (Proc Mixed, SAS Institute, 2008). In 2010 and 2011, fixed effects were plant date, number of days in the field, location, cultivar, tuber size, insecticide active ingredient, and cultivar by tuber size; random effects were field, field by proximity to ditch, and field by proximity to ditch by harvest date. Backtransformed means are presented in all figures and tables. Mean separations were based on Tukey-Kramer, where treatment means differed significantly at α A correlation procedure was conducted in SAS v9.2 (Proc Corr, SAS Institute, 2008) to determine whether a correlation existed between the number of wireworms captured in oat baits and wireworm damage incidence in potato. Data Analysis - Small plot study evaluating insecticide efficacy and harvest date All percent data were subjected to arcsine square root transformation and severity (holes per tuber) data were subjected to square root transformation to achieve homogeneity 16

29 of variance prior to analysis. Transformed data were analyzed using a mixed model analysis of variance in SAS v9.2 (Proc Mixed, SAS Institute, 2008). The fixed effect was treatment and random effects were replicate and replicate by treatment. For the trial conducted at TRS in 2011, only the harvest date of 29 June will be used in insecticide efficacy analyses. Untransformed means are presented in all figures and tables. Mean separations were based on Fisher s least significant difference (LSD), where treatment means differed significantly at α Results Evaluating risk factors that influence species composition and abundance Melanotus communis and Glyphonyx spp. were the most abundant wireworm species in both years (Fig. 1.1). A total of 248 wireworm larvae including 107 M. communis (43%), 66 Glyphonyx spp. (27%), 29 Aeolus spp. (12%), 11 C. vespertinus (4%), 4 C. lividus (2%), 3 C. scissus (1%), 27 C. bellus (11%), and 1 other (<1%) were collected from 729 oat baits in 2010 across all fields (Table 1.1). A total of 561 wireworm larvae including 235 M. communis (42%), 172 Glyphonyx spp. (31%), 30 Aeolus spp. (5%), 49 C. vespertinus (9%), 30 C. lividus (5%), 5 C. scissus (1%), and 18 C. bellus (3%) were collected from 1079 oat baits in 2011 across all fields (Table 1.1). Glyphonyx spp. were the most abundant species in soybean in 2010 and 2011; species composition in corn was similar to that of potato (Table 1.1). All Glyphonyx spp. specimens that were sent to the expert for identification were determined to be Glyphonyx bimarginatus (Shaeffer). 17

30 In 2010, insecticide active ingredient had a significant effect on the mean number of wireworm larvae collected per bait in potato (F 6, 23 = 3.74; P= ; Fig. 1.2). The highest number of total wireworm larvae was collected in potato treated with clothianidin or imidacloprid. Fewer total wireworms were collected from fields treated with thiamethoxam or no insecticide than all other insecticides. Insecticide active ingredient had no effect on the mean number of M. communis larvae collected per bait (F 6, 23 = 2.35; P= ; Fig. 1.2). The mean number of Glyphonyx spp. larvae collected per bait was affected by insecticide active ingredient (F 6, 23 = 6.54; P= ; Fig. 1.2). More Glyphonyx spp. were collected in fields treated with imidacloprid than any other insecticide treatment. In 2011, insecticide active ingredient had a significant effect on the mean number of wireworm larvae collected per bait (F 5, 224 = 4.00; P= ; Fig. 1.2). More total wireworm larvae were collected in baits from potato fields treated with clothianidin, thiamethoxam, or bifenthrin than any other treatment in In 2011, more M. communis larvae were collected per bait in fields treated with thiamethoxam or bifenthrin (F 5, = 3.64; P= ; Fig. 1.2). Insecticide active ingredient did not influence the mean number of Glyphonyx spp. larvae collected per bait in 2011 (F 5, = 1.35; P= ; Fig. 1.2). In 2010, the distance from drainage ditches had no effect on the mean number of total wireworms collected per bait (F 2, 58 = 1.63; P= ; Fig. 1.3). There was no effect of distance from the ditch on mean number of M. communis collected per bait (F 2, 58 = 2.85; P= ; Fig. 1.3), or mean number of Glyphonyx spp. collected per bait (F 2, 58 = 1.87; P= ; Fig. 1.3). The distance from the ditch did however influence the mean number of 18

31 total wireworms collected per bait in 2011 (F 2, 224 = 4.04; P= ; Fig. 1.3). Significantly fewer total wireworms were collected near the drainage ditch than any other part of the field. Significantly fewer M. communis larvae were collected near the drainage ditch (F 2, = 6.82; P= ; Fig. 1.3).The distance from drainage ditch had no effect on the mean number of Glyphonyx spp. collected per bait in 2011 (F 2, = 0.95; P= ; Fig. 1.3). Evaluating risk factors of wireworm damage to potato In 2010, wireworm damage incidence was higher in early planted potatoes than late planted potatoes (F 1, 15.4 = 7.36; P= ; slope = 0.012). Length of time tubers remained in the field had no effect on wireworm damage incidence (F 1, 84.3 = 1.85; P= ). In 2011, plant date had no effect on wireworm damage incidence (F 1, 67 = 0.34; P= ); however, as the time tubers remained in the field increased, tuber damage incidence significantly increased (F 1, 82.6 = 8.32; P= ; slope = ). Proximity to the field edge/drainage ditch had no effect on wireworm damage incidence in potato during either year (F 1, 27 = 0.02; P= , F 1, 40.1 = 0.92; P= , 2010 and 2011, respectively). Cultivar did not influence wireworm damage incidence in 2010 (F 5, 15.2 = 2.61; P= ; Fig. 1.4), but did influence wireworm damage incidence in potato in 2011 (F 5, 57.5 = 3.83; P= ; Fig. 1.4). There was no difference in the incidence of damaged tubers in Frito Lay 1867, Dark Red Norland, and Yukon Gold cultivars in The incidence of damage to Dark Red Norland and Yukon Gold was not different from that observed in Atlantic, Snowden, or Superior cultivars. Wireworm damage incidence differed statistically between insecticide active ingredients in 19

32 2010 (F 6, 14.4 = 3.30; P= ; Fig. 1.5), but no effect was observed between insecticides in 2011 (F 5, 51.4 = 0.61; P= ; Fig. 1.5). Previous crop had no effect on wireworm damage to potato in either year (F 1, 14.6 = 2.75; P= ; Fig. 1.6, and F 1, 44.5 = 0.10; P= ; Fig. 1.6, 2010 and 2011, respectively). Wireworm damage was influenced by tuber size in both years (F 2, 161 = 29.17; P< ; Fig. 1.7, and F 2, 169 = 26.31; P< ; Fig. 1.7, 2010 and 2011, respectively). Wireworm damage incidence was significantly lower in tubers with a diameter of less than 4.75cm than larger tubers in both years. There was no difference in wireworm damage incidence in tubers with a diameter between 4.75cm and 6.5cm and those with a diameter greater than 6.5cm. In 2010, an overall weak negative correlation existed (Pearson Correlation Coefficient = 0.185, n=159, P=0.0193) and no correlation existed in 2011 between wireworm capture and tuber damage incidence (Pearson Correlation Coefficient = 0.074, n=163, P=0.3478). Small plot study evaluating insecticide efficacy and harvest date In the insecticide efficacy trial conducted at TRS in 2010, all insecticide treatments provided a significant reduction in wireworm damage incidence (F 9, 29.5 = 2.60; P=0.0241; Table 2) and severity (F 9, 29.7 = 2.84; P= ; Table 1.2) compared to the untreated check. At CRS in 2011, insecticide treatments influenced wireworm damage incidence (F 8, 23.1 = 5.65; P= ; Table 1.3), and severity (F 8, 23.5 = 2.78; P= ; Table 1.3). Damage incidence to potato tubers was significantly reduced compared to the untreated check by all 20

33 insecticide treatments except thiamethoxam. Damage severity in phorate, thiamethoxam, and imidacloprid treatments was not different from the untreated check. At TRS in 2011, 10 out of 13 insecticides had a significant effect on wireworm damage incidence (F 13, 39 = 3.65; P= ; Table 1.4) and/or severity (F 13, 42 = 3.43; P= ; Table 1.4). At-plant applications of bifenthrin plus a drag off application of bifenthrin + imidacloprid, or an at-plant application of either phorate, thiamethoxam, or imidacloprid did not reduce wireworm damage incidence compared to the untreated check. Treatments consisting of: an at-plant application of bifenthrin plus a drag off application of bifenthrin + imidacloprid, an at-plant application of bifenthrin + clothianidin, or thiamethoxam or imidacloprid did not reduce wireworm damage severity compared to the untreated check. There was no insecticide treatment by harvest date interaction for damage incidence at TRS in 2011, therefore all insecticide treatments were combined to analyze the effect of harvest date on wireworm damage incidence. Percent of wireworm damaged tubers increased significantly over three harvest dates (F 2, 4167 = 56.04; P<0.0001; Fig 1.8). Discussion Evaluating risk factors that influence species composition and abundance Wireworm species composition varies greatly by geographic region, and one goal of this study was to determine the overall species composition of wireworms in potato cropping systems of northeastern North Carolina. Information about the biology of the most abundant wireworm species in a region could facilitate the development of management strategies 21

34 directed toward only the most economically important species. Knowledge of wireworm species composition in North Carolina sweetpotato led to the development of management strategies targeting the most abundant species and resulted in significant reductions in damage and insecticide use (Abney and Kennedy, 2008). Previous wireworm population studies in North Carolina were conducted in tobacco, peanut, and sweetpotato cropping systems (Deen and Cuthbert Jr., 1955; Herbert Jr. et al. 1992; Willis et al. 2010b). Knowledge of the wireworm species present in potato in North Carolina is needed to develop management strategies for wireworm in the crop. Potato is produced in the northeastern region of North Carolina, which is geographically distant from all other areas where extensive wireworm study has been conducted. In 2010, M. communis and Glyphonyx spp. comprised 70 percent of the total wireworm population, and in 2011, the two wireworm species made up 73 percent of the overall wireworm population. The remainder of the wireworm species composition was comprised of wireworms from the genera Aeolus and Conoderus. Melanotus communis is found in agricultural production systems throughout much of the central and eastern United States (Fenton, 1926; Jansson and Lecrone, 1989, 1991; Kuhar et al. 2003; Riley and Keaster, 1979; Riley et al. 1974; Wilson, 1940). It is a well-known pest of corn (Fenton, 1926; Riley et al. 1974; Riley and Keaster, 1979), and the most economically important pest of sugarcane in Florida (Cherry and Stansly, 2008). Melanotus communis accounted for seventy percent of wireworms collected on the Eastern Shore of Virginia (Kuhar and Alvarez, 2008). Jansson and Lecrone (1989) reported grade reductions 22