Refined Management of Lygus Bugs in Baby Lima Beans Larry Godfrey 1, Rachael Long 2, Steve Temple 3, Evan Goldman 1, and Mohammad-Amir Aghaee 1

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1 Refined Management of Lygus Bugs in Baby Lima Beans Larry Godfrey 1, Rachael Long 2, Steve Temple 3, Evan Goldman 1, and Mohammad-Amir Aghaee 1 ABSTRACT The western tarnished plant bug, Lygus hesperus, is one of the most challenging pests to manage in California s agroecosystem and production of several types of beans are affected by this pest. Bean yield and bean quality can both be severely impacted. Studies in 2011 were designed to develop improved management options for this pest on baby lima beans in central California. In summary, experimental insecticides were evaluated against this pest in small plots and two materials were found to be very effective. These results support those from Rimon, a recently registered product was tested in grower fields and found to be an effective insect growth regulator that will help suppress lygus nymphs in lima bean production. However, since Rimon does not control adult lygus, it must be tank mixed with another insecticide that has efficacy on adult lygus bugs, which may not be economical in dry bean production. Haskell, a vine baby lima variety with a level of resistance to lygus bugs, was grown with various insecticide regimes intended to supplement the level of pest control under high lygus pressure. The variety performed well but the lygus bug pressure in 2011 was not high enough or occurring early enough in the season to stress the variety and to fully fulfill this objective. Finally, procedures were developed to aid in sampling lygus bug populations in vine baby lima beans with the goal of making treatment decisions. The dense crop canopy of the vine beans inhibits the effectiveness of the sweep net as a sampling tool. However, by adding a correction factor, i.e., doubling the number of lygus captured with the sweep net in vine beans, this tool can be used in pest management and the results are comparable to those from bush beans. Key Words: pest management, lygus bugs, chemical control, host plant resistance INTRODUCTION The western tarnished plant bug, Lygus hesperus, is one of the most challenging pests to manage in California s agroecosystem. Several factors contribute to the severe pest status including 1.) wide host range of more than 200 plants, 2.) high mobility being able to move (fly) several miles, 3.) readily develops resistance to insecticides, 4.) inflicts economic damage to crops such as beans at fairly low numbers, 5.) affects both crop yields and commodity quality, 6.) damaging in both the nymph and adult stages, and 7.) is not well controlled with natural enemies. This is a native insect to the western part of the U.S. and flourishes in our Mediterranean climate. Research has been conducted on this pest for 50+ years but management plans must be constantly refined and updated as conditions change. For a robust management program, biological, cultural, host plant resistance, and chemical controls should all play a role in lygus bug management. Lygus bug management is ranked at the top of the research priority list for large limas, baby limas, and blackeyes, so the importance to the industry is recognized. 1 Larry Godfrey, Evan Goldman, and Mohammad-Amir Aghaee, Dept. of Entomology, Univ. of California, Davis, CA 95616; 2 Rachael Long, Cooperative Extension Yolo County, 70 Cottonwood Street, Woodland, CA 95695; 3 Steve Temple, Dept. of Plant Science, Univ. of California, Davis, CA 95616; ldgodfrey@ucdavis.edu, In: University of California Dry Bean Research: 2011 Progress Report published by the California Dry Bean Advisory Board, 531-D North Alta Avenue, Dinuba CA

2 This research group has been researching lygus bug management in beans for the last several years. The majority of the research has been in small plots on the UC-Davis Agronomy Farm on baby limas and at the UC Shafter REC (up through 2009) on blackeyes. We have made progress on this research as evidenced by the inclusion of studies in 2011 in grower fields (conducted by Rachael Long in the Sacramento Valley on baby limas and by Carol Frate, under a separate proposal, on blackeyes in Tulare County). Godfrey and Temple, facilitated by Long, continued to investigate and refine lygus management on bush and vine baby limas in plots on the UC-Davis Agronomy Farm. Four areas were researched in 2011 as detailed in the objectives below. Objectives: 1.) To compare the performance of registered and experimental insecticides against Lygus bugs in dry beans ( Luna bush baby limas). The results will be evaluated in terms of lygus control, effects on natural enemies, and protection of bean yield/quality. 2.) To compare the efficacy a newly-registered insecticide, Rimon, versus a grower standard against a natural infestation of lygus bugs under a grower field situation. The ability to research this product in a larger area (~5 acres) will allow for more meaningful evaluation. 3.) To investigate and develop a pest management program for Haskell vine baby limas. a.) incorporating the host plant resistance characteristics to lygus bugs of this cultivar with targeted insecticide use. Various insecticide timings, frequencies of application, products, etc. will be evaluated. b.) designing appropriate ways to sample lygus bugs in Haskell vine baby limas such that populations can be evaluated for treatment decisions. 4.) Extend results to the bean industry via field days, updates to the UC Pest Management Guidelines, etc. such that the most up-to-date information is available. Objective 1: Performance of registered and experimental insecticides against Lygus bugs Insecticidal control for lygus bugs is needed in many fields to protect the crop. Insecticides are an important production cost and can have other drawbacks including destruction of populations of beneficials possibly leading to outbreaks of other pests, environmental consequences, regulatory challenges, etc. An integrated approach, where insecticides are one tool used, is desired but the severe pest status of lygus mean that insecticides are the primary method of management in many cases. We are searching for more effective and environmentally-friendly insecticides for lygus bugs and that has been a goal of this project. Lygus bugs have the ability to develop resistance to frequently-used insecticides which emphasizes the need to search for new active ingredients. In recent years, lygus bugs have developed into a significant pest in the eastern half of the U.S. (different species) and in California on many crops (cotton, strawberries, vegetables, etc.) thus companies have placed a renewed effort into identifying new products for lygus bugs. Getting these products into the California bean market is the challenge. In 2010, fourteen different treatments were evaluated for lygus control, effects on natural enemies, and protection of yield/quality on Luna bush baby limas. Rimon is a newly-registered product for beans and that could be useful and was evaluated in Objective 2. The experimental products, sulfoxaflor, Carbine, and Belay also showed promise in 2010 and warranted further evaluations in Several of these new chemistries have unique modes of action which fall into the reduced risk category. This facilitates registration but can necessitate changing application timing and practices. These products do not necessarily quickly kill lygus bugs but disrupt the development and build-up of populations. 27

3 Methods Research was done at the UC-Davis Research Farm. Treatments were applied with a backpack CO 2 sprayer at 30 GPA. Lygus populations are traditionally very high at this location and we generally plan on using three applications per season in an attempt to protect the crop. In 2011, these dates corresponded to 4 Aug. (beans in early bloom stage), 22 Aug. (beans in full bloom), and 2 Sept. (beans in pod fill stage). Plots of furrow irrigated Luna bush baby lima beans were 6 rows x 30 feet with 4 replications in a randomized complete block design. Weekly sweep net sampling of lygus populations was conducted and lygus bugs adults and nymphs and beneficial insects and spiders were counted. Plots were harvested on 26 Sept. and yields were calculated. Bean quality, stings, etc., was determined from a subsample. The following treatments were evaluated in Table 1. Treatment list for lygus bug management study, 2011 Product per A Product * (oz.) 1 Rimon 0.83EC 12 fl. oz. 2 Rimon 0.83EC + Warrior II fl. oz. 3 Rimon 0.83EC + Brigade 2EC fl. oz. 4 Warrior II 1.92 fl. oz. 5 Rimon 0.83EC + dimethoate 4EC fl. oz. 6 dimethoate 4EC 16 fl. oz. 7 Brigade 2EC 6.4 fl. oz. 8 Untreated Carbine 50WG 2.8 oz. 10 Steward EC 11.3 fl. oz. 11 Belay 2.13EC 4 fl. oz. 12 Mustang Max EW 4 fl. oz. 13 Closer 3 fl. oz. 14 Closer 4.5 fl. oz. * Treatments % SilWet L-77 added Results Arthropod Numbers Lygus bug populations were high during the early-mid bloom stage and peaked at about 4 per sweep; the threshold value during this period is ~2 per sweep. During the peak population period, the ratio of adults to nymphs was about 60:40. Populations declined significantly near the end of August and for the last three samples dates and last two applications the populations were less than 1 per sweep. The reason for this decline is unknown. There was a block of later planted beans west of this plot area and this may have attracted the lygus. Generally at this location, lygus bug populations persist until bean maturation. Populations of beneficials were present at moderate levels throughout this study. At 8 DAT, numbers of lygus adults were significantly reduced by Rimon + Warrior, Rimon + Brigade, Warrior, dimethoate, Brigade, Carbine, Belay, Mustang Max, and Closer (both rates). Reductions 28

4 ranged from 42 (Carbine, Mustang Max) to 78% (Rimon + Warrior) (Fig. 1). Nymphal populations were significantly reduced by all treatments by at least 64%. Closer (4.5 oz.) and Rimon + Brigade showed the greatest reduction in nymphal levels. These same two treatments provided the greatest reduction in total lygus levels. At 15 DAT, only four treatments significantly reduced the number of lygus adults. These were Rimon + Warrior, Warrior, Steward, and Closer (3.0 oz. rate). Warrior was most effective with a 56% reduction. Nymphal lygus were the most numerous on this date of any in the study. Rimon, Rimon + Warrior, Rimon + Brigade, Warrior, Rimon + dimethoate, dimethoate, Belay, Mustang Max, Closer (4.5 oz. rate) significantly reduced numbers. Several treatments reduced levels ~70% and this was the greatest reduction seen relative to the untreated. All treatments except Brigade, Carbine, and dimethoate reduced numbers of total lygus at 15 DAT. Following the second application (4 days), lygus levels had overall declined (Fig. 2). Rimon + Brigade, Rimon + dimethoate, Steward, Belay, and Closer (both rates) significantly reduced numbers of lygus adults by at least 73%. All treatments except Brigade and Mustang Max significantly reduced numbers of lygus nymphs relative to the untreated. At 10 days after the second application, lygus adult populations were low and there were no significant differences among treatments. Lygus nymph populations were even lower but five of the treatments zeroed the population. On 9 Sept. (7 days after the third application), levels of Lygus adults, nymphs, and totals were significantly reduced by all treatments except Brigade. Most reductions were in the % range. Seasonal totals showed the fewest lygus adults in the Rimon + Warrior and Warrior treatments (Fig. 3). Lygus nymph populations were lowest in the Closer (4.5 oz.) and Rimon + dimethoate treatments. On most dates, there were few significant differences among treatments for numbers of beneficials. At 8 days after the first application, only populations in Belay and both Closer rates differed (were lower) than those in the Warrior treatment (highest). On the next sample date, populations were lowest in the untreated and highest in Rimon. The numbers of beneficials can be impacted directly by the treatment active ingredient but also indirectly if the insecticide reduces numbers of some food item the predators are feeding on, i.e., with a low level of food reserves they may move. This food item can be some secondary pest such as aphids, leafhoppers, etc. This makes looking at populations on individual sample dates difficult. Examination of the seasonal total showed ~ a 2x differences in numbers of beneficials among the treatments being highest (Rimon) and lowest (Closer [4.5 oz. rate]). Yield and Quality Lima bean yields ranged from 3677 to 4096 lbs./a (Table 2). This range is much less than in past years. Yields in Closer (3.0 oz.) and Belay were significantly lower than those in Rimon, Rimon + Warrior, Rimon + Brigade, Warrior, dimethoate, Brigade, and untreated; the other treatments had intermediate yields. Bean damage (stings) was significantly lowest in the dimethoate treatment and highest in the Steward and Mustang Max treatments. Again the range in damage values was low from 5.4 to 9.6%. The lygus bug populations at this location were lower than normal, of shorter duration, and developed later in the season thus the impact on yield and quality was minimized. 29

5 Figure 1. Lygus bug (adults and nymphs) levels pretreatment and at various days after first application. Figure 2. Lygus bugs (adults and nymphs) at various days after second application,

6 Figure 3. Seasonal means for lygus bug nymphs, adults, and total and for number of beneficials, Table 2. Yield and bean quality results from lygus bug efficacy study, Product Formulation per A (oz.) Lima Bean Yield (lbs./a) % Stung Beans Rimon 0.83EC a 8.0 ab Rimon 0.83EC + Warrior II a 5.7 ab Rimon 0.83EC + Brigade 2EC a 6.3 ab Warrior II a 9.2 ab Rimon 0.83EC + dimethoate 4EC ab 7.3 ab dimethoate 4EC a 5.4 b Brigade 2EC a 7.0 ab Untreated a 7.0 ab Carbine 50WG ab 8.1 ab Steward EC ab 9.5 a Belay 2.13EC b 9.4 ab Mustang Max EW ab 9.6 a Closer b 8.4 ab Closer ab 7.8 ab 31

7 Objective 2: Efficacy of Rimon insecticide under grower field conditions. Rimon is an insecticide that was recently (2010) registered for beans. This product was evaluated in small plots studies from 2008 to 2010 and the performance was moderate. In 2011, a study was undertaken in Yolo and Sutter counties to look at the impact of Rimon (Novaluron) for Lygus bug control in lima bean production. Rimon is an insect growth regulator that prevents nymphs from molting. It does not have efficacy on adult insects. Insect control is by both ingestion and contact. This insecticide is a low risk material and it has minimal impacts to non-target organisms. This same active ingredient (Novaluron) is commonly-used and very effective against lygus bugs in the southern U.S. Their pest lygus is a different species than the one in California but the behavior and damage is very similar to the western species. Methods The trial was a split block design with Rimon at 12 oz/ac tank mixed with a pyrethroid (Warrior at 1.92 oz/ac or an organophosphate, OP) applied to one part of the field (27 acres) and a pyrethroid (Warrior at 1.92 oz/ac) or an organophosphate to the rest of the rest of the field (about 50 acres). Treatment was on July 29 in Yolo County and August 1 in Sutter County. Lygus bugs were sampled with a standard sweep net in each field by taking ten sweep net samples in four different areas of the field, in each treatment block. Insect samples were taken prior to treatment and then about every three days for two weeks post treatment. Brigadier at 5.6 oz/ac (bifenthrin+imidacloprid) was over sprayed over the entire field in Sutter County about a week after Rimon was applied (August 8). At each field and treatment site, 300 pods were collected, at random after the fields were cut and windrowed (around August 29 in Yolo County and September 9 in Sutter County). A subsample of 300 beans were taken from the pods, weighed and scored for lygus stings. Results Rimon suppressed nymphs by about 30% in both sites at peak counts compared to the Warrior or organophosphate treatments (Figures 4 and 5). However, adult counts were about the same in both treatments (Figures 6 and 7). The number of stings per 300-beans was less than 0.5% in both treatments at each site. There were no differences in seed weights between the Rimon and Warrior or organophosphate treatments. Sutter County weights were 93±1 grams for both treatment plots and Yolo County weights were 101±1 grams for both treatment plots. In conclusion, Rimon is an effective insect growth regulator that will help suppress lygus nymphs in lima bean production. This material is used with good success in controlling lygus in strawberries and cotton. However, since it does not control adult lygus, it must be tank mixed with another insecticide that has efficacy on adult lygus bugs, which may not be economical in dry bean production. 32

8 Figure 4. Number of lygus nymphs per sweep, lima beans, Yolo County Field treated with Rimon+Warrior and Warrior alone. Figure 5. Number of lygus nymphs per sweep, lima beans, Sutter County Field treated with Rimon+organophosphate and an organophosphate alone. Entire field over treated with Brigadier about a week after the Rimon treatment. 33

9 Figure 6. Number of adult lygus bugs per sweep, lima beans, Yolo County Field treated with Rimon+Warrior and Warrior alone. Figure 7. Number of adult lygus bugs per sweep, lima beans, Sutter County Field treated with Rimon+organophosphate and an organophosphate alone. Entire field over treated with Brigadier about a week after the Rimon treatment. 34

10 Objective 3: Develop a pest management program for Haskell vine baby limas The host plant resistance characteristics to lygus bugs in Haskell cultivar, with targeted insecticide use, were studied in replicated field plots. Host plant resistance is an excellent pest management tool for many crops/pests. The lower cost of this type of management, lack of environmental hazard and reduced incidence of other problems such as insecticide resistance, compared with insecticides, are significant advantages. Host plant resistance often does not provide complete pest control and, with cases such as lygus bugs, the remaining pests may still inflict economically-important damage. This can be mitigated by combining host plant resistance with other approaches including the targeted use of insecticides. This is the approach examined in this study. Supplemental lygus control with insecticides. Haskell is a vine baby lima bean that has been developed by Steve Temple. This cultivar may function as a stand-alone tool under low to moderate lygus pressure and under high pressure it may be combined with an insecticide to provide cost-effective lygus management. Given the properties of this cultivar to withstand lygus bugs, the highest level of supplemental control from an insecticide is not needed. Instead, a bump in lygus bug control may be a more viable approach when growing Haskell and this added control can be achieved with a soft insecticide. Some soft insecticides will provide excellent control and some of those are under development and a few years from any possible registration. One product, Steward, is in the registration pipeline and, from my experience, one can expect ~60-70% lygus control from an application. The advantage of soft products is that they help to preserve populations of natural enemies predators and parasitoids. Therefore, this may avoid the use of treatments for bean aphids, armyworms, leafminers, and spider mites (these pests often build-up where natural enemies populations have been compromised). Spider mites and aphids have been severe pests of limas in some parts of the SJV and this has resulted in additional costs to control these pests. The severity of the lygus infestation and likelihood of aphids and/or spider mites in each particular location will influence the management regime. Methods Vine lima beans were planted on the UC Davis Agronomy Farm on 27 May; two bean cultivars were compared Mezcla and Haskell. Within each cultivar, various insecticide management approaches were used, 1.) a broad-spectrum insecticide, Mustang Max EW at 4.0 fl. oz. per A and 2.) a reduced risk insecticide, Steward EC at 11.3 fl. oz. per A. In terms of lygus bug control, Mustang Max will likely be better than Steward but coupled with the host plant resistance characteristics of Haskell, Steward is hypothesized to provide adequate lygus management. The third factor considered was application intensity. In 2011, the two insecticides were applied at bloom (4 Aug.) or pod fill (2 Sept.) or with both timings. Treatments were applied to plots measuring 6 rows wide by 30 feet long with each treatment replicated four times. Applications were done with a CO 2 backpack sprayer at 30 GPA. Lygus bug populations were sampled with a sweep net at approximately weekly intervals with bugs separated into nymphs and adults. Finally, bean yields and seed quality were determined from two rows of each plot on 13 October. 35

11 Results Lygus bug populations were slow to develop in this vine baby lima bean plot. The first treatment timing was on 4 Aug. when lygus bug levels were increasing. As shown in Fig. 8A, the untreated plots (designated as late or untreated) had ~0.5 lygus per sweep. The population was ~75% adults and 25% nymphs. There were no obvious differences in numbers between the two cultivars. For the two insecticides, Mustang was slightly more effective than Steward but both showed activity (the early and both treatments received insecticides 8 days before these data were collected). Data from 26 Aug. (Fig. 8B) were collected during the period of peak lygus populations. However, the highest numbers were ~1.5 lygus per sweep with more adults and nymphs. Fig. 8C data were collected 1 week after the late application timing (the both treatment also received insecticides). Lygus bug populations were overall reduced and mostly comprised of nymphs. Yields varied among treatments but there were no trends or significant differences (Fig. 9). The only significant difference in yields was among blocks, i.e., position in field. Block 4 (west end of field) had lower yields than the other three blocks. These yields from block 4 were deleted and the results shown in Fig. 9 are from blocks 1 to 3. The reason for this difference is unknown. Overall, the lygus bug population was not high enough and did not occur early enough in the season the impact bean yields. The highest population seen was ~1.5 lygus per sweep in late Aug. This level of the pest at this late date (3 months after planting) would not have any effects on bean set and yield. Sampling Lygus bugs in vine baby limas. The second area of research for vine baby limas ( Haskell ) was determining how to best sample the vine canopy for lygus bugs. The lygus resistance bred into this variety means that an automatic lygus insecticide treatment will not be needed in many cases. This places an added importance on being able to assess the lygus population in order to make a treatment decision. The sweep net is the standard way to assess lygus bug levels in beans. But the dense, interwoven growth of the vine limas makes the net difficult to efficiently use for sampling, compared with bush baby lima beans. Methods Plots (10 rows wide x ~80 ft. long with 3 reps) of Haskell vine and Beija-Flor bush baby limas were grown on the UC Davis Agronomy Farm and were naturally infested with lygus bugs. Three sampling methods were conducted in each bean type at about weekly intervals from 22 July to 9 Sept., 1.) the standard sweep net (6 sets of 10 sweeps), 2.) D-vac suction samples a method which should better remove lygus bugs from deep in the canopy (6 sets of suctions each covering 10 sq. ft.), and 3.) a 2 sq. meter confined area that was destructively sampled and all plant material and therefore lygus bugs counted. Collected lygus bugs were separated by sex and adults vs. nymphal instars. The sampling as described above was all done mid-morning (~9 to 10 am) and mid-afternoon (~2 to 3 pm). The comparison and relationship between populations sampled in the vine and bush beans will allow us to better design and sampling method/program to be used by PCAs. 36

12 A B C Figure 8. Lygus bug populations per sweep (separated by nymphs and adults) in vine baby lima bean management study. Bean cultivar, treatemnt timing and insecticdes were examined as factors. A.) data from 12 Aug., B.) data from 26 Aug., C.) data from 9 Sept. 37

13 Figure 9. Vine baby lima bean yields with two cultivars, and a range of insecticide treatments for lygus bug management, Results Results, summarized across all sample dates, are shown in Fig. 10. Numerous factors were examined in this study so I will briefly summarize the most important findings. Comparing the sweep net results vs. the vacuum in the two bean types shows that the vacuum sampler captured slightly more lygus than the sweep net in the vine beans. The vacuum machine should operate independent of bean crop canopy and give a better representation of the real number of lygus in the beans. It appears the vine beans have more lygus per unit area than the bush beans (the soil surface areas sampled were the same between the two bean types but the additional height of the vine canopy must be considered). Looking at the sweep net data, the opposite results are seen. There were more lygus captured with the sweep samples in the bush beans than in the vine beans about twice the number. The sweep net cannot efficiently sample the lygus popualtion in the vine beans due to the canopy. It appears that numbers obtained with the sweep net in vine beans should be doubled to equilibrate to numbers collected with the sweep net in the bush beans. This could be important for treatment decisions. The second conclusion is that the numbers with the sweep net are lower, by about 25%, with an afternoon sampling than a morning sampling in both bean types. This difference is due to capturing fewer adult lygus in the afternoon than the morning. The reason for this is either the adults move deeper in the canopy in the afternoon to avoid the heat or that they are more active and avoid (fly off) the sweep net. The same result was not seen with the nymphs or in the vacuum samples. This small difference may not be critical for treatment decision but should be considered. 38

14 Figure 10. Influence of bean type (vine versus bush) and time of day of sampling lygus bugs and assessing populations for a treatment decision, Objective 4: Extending results to the bean industry. The UC pest management guidelines will be modified at the next opportunity for revision. Results and field plots were highlighted to the industry during the UC Davis bean field day in Aug organized primarily by Long and Temple. Other venues for getting results to the industry will be explored and conducted as appropriate. SUMMARY Host plant resistance, biological control, and reduced risk insecticides are fundamental aspects of integrated pest management programs and research was conducted to strengthen all of these in the baby lima bean system. The key pest, western tarnished plant bugs (lygus bugs) was the target of this research. Registered and experimental insecticides were evaluated in small field plots and in grower fields. Several products effectively controlled a low to moderate level of the pest in research plots. Two experimental products, Carbine and Closer, which also had minimal effects on populations of natural enemies (predators and parasitoids) were most promising for the future. Rimon is an insect growth regulator that was recently registered on beans. In large grower field plots, this active ingredient showed activity on lygus bug nymphs. However, the need to tank-mix Rimon with an adult-active product (since both lygus stages are always present in bean fields) appears to limit the cost-effectiveness. Haskell vine baby lima beans appear to have a significant role in lygus bug management because of its ability to withstand a low of lygus bug infestation. Under heavy lygus bug pressure, this variety is not a stand-alone tool and control must be supplemented by insecticides. The selection of insecticides and appropriate timing were explored in field plots but a late-developing lygus bug population limited the appropriateness of this work. Sweep net sampling for lygus bugs to make informed treatment decisions in vine baby limas was also studied. A 2X adjustment factor needs to be used in vine baby limas to compensate for the inefficiency created by the dense canopy. Acknowledgements. The technical assistance of Chip Morris was critical as well as the Godfrey lab student assistants, Jennifer Mueller, Tara Trakin, Brandon Anson, and Gregory Finkelstein. 39