Biology and Management of Bark Beetles (Shothole Borer) in Stone and Pome Fruits

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1 Biology/Phenology Biology and Management of Bark Beetles (Shothole Borer) in Stone and Pome Fruits Mike Doerr 1, Jay Brunner 1 and Tim Smith 2 1 Washington State University Tree Fruit Research and Extension Center, Wenatchee, WA 2 Washington State University Extension, Wenatchee, WA Keywords: bark beetles, shothole borer, Scolytus rugulosus, ambrosia beetle, Coleoptera, Scolytidae, biology, management, insecticides, monitoring, stone fruit Abstract: We will report results from a one-year project that focused on biology and management of bark beetles in Washington State. The dominant bark beetle found throughout the state was the shothole borer (SHB), Scolytus rugulosus Müller (Coleoptera: Scolytidae). An ambrosia beetle, the European shothole borer (AB) [Xyleborus dispar Fabricius (Coleoptera: Scolytidae)] and/or the lesser shothole borer [Xyleborinus saxeseni Ratzeburg (Coleoptera: Scolytidae)], was present in high numbers at only one location, a cherry orchard abandoned for several years. At many locations more than one species of Scolytidae were detected. The life histories as determined in 2003 indicated one prominent flight that peaked in e and a probable but less prominent second flight of SHB and possibly two flights of ambrosia beetle. These data were inconsistent with some of the reported literature. Beetles appeared to live a long time or emerge from various hosts over a long time. Woodpiles of apple and cherry are the main sources of high beetle densities that cause injury to commercial orchards. Cherry trees were most heavily attacked and in some cases severe injury resulted to trees adjacent to woodpiles. Injury accumulation continued well after peak flight, with damage occurring through September. Traps were useful in identifying peak activity periods but it was not clear whether they would be useful in setting thresholds for treatments. Available attractants enhanced trap captures but it seemed likely that with some effort better attractants could be found. A preliminary protocol was developed for screening insecticides against SHB adults. The method seemed to work well when tested against older adults but needs to be validated against adults of a known age distribution. Introduction Bark beetles (Coleoptera: Scolytidae) have been reported historically as pests of pome and stone fruit. The use of synthetic organic insecticides reduced problems with bark beetles, and until recently they have been considered sporadic and localized pests. Bark beetles generally attack weakened trees and cause limb or even tree death if present in high enough numbers. Sanitation has been touted as the key to control, with brush piles identified as sources of beetles that attack other trees. In recent years the reported incidence of injury from bark beetles has been increasing. Injury has been attributed to beetles moving into orchards, especially cherry orchards, and attacking healthy trees. The damage most often noted is from beetles boring at the base of buds, causing the buds to die. The downturn in the economy of the fruit industry has no doubt contributed to the bark beetle problem. In 2003 the Washington Tree Fruit Research Commission supported a one-year project to examine the bark beetle problem. Based on the results of this one-year study, the results of which

2 are reported here, it seems clear that the available information on bark beetle biology does not accurately describe the situation in Washington. While the beetle species that is the primary cause of injury in orchards does appear to be the shothole borer (fruit-tree bark beetle), Scolytus rugulosus Müller, there is at least one other species of bark beetle involved in attacking live cherry trees, the ambrosia beetle, possibly Xyleborus dispar (Fabricius) or Xyleborinus saxeseni (Ratzeburg). Timing of insecticides for pests invading orchards from external sources is always a challenge. It requires not only knowledge of the pest s development in host plants but also of its ability to move from these plants into orchards. Monitoring of this activity can be helpful, and more research is needed on how traps can be best used to indicate the beginning of beetle movement into orchards. In addition, some mechanism is needed to determine how much of an orchard needs to be protected and over what time period. Of course, knowledge of which insecticides are effective and for how long is critical to management of bark beetles. Materials and Methods Species identification: Bark beetles were reared from infested stone and pome fruit. Infested fruitwood was collected from many SHB infested areas throughout central Washington. Care was taken to record the plant host and describe the galleries as these can be important characteristics in identifying scolytids. Adult SHB and associated natural enemies were collected from the infested wood by laboratory rearing in darkened cardboard cages having a vial to catch emerged beetles and parasitoids that fly toward the light. Adult beetles were preserved and identified using established keys. Samples were collected from spring through fall and from all locations of the state. Verification of the identifications will be done by submitting specimens to experts. In addition to beetles, the natural enemies collected from the beetles or emerging from galleries will be identified. Life history/monitoring: At selected locations, the seasonal life history (adult activity) was examined. Beetle emergence from wood was monitored by collecting infested wood and rearing SHB to the adult stage in a cage. Cages were examined at regular intervals and beetle adults collected. The beginning of new beetle egg laying galleries was recorded and tagged. Any second-generation emergence was followed in these cages. The timing and longevity of adult beetle activity were compared to historical information on bark beetles. There are established monitoring methods for bark beetles. These are generally associated with monitoring in forest systems but could have some potential for use in orchards. We monitored infested sites with yellow sticky traps (unbaited Pherocon AM Trap, Trécé, Inc.) along with commercially available interception traps (Pane Intercept Trap, IPM Technologies; 8- funnel Lindgren Funnel Trap, Phero Tech, Inc.). Several side-by-side comparisons were made with the available trapping systems to determine the most effective way of monitoring SHB infestations. All traps were placed when beetles began to emerge in the spring (April) and were checked at regular intervals throughout the season. The sites used for species identification described above were used for monitoring studies.

3 Distribution of feeding damage from a source adjacent to a cherry orchard was monitored at one orchard with a particularly severe infestation. The situation was a burn pile started in the spring of 2003 next to a firewood pile that was added to annually. A five-yr-old Rainier cherry (Prunus avium L., Rainier ) orchard was planted adjacent to this source. Ten trees from each row were sampled in September and the percentage of shoots with visible signs of SHB adult feeding was recorded. Samples were taken from each row moving from the border row near the source toward the interior of the orchard until no further SHB damage was noted. Laboratory rearing: In order to make progress on the understanding of bark beetle control and biology it will be very helpful to establish a laboratory colony. This would allow for year-round research activity and the possibility of developing an insecticide screening method for beetles. Various sources of wood types and conditions will be used to initiate colonies. SHB adults were collected from infested wood as described in the Species Identification section. The majority of adults were collected following peak flight noted in e. Adults were exposed to drying wood in a cage, as well as cups of artificial scolytid diet (Southern pine beetle diet #F9761B, Bio-Serv, Inc.). The rearing arenas were examined regularly for SHB feeding activity and any sign of reproduction. Insecticide screening: Adult SHB were collected from infested wood that was returned to the laboratory in August and September. The insecticide screening methodology was designed to analyze the efficacy of field-aged residues. Mature 'Delicious' apple trees at WSU-TFREC were treated with recommended rates of various insecticides commonly used in apple and cherry production. An effort was made to screen candidate insecticides from as many classes of insecticides as possible. Treated apple branches were collected at 1, 7, 14 and 21 days after treatment (DAT) and returned to the laboratory. Approximately 6-inch sections of 1- to 2-yr-old wood were added to 32 oz. deli cups (Anchor Packaging, Inc.). Untreated apple branches were used as a control at each evaluation date. SHB adults were added to the deli cups and survival was recorded at 1 day and 3 days. All of the chemicals screened were neuro-active insecticides with relatively quick modes of action. Consistent data were collected at the 3-day evaluation and it was not necessary to run the test for a longer time period. Five cups with five SHB adults were set up for each insecticide at each evaluation period (25 SHB adults/treatment). Results and Discussion Species identification: Infested fruitwood was collected from 10 sites ranging from the Wenatchee Valley to Oroville. Several hundred adult scolytids were collected and preserved for identification. At the time of this report, verification of our initial identifications had not been completed. The dominant bark beetle found throughout the state was the shothole borer (SHB), Scolytus rugulosus Müller. An ambrosia beetle, the European shothole borer (AB) (Xyleborus dispar Fabricius) and/or the lesser shothole borer (Xyleborinus saxeseni Ratzeburg), was present in high numbers at only one location, a cherry orchard abandoned for several years. At many locations more than one species of scolytids was detected. Further, many other wood decomposing beetles were reared from infested wood. In fact, the majority of beetles collected were associated with dry, dead wood. Buprestids, bostricids and powderpost beetles (Lyctidae)

4 were the primary families of beetles associated with dry, dead wood. The SHB and AB appear to be the primary attackers of live and weakened trees. Initial observations at the time of collection show that there was fairly significant parasitism (up to 50%) of SHB larvae by hymenopteran parasitoids. These have been submitted for identification. Life history/monitoring methods: It was believed that AB overwinter as adults while SHB overwinter as mature larvae or pupae. Thus it would be expected that AB adult flight would be detected first, as temperatures warmed and the adults became active. This appeared to be the case in 2003, as AB adults were detected in the traps prior to SHB (Fig. 1). AB activity began in early and peaked in mid-. A possible second flight was observed during July. The life histories as determined in 2003 indicated one prominent flight of SHB that peaked in e. Adult activity was noted with both species throughout the entire summer. These data were inconsistent with some of the reported literature for both species. It should be noted that our traps were not placed until the middle of April, and it is known that some scolytids, and possibly SHB, could be active during March. It is not known if this is the case in Washington, but initiating trapping earlier in 2004 should resolve that question. Beetles appear to live a long time or emerge from various hosts over a long time. The exact biology and life history of these insects were difficult to determine with one year s research. It was possible that a portion of the summer SHB larval population emerged during the late summer, while the majority of larvae ceased development, remaining in a diapause-type condition. These individuals likely will overwinter and emerge next spring or early summer. Woodpiles of apple and cherry are the main sources of high beetle densities that cause injury to commercial orchards. Cherry trees are most heavily attacked, and severe injury can occur to trees closest to woodpiles (Fig. 2). Injury appeared to be closely associated with the population source and extended only a short distance into an orchard. Younger trees were more susceptible to severe injury. A high incidence of beetle attacks to a young tree can result in injury that can result in tree death. Injury accumulations from SHB, as noted by adult feeding at the base of buds, began during the peak flight in e and continued through October. Traps were useful in identifying beetle activity but it was not clear whether they would be useful in setting thresholds for treatments (Fig. 3). There did not appear to be any significant difference in commercially available trap types. Either the Pane Intercept trap or the Lindgren Funnel Trap was a suitable trapping system to monitor both SHB and AB adult activity. Available ethanol attractants enhanced trap captures, but the lure system seemed to be a limiting factor in the monitoring system (Fig. 4). It is likely that better attractants could be found. Laboratory rearing: Laboratory rearing of multiple generations was not successful. Adult SHB survived a short time on an artificial diet, however, no reproduction occurred. Adults were collected and reared in large numbers from larval infested wood in the laboratory. Rearing of beetles on cherry or apple wood should present no problems but it would take a lot of space to rear numbers needed to conduct additional studies of development or pesticide screening. An efficient means of rearing insects in the laboratory or under controlled environments would make precise observations of phenology possible.

5 Insecticide screening: The average survival of SHB adults introduced to a rearing arena with untreated sections of apple wood (4 inches by 0.5 inches diameter) was greater than 80% through three days of observation. Lower levels of beetle survival when exposed to wood sections treated with insecticide could be attributed to the effects of the insecticide. Our results indicate that the most toxic insecticide against SHB adults was the pyrethroid Asana XL (Table 1). It is likely that other products from this same class of insecticides (e.g., Warrior or Danitol) would also be highly toxic to beetles. There was some evidence that the organophosphate insecticides tested may not provide the level of control that a grower would like. Surviving beetles were detected at each bioassay interval with Guthion (azinphosmethyl). Malathion was very active as a 1- and 7-day-old residue, but further studies are required to characterize the residual life of this insecticide. There are additional organophosphate insecticides, including diazinon, that need to be evaluated in Thiodan (endosulfan) appeared to be similar in toxicity levels to Guthion. It appeared that the chloronicotinyls, Assail (acetamiprid) and Actara (thiamethoxam), the carbamates, Carzol (formetanate hydrochloride) and Sevin (carbaryl), Avaunt (indoxacarb), and Success (spinosad) were not highly toxic to SHB. However, mortality was noted with all these products for up to 7 days, and their repeated use during the early growing season as cherry fruit fly controls may be responsible for maintaining SHB and AB populations below damaging levels in most commercial orchards. Cherry orchards probably become most susceptible to injury from bark beetles in the postharvest period when insecticide programs for cherry fruit fly and leafroller have ended. The bioassay method worked well when tested against SHB adults collected in late summer but the age of beetles can add variability to a test of this kind. Therefore we will validate the test results using adults collected during the peak flight of late spring, when the age distribution of the beetle population is likely to be less variable. Summary The most effective way to control bark beetle infestations in an orchard is through good sanitation practices. If an orchard is experiencing injury from bark beetles, then the source needs to be located and eliminated. Damage detected in orchards is often closely associated with an outside source, e.g., firewood or brush piles. If the infestation is from within the orchard, it must be treated aggressively by removing all infested wood or unhealthy trees. Once established within an orchard, bark beetles can continue to weaken trees and reproduce successfully. Healthy, vigorous trees have a natural defense against bark beetle attacks. When healthy trees are attacked, sap flows into the excavation holes made by adult beetles, destroying the oviposition galleries, thus preventing reproduction. However, repeated attacks eventually weaken trees or branches, allowing for successful colonization. Insecticide management of bark beetles will be most successful at reducing the population if targeted during peak adult activity, late through e. Only adults that contact or ingest insecticide residues will be affected. To our knowledge there are no insecticides that can affect larvae within the tree. Late through e is an insecticide-intensive period in stone fruit management. Success applied for control of leafrollers, Thiodan applied for aphids, rust mites or stink bugs, Diazinon applied for scale and a variety of cherry fruit fly sprays (e.g., Malathion, Guthion, Sevin, Diazinon or Success) should all coincidently suppress SHB damage

6 originating from the first flight. However, after cherry harvest insecticide use is greatly reduced, and the trees are susceptible to damage in August and September. A grower s best option to protect trees at this time may be Thiodan or Guthion. Based on our research today a softer, more specific insecticide is not available for controlling SHB. The synthetic pyrethroid Asana is available after harvest but each grower will have to evaluate his tolerance for a possible disruption to integrated mite control, as these products have been implicated repeatedly in mite flare-ups.

7 35 30 SHB AMBROSIA Avg SHB/trap Avg AB/trap Apr Jul 23- Jul 6- Aug 20- Aug 0 Fig. 1. Average adult scolytid captures in ethanol-baited traps, 2003.

8 % SHB infested shoots Row 1 Row 2 Row 3 Row 4 Row 5 Row 6 Row 7 Rows from SHB source Fig. 2. Distribution of adult SHB feeding damage by distance from a source adjacent to a cherry orchard, 2003.

9 30 Lindgren Pane Intercept 25 Avg SHB adults/trap Apr Jul 23- Jul 6- Aug 20- Aug Fig. 3. Adult SHB trap captures in commercially available scolytid traps, 2003.

10 35 30 Lure No Lure 25 Avg SHB adult/trap Apr Fig. 4. Relative effect of an ethanol lure on adult SHB trap captures, 2003.

11 Table 1. Relative effect of field-aged insecticide residues against SHB adults, September 2003 Rate Avg corr % mort. (3-day evaluation) Insecticide (form/a) 1 DAT 1 7 DAT 14 DAT 21 DAT Asana XL 8 fl oz Malathion 50% 1.5 qt Guthion 50WP 2 lbs Thiodan 3E 3 qts Actara 25WDG 4.5 oz Success 2SC 6 fl oz Avaunt 30WDG 6 oz Assail 70WP 3.4 oz Carzol 92SP 1.25 lb Sevin XLR 1 qt Bioassays not run for these insecticides on these dates due to lack of beetles.