Washington Tree Fruit Research Commission

Research Reports

Lygus Bug and Western Flower Thrips Ecology in Washington State Apple Orchards (2003)

WTFRC Project #13C-3343-3123
YEAR 0/0
Organization Project #
Title:Lygus Bug and Western Flower Thrips Ecology in Washington State Apple Orchards
PI:D.B. Walsh
Organization:Agrichem/Environ. Educ. Spec, WSU-Prosser
 PDF version of report


R. Wight, Field Research Dir., IR4 Project, WSU-Prosser


1.                    Compare Lygus sampling techniques for Lygus on the orchard floor.
2          Develop an early season threshold for Lygus on apples in the Columbia Basin.
3.         Attempt to fine tune the UC Lygus phenology model to fit eastern Washington Lygus populations

OBJECTIVES new for 2002: Thrips
1.         Evaluate edge effects of migration of thrips from adjacent riparian habits into apple orchards.

OBJECTIVES: Riparian Buffers
1.         Develop riparian habitats that will not serve as a point source for Lygus and thrips infestations.

Significant findings

Orchard Sampling. In 2001 and 2002 several Lygus abundance sampling techniques were investigated. Techniques tested included sweep net samples of the orchard floor, beat sampling of trees, and colored sticky cards. Sweep net sampling of the orchard floor was the only technique tested that caught substantial numbers of Lygus.  Tree beating was inefficient at capturing Lygus after a cover spray for codling moth had been applied. Additionally the use of colored sticky cards in measuring abundance of Lygus was not efficient at capturing Lygus.


Lygus Damage. Branch cage studies in 2001 and 2002 helped quantify proportional Lygus abundance to fruit damage. Lygus feeding in April resulted in greater proportional amount of fruit injury then Lygus feeding in feeding in May 2002 or July 2001. Weekly assessments of Lygus infestation at high abundance of Lygus (1 bug per fruit) resulted in high levels of damage when cages were established on any given week from 17 June through19 August 2002. A cover spray of azinphos methyl applied to our research orchard on 5 July effectively killed the Lygus we caged on 8 and 15 July. Azinphos methyl’s residual effectiveness for suppression of Lygus appeared to break down after 15 to 17 days after application.


Phenology Model. A phenology model currently used in California proved effective at predicting the first generation hatch of Lygus in late-May in Eastern Washington and the subsequent peak hatch event in mid-July. However, the model lost predictive accuracy as the season progressed and would provide little predictive value for when adult migration into orchards might occur in April or May.


Biological Control. A parasite Peristenus spp. attacks the nymph stages of Lygus and keeps individuals from reaching sexual maturity by emerging in the late instar nymph or early adult stage. 
Extensive surveys conducted by Walsh in 2002 determined the presence of Lygus parasitism by Persitenus spp. in several important fruit production regions in Washington State. However, the results of the survey were disappointing in that levels of parasitism were low or not detected in several important apple growing areas


Flight monitoring. Thrips flight activities were monitored in apple orchards with yellow and blue sticky card traps in 2001. Blue cards are proving to be significantly (P<0.01) more effective then yellow cards at catching western flower in apple orchards. We have observed a definite orchard edge effect with the orchard floors bordering riparian buffers having significantly (p<0.01) greater populations of thrips then the orchard floor 92 meters inside the orchard. There were also significant (P<0.05) differences in abundance of thrips as measured by blue sticky card with the orchard floor having a greater abundance of thrips then cards placed in the canopy on both the orchard edge and 92 meters in. 


We have developed considerable evidence that riparian areas are confirmed sources of hemipteran and thrips pests. These include both stinkbugs, (Jay Brunner, personal com), and Lygus bugs and flower thrips (Walsh unpl. data). We have also documented an increase in the populations of several beneficial arthropods in riparian buffers. We have also identified host plant on which Lygus can complete development and feral plants around which the abundance of flower thrips are greater then plants that appear to be non-hosts for flower thrips



Methods Lygus
1.       We compared several sampling techniques to assess Lygus populations in the riparian sites and nearby orchard floors in spring and summer 2001 and 2002 to assess when Lygus adults become active and to determine when subsequent generation’s egg hatch takes place. Sampling techniques tested included sweepnet, and colored sticky cards.
2.       Lygus damage thresholds. Sleeve cages were sewn in 2001 that covered 1 meter lengths of apple branch. Fruit was thinned so that constant ratios of Lygus to apples can be maintained. Adult Lygus were introduced into the sleeve cages at ratios of 0.25, 0.17, 0.125, 0.083, 0.056, 0.033, and 0.015 Lygus per fruit. Each cage treatment was replicated 4 times on Fuji fruit set in April and mid-season in July 2001. These same trilals were repeted on May 24, 2002. Cages were left on the trees for 2 weeks and when they were removed the branches were treated with acephate. A damage assessment was taken just prior to commercial harvest. Additional cage studies were conducted in 2002 in which 3 replicate cages were established weekly from 17 June to 19 August in which fruit was thinned to 10 and in each cage 10 Adult Lygus bugs were placed. This was to determine if damage impacts from Lgus feeding changed over time as the growing season progressed.
3.       Phenology model. Data collected in section below “Riparian habitats” was used for this study. Collected weather information from  Washington Public Access Weather Systems data and imported the data into the UCIPM degree day calculator/ phenolgy model calculator for Lygus bugs in a single-sine model with 54°F serving as a horizontal developmental threshold. An arbitrary biofix date of January 1 was used in 2001 and 2002 as the biofix for potential egg-laying.
4.       Biolgical control.  Extensive surveys in 2002 determined the presence of Lygus parasitism by Persitenus spp. Sites were chosen opportunistically during a 2 week period of driving-about eastern Washington State. Sweep nets were used to capture Lygus nymphs from crop plant/ weed hosts. Lygus nymphs were then aspirated into vials and placed on ice and transported to a freezer located at IAREC in Prosser. As time permitted laboratory assistants dissected the nymphs to determine and quantify the presence of the nymphal parasitoid Peristenus spp. In total over 40 sites were surveyed and over 4,000 Lygus were dissected.


Methods- Thrips
We have evaluated thrips population abundance in riparian buffers and on the the perimeter of orchards and in the interior of orchards within the tree canopy and on the orchard floor with blue and yellow sicky cards that were placed every 2 weeks in May through October 2001.


Methods- Riparian habitats. 
1.         In 2001 we identified apple orchards in the Yakima Valley that had “protected waterways” running in, next to, or near their orchards. In early spring 2001 we established 3 field survey sites of 180’ by 180’ along protected waterways and monitored; a) the ambient plant species, and b) the abundance of Lygus, thrips and other pest and beneficial insects present at each respective riparian site and at the adjacent orchard edge and orchard interior every 2 weeks from March through November. In spring 2002 we identified or established distinct specific plant stands on which we began monitoring for the ambient insects present on these plants.

Results and discussion

Orchard Sampling.  In 2001and 2002 several Lygus abundance sampling techniques were investigated during spring. Techniques tested included sweep net samples of the orchard floor, beat sampling of trees, and placement of yellow and blue colored sticky cards on the orchard edge and interior of 4 apple orchards on the Roza near Prosser, WA. Sweep net sampling of the orchard floor was the only technique tested that caught any observable numbers of Lygus.  Tree beating at 10 beats per sample site was ineffective at quantifying Lygus and both colors sticky cards of sticky cards were ineffective. Lygus nymphs were observed on orchard floors but were never directly observed within the tree canopy. At each orchard site application of cover-insecticide sprays resulted in an  observed reduction of Lygus abundance.


Lygus Damage. Lygus feeding has been likened to chemical injury. Lygus feeding damage in apple orchards is a significant concern after fruit set, however feeding damage can result in fruit disfigurement during the fruit growing season. Branch cage studies in 2001 and 2002 have helped quantify proportional Lygus abundance to fruit damage. Three sets of sleeve ages were placed on branches of Fuji trees. Fruit was hand thinned within each cage and specific numbers of adult Lygus were added to each cage to produce specific ratios of fruit to Lygus bug in each respective cage. Ratios of fruit to Lygus per cage included 0, 4, 6, 8, 12, 18, 30, & 60 fruit per Lygus. Cages were left on for 2 weeks at each cycle in April and July 2001 and May 2002 and then removed.  Each caged tree branch was then treated with acephate (Orthene) to prevent subsequent feeding injury from occurring. On August 30, 2001 an.d September 10, 2002 ten fruit were removed from each cage site and peeled with a paring knife. Lygus damage was noted if necrotic feeding spots were present below the fruit skin surface. Our estimates for fruit damage are much higher then typical consumer standards. A majority of Lygus feeding damage was not observable above the fruit skin surface. However, April feeding injury was greater then feeding damage in May or July (Figures 1, 2, & 3) . In 2002 we also designed a sequential sample experiment in which we established cages weekly from June 17, 2002 through August 19, 2002. One meter sleeve cages were placed over tree branches on which fruit had been thinned to a total of 10. Ten adult Lygus bugs were placed into 3 replicate cages each week. On September 10, 2002 ten fruit were removed from each cage site and peeled with a paring knife. Lygus damage was noted if necrotic feeding spots were present below the fruit skin surface. Lygus feeding damage was greatest on the June 17 establishment date at about 70% of the fruit being damaged (Figure 4). Damage for every other week appeared to be fairly consistent.  A application of azinphos methyl was made on July 5 and this effectively reduced Lygus feeding in tyhes cages on the July 8 & 15 cage-loading dates. This indicates that a benefit of azinphos-methyl application includes a 2 week suppression of Lygus fe


Figure 1. April 2001 Feeding Injury

Figure 2. July 2001 Feeding Injury




Figure 4. Sequential feeding injury 2002



Figure 3. May 2002 Feeding Damage




Phenology Model. Lygus overwinter as adults in plants and plant debris. Russian, thistle, Kochia, smotherweed, mullein, horseweed, sweetclover, wild mustards, ragweed, and sagebrush are among many plants that will serve as good overwintering hosts for Lygus. Overwintering Lygus adults became active as temperatures warmed in spring 2001 and 2002. We observed our first adult Lygus on 2 April and 9 April 2001 and 2002, respectfully. A phenology model Walsh helped develop in 1990 (www.ipm.ucdavis.edu) proved effective at predicting the first generation hatch of Lygus in spring in Eastern Washington (Table 1) and the subsequent peak hatch periods for the 2nd and 3rd generations of Lygus in 2001 and 2002. In running this model degree days are accumulated starting on January 1 with 54° Fahrenheit serving as a horizontal lower-cutoff for development. Eggs laid by adult Lygus in late-fall or winter will require approximately 252 degree days in order to hatch. This corresponded well with when we observed our 1st generation of nymphs in both orchards and in our riparian survey sites (Table 1) in mid to late May.  Although this model proved fairly effective at predicting hatch periods for Lygus during the summer months it has been our observation that the majority of feeding injury on apples is caused by adult Lygus and that this model does little towards predecting when adult Lygus will migrate into apple orchards. Rather rainfall patterns and the subsequent dry-down of the over-wintering hosts of Lygus is a prime cause of spring movement of adult Lygus. During the summer months harvest of field and forage crops (primarily alfalfa) also contributes to movement of adult Lygus. A highly preferred weed-host for adult Lygus during the summers of 2001 and 2002 was Kochia. Preferred hosts for nymph development included alfalfa and wild mustards.


Table 1. Cumulative degree day accumulations 54°,  predicted dates of peak hatch for the 1st, 2nd, and 3rd generations of Lygus and the actual average number of Lygus caught per sweep among all of the samples taken bi-weekly at our riparian survey sites (n=72) for 2001 and 2002.


                                                2001                                                                2002
                        Cumulative        Model              Nymphs            Cumulative        Model              Nymphs
Date                  DD                  Prediction         /sweep               DD                 Prediction         / sweep
March 1                0                                          n/a                      9                                            n/a
April 1                 35                  (1st adult 4/2)     n/a                     27                    (1st adult 4/9)     n/a
April 15               39                                          n/a                     69                                            n/a
May 1               109                  1st hatch            0                      119                    1st hatch            0
May 15              198                  (May 23)          0                      166                    (May 27)          0
June 1               384                                          1.6                  307                                            0.6
June 15              484                                          1.2                   473                                            1.3
July 1                675                                          0.8                   713                                            1.5
July 15               961                  2nd hatch           1.3                   991                    2nd hatch           3.2
August 1          1196                  (July 23)           3.0                 1351                    (July 17)           0.3
August 15        1513                                          1.6                 1573                                            1.2
September 1     1784                  3rd hatch           0.7                 1870                    3rd hatch           3.4
September 15   1984                  (Sept. 8)           2.4                 2052                    (Sept. 8)           1.3
October 1        2153                                          1.2                 2173                                            1.3
October 15       2215                                          0                    2235                                            0
November 1     2237                                          0                    2272                                            0


Biolgical control.  A parasite attacking Lygus spp. was discovered in 1995 in Washington State and subsequent collections in Parma, Idaho in 1996 and 1997 showed that the parasite was present (Mayer unpublished data).  The parasite has been described as Peristenus howardi Shaw (Hymenoptera: Braconidae), a new species . Previously, Peristenus pallipes Curtis was reported from Idaho. However, recent taxonomic work on the genus indicates that these may have been misidentified.  Peristenus spp. attacks the nymph stages of Lygus and keeps individuals from reaching sexual maturity by emerging in the late instar nymph or early adult stage.


Collections made in 2000 (Mayer, unpublished data) did not document  the parasite’s presence beyond the Touchet, Washington and Parma, ID regions. Extensive surveys conducted by Walsh (unpublished data/ Table 2) in 2002 determined the presence of Lygus parasitism by Persitenus spp. in several important fruit production regions in Washington State. However, the results of the survey were disappointing in that levels of parasitism were low or not detected in several important apple growing areas (Table 2).Extensive surveys in 2002 determined the presence of Lygus parasitism by Persitenus spp. In total over 40 sites were surveyed and over 4,000 Lygus were dissected to determine if Peristenus spp were present. Parasitism by of Lygus by Peristenus was greatest in areas that were less disturbed by human activity.



         Table 3. Presence of the parasitoid Peristinas spp. attacking the nymphs of Lygus bugs- July 2002.
                                                                                                                                                      Number of nymphs                   Percent
Location                                                              State                   Vegetation Type                  examined                                                                             parasitized       parasitism
Touchet                                                                 Washington          mustard                                            320                101                                                                              32%
Crosby Site                                                            Washington          yellow mustard                                 88                  11                                                                                13%
Yakima Valley Hwy near Donald and Snokist          Washington          alfalfa                                              140                15                                                                                11%
Naches                                                                 Washington          alfalfa                                              90                  8                                                                                 9%
Monitor                                                                 Washington          wild alfalfa                                       207                14                                                                                7%
Roza                                                                     Washington          alfalfa                                              45                  3                                                                                 7%
Corner Snipes & Wilson (Sunnyside)                       Washington          feral alfalfa                                       168                9                                                                                 5%
Sommers Rd.: 4 Miles North of Hwy 26                  Washington          canola                                              70                  3                                                                                 4%
Hwy 12: Burbank                                                   Washington          alfalfa                                              25                  1                                                                                 4%
Chaffee Rd. near Kershaw Hgts.                           Washington          mustard                                            153                5                                                                                 3%
Othello Station                                                       Washington          alfalfa                                              81                  2                                                                                 2%
Othello                                                                  Washington          alfalfa                                              167                4                                                                                 2%
Factory Rd: Sunnyside                                            Washington          alfalfa                                              196                4                                                                                 2%
Corner of Robblard and Komnawack Pass Rd         Washington          alfalfa                                              275                4                                                                                 1%
4 Miles West of Pomeroy                                       Washington          alfalfa                                              142                1                                                                                 1%
Wapato: Hwy 97 & Branch Rd.                              Washington          alfalfa                                              145                1                                                                                 1%
3735 "Bjur Ranch" near Clark Rd.                           Washington          dry alfalfa                                         181                1                                                                                 1%
10 Miles Southwest of Ephrata: Hwy 282                Washington          alfalfa                                              186                1                                                                                 1%
Kahlotus Hwy Pasco                                             Washington          Russian thistle/ mustard                     134                0                                                                                 0%
6 Miles East of Othello                                           Washington          mustard on edge of alfalfa                 209                0                                                                                 0%
15 Miles East of Ephrata                                        Washington          lettuce seed                                      133                0                                                                                 0%
Road 0: 10 Miles East of Quincy                             Washington          alfalfa by apples                                175                0                                                                                 0%
Across from Trinidad                                             Washington          apple orchard floor/ferrel alfalfa         129                0                                                                                 0%
Wenatchee: Wildwood/Hwy 97                               Washington          alfalfa next to apple orchard              343                0                                                                                 0%
Wenatchee: TFREC                                               Washington          Russian thistle & knapweed               69                  0                                                                                 0%
Sunnyside                                                              Washington          concord grapes/mustard                    129                0                                                                                 0%
IAREC HQ                                                           Washington          alfalfa                                              124                0                                                                                 0%
Orondo Orchard: 2 Miles North of Orondo              Washington          feral alfalfa next to apples                 112                0                                                                                 0%
Corner Scoon & Williamson (Outlook)                    Washington          alfalfa                                              90                  0                                                                                 0%
Yakima: Birchfield & SR24                                    Washington          alfalfa                                              43                  0                                                                                 0%
Hwy. 22 & Fisher (Mabton)                                   Washington          alfalfa                                              77                  0                                                                                 0%

Thrips Sampling. Thrips flight activities were monitored in 3 apple orchards in the Yakima Valley near Prosser. This study was run in conjunction with other projects that we had ongoing on thrips management on several other crops comparing blue or yellow sticky cards as a monitoring tool. At present thrips abundance has been counted on 756 blue cards and 886 yellow cards. Blue cards are proving to be significantly (P<0.01) more effective then yellow cards at catching western flower thrips across a range of crops.

Thrips per 3” by 5” sticky card



In apple orchards this same pattern remained consistent as well across 3 sample dates

Thrips per 3” by 5” sticky card


Riparian Habitats.  Populations of agricultural pest insects in Pacific Northwest irrigated agro ecosystems are increased by the improper establishment and maintenance of riparian buffers adjacent to ephemeral creeks that can serve as habitat corridors. Exotic species of plants persisting in degraded riparian buffers are serving as better hosts to problematic species of arthropods including Lygus than are native plant species. In contrast, populations of some beneficial arthropods including spiders and Carabid beetles increase in the presence of several exotic plant species. Arthropod and vegetation surveys conducted in representative riparian buffers adjacent to apple orchards have determined trends in associations between pest and beneficial arthropods and the species of plants they persist on. Pending regulations that may arise from the Endangered Species and Clean Water Acts necessitate these studies. Knowledge of insect plant associations when re-habilitating degraded riparian buffers will enable land owners/ growers to establish plant species that are more likely to host beneficial predacious arthropods, and less likely to serve as hosts to pest insects. From our studies we have identified plants which appear to serve as host for the pest arthropods Lygus and Thrips. In spring 2002 we established distinct plant stands of selected plants (Table 4) on which we will conduct comprehensive study in future years.
Our  research on riparian habitats clearly points out the need for weed and insect management tools during the establishment/rehabilitation phase of  riparian buffers.


Table 4. Plants selected for comprehensive study in 2002/2003 field studies


Common Name                               Genus species                           Native              Pest host
Reed canary grass                           Phalaris arundinacea              Yes                  Yes
Wild rose                                        Rosa spp.                                 Yes                  Yes
Big sagebrush                                 Artemesia tridentata                 Yes                  Yes
Canada thistle                                 Cirsium arvense                       No                    No
Perennial pepperweed                     Lepidium latifolium                  No                    No
Kochia                                            Kocia scoparia                                    No                    No
False london rocket              Sisymbrium loeselii                              No                    No
Spike rush                                       Eleocharis spp.                                    Yes                  No
Gray Rabbit brush                           Chrysothamnus nauseosus       Yes                  No
Common mullein                              Verbascum thapsus                  No                    ?
Cheat grass, downy brome   Bromus tectorum                                  No                    ?
white horehound                              Marrubium vulgare                 No                    ?
Tumble mustard                              Sisymbrium altissimum             No                    ?
Yarrow                                           Achillea millifolium                  Yes                  ?
Stinging nettles                                Urtica dioica                           Yes                  ?
Prickly lettuce                                 Lactuca serriola                      No                    ?
Saltbush                                          Atriplex patula                         Yes                  ?


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