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Pear Psylla Integrated Pest Management

Updated by Tianna DuPont, WSU Extension; Louis Nottingham, Robert Orpet, WSU Entomology. May 2022. Adapted from by Everett C. Burts, Helmut Riedl, and John Dunley, originally published 1993.

Cacopsylla pyricola (Foerster) (Hemiptera: Psyllidae)

Pear psylla is an important pest of pear in Washington. Honeydew produced by pear psylla causes fruit russet, and serious infestations can stunt and defoliate trees.


Pear psylla likely arrived in the United States along with shipments of pear nursery stock from western Europe. It was first found in Connecticut in 1832 and spread to Washington State by 1939 (Westigard et al., 1979). Within a few years it became a serious pest throughout all pear growing areas in the Pacific Northwest.


In the Pacific Northwest, pear psylla is a pest only of pear. Several other plants are transitory hosts and overwintering sites for winterform pear psylla adults. Pear psylla adults may feed on other deciduous fruit trees including apples, conifers and shrubs as they disperse from pear orchards in the fall and return in the spring (Horton et al., 1994). However, pear psylla does not reproduce on these transitory hosts (Kaloostian, 1970; Cooper et al., 2019). In addition, pear psylla adults will colonize ornamental Bradford pears (Pyrus calleryana) and nymphs can develop, however, populations in Bradford pears are much lower than commercial cultivars and it is unclear if immatures can complete development.

Life stages


The egg, shaped like a grain of rice, is attached to the host by a small protrusion extending from the rounded end. The egg is creamy white when laid but turns yellow to orange as it develops (Figure 1).


The nymph passes through five instars each of which ends in a molt. The first instar is translucent yellow. It is long, cylindrical and about the size of the egg. Each successive instar is larger, flatter and more oval than the last. The fourth instar nymph is yellow-green to light tan. The fifth instar is dark green to dark brown. Third, fourth and fifth instars have progressively larger wing pads (Figure 1–3).


There are two adult forms: winterform and summerform. Both forms of adults hold their wings roof-like over the abdomen. Adults have reddish brown bodies with black markings, and winterforms may appear almost black. The winterform is larger (wing length 2.3 to 2.5 mm) than the summerform (wing length 1.6 to 1.8 mm) (Figure 4–5) (Slingerland, 1892; Wong and Madsen, 1967; Burckhardt and Hodkinson, 1986).

early instar nymph and eggs on leaf
Figure 1. Pear psylla eggs and early instar nymph (E. Beers).
nymph and young on a leaf
Figure 2. Pear psylla nymph hardshell (left), young nymph (right) (T. DuPont).
circular drawing of the developmental cycle
Figure 3. Developmental cycle of the pear psylla. Adapted from Tougeron et al. 2021.
 adult on leaf
Figure 4. Pear psylla summerform adult (T. DuPont).
closeup of adult
Figure 5. Pear psylla winterform adult (E. Burts).

Life History

Pear psylla overwinter as winterform adults in a state of reproductive diapause. They begin laying eggs when pear buds begin to swell. First eggs are deposited on the wood, generally at the base of fruit and leaf buds (Horton, 1999). Offspring of the overwintered generation become summerform adults first appearing in mid-May. Pear psylla has two to four summerform generations in most pear growing regions, with generally two complete summerform generations occurring in Washington (Horton, 1999). Summerform adults tend to lay eggs on rapidly growing leaf tissues often placing eggs along the leaf mid-vein (Horton, 1990).


Fruit russet

Nymphs and adults are phloem feeders. Honeydew, produced by nymphs, drips or runs onto fruit, causing dark russet blotches or streaks and fruit downgrading (Figure 6). The damage may be exacerbated by a sooty mold fungus that colonizes the honeydew and also marks fruit (Burts, 1970).

pear with russet on a tree branch
Figure 6. Fruit russet caused by psylla honeydew (T. DuPont).

Psylla shock

In large numbers, pear psylla can stunt and defoliate trees and cause fruit drop. A carry-over effect may reduce fruit set the following year. These symptoms, called psylla shock, are caused by toxic saliva injected into the tree by feeding nymphs (Westigard et al., 1979).


Pear psylla also transmit a mycoplasma disease organism (Pear decline phytoplasma) through its saliva. The disease damages sieve tubes in the phloem. This damage prevents nutrients from moving down the tree and results in root starvation. Trees grafted on Ussurian pear (P. ussuriensis) and Asian pear (P. pyrifolia, synonymous with P. serotina) rootstocks are the most susceptible. Trees grafted on P. communis, P. betulifolia, P. calleryana, and Cydonia oblongata (quince) rootstocks become infected but are tolerant and display reduced decline symptoms (Teng et al.; Blomquist and Kirkpatrick, 2002). Most pears in Washington and Oregon are grafted to tolerant P. communis (Elkins et al., 2012).



Monitor adults with beat tray sampling (Figure 7). Hold an 18-inch-square tray with a white cloth cover one foot below a 0.75 to 1.5-inch diameter limb with an average number of spurs and branches. Tap the limb firmly three times with a stiff rubber hose. Count adults jarred from the limb onto the tray. Thirty trays at random through the sampling area is standard for a pear block of ten to twenty acres.

Subsequent generations of eggs and nymphs should be sampled on new shoot growth. Collect a total of ten leaves from each of ten randomly selected trees. Select five leaves from the lower canopy with two in the center of the canopy near the crotch of the scaffold limbs and three in the middle of each of two scaffold limbs (Figure 8). Include 1-2 leaves that may not receive good coverage in the center of the tree. Use a telescopic pruner to collect five leaves across two clusters or shoots in areas which are difficult to spray such as the upper canopy and the back side of limbs.

diagram showing dimensions
Figure 7. Beat tray sampling.
tree showing where to collect leaves
Figure 8. Leaf sampling.

Biological control

Important biological control organisms in Washington pear orchards are the parasitic wasp Trechnites insidiosus; true bugs Deraeocoris brevis, Campylomma verbasci, and Anthocoris spp.; lacewings Chrysoperla carnea, Chrysopa nigricornis, Hemerobius spp.; and the earwig Forficula auricularia.

Trechnites insidiosu

Trechnites insidiosus can parasitize pear psylla at rates that exceed 70% in unsprayed orchards and 50% in organic orchards (Beers et al., 1993). Maximum parasitism rates in a review of nineteen field studies range from 1.7% to 100% (Tougeron et al., 2021). Over half of studies report parasitism rates exceeding 40% (Tougeron et al., 2021). Trechnites insidiosus generally has four generations per year in Washington orchards. Many emerge from the overwintering generation of psylla hosts at approximately bloom time (Figures 9-10).

Trechnites adult.
Figure 9. Trechnites adult (R. Schmidt-Jeffris).
graph with single blue line showing relative average occurrence
Figure 10. Relative average occurrence of Trechnites adults captured on yellow sticky cards in North Central Washington orchards. Pear psylla degree days (PDD). (DuPont, S. T., Strohm, C.).

Deraeocoris brevis

Deraeocoris brevis is an abundant predator found in Pacific Northwest apple and pear orchards. Deraeocoris brevis has can consume 175 to 226 psylla eggs per day (Booth, 1992). Overwintering adults can be found active in orchards starting in early March. They will lay eggs in April or May, and first-generation nymphs generally occur starting in mid-May (Yakima) to early June (Wenatchee) (Horton et al., 2012; DuPont and Strohm, 2020).  Deraeocoris brevis has two to three generations per year in Washington and are present from April to October (DuPont and Strohm, 2020) (Figure 11-12).

adult from above on pear
Figure 11. Deraeocoris adult on pear (T.J Mullinix, T. DuPont, C. Strohm).
graph with red line for immatures and blue line for adult
Figure 12. Relative average occurrence of Deraeocoris in beat tray samples in North Central Washington Orchards (DuPont, S.T., Strohm, C.). Pear psylla degree days (PDD).

Anthocoris spp.

Anthocoris spp. are well-adapted to feed on pear psylla and can play a major role in the biological control of this pest. Anthocoris spp. are found occasionally in Wenatchee River Valley orchards and commonly in the Yakima valley in Washington (Horton and Lewis, 2000; Horton et al., 2012; Horton and Lewis, 2014). They overwinter as adults in multiple habitats. They have multiple generations per year and tend to be active very early. They have a strong preference for psyllids. They are common outside of orchards and have a strong preference for trees and shrubs such as willow, alder, poplar, bitterbrush (Horton and Lewis, 2000).

Campylomma verbasci

Campylomma verbasci is known as a pest in apple, but it is a beneficial predator of pear psylla and is not known to cause economic injury in pears. Campylomma can consume 179 pear psylla eggs per day in the laboratory (Booth, 1992). Campylomma also consume young pear psylla nymphs and hardshells, eating an average of 4 eggs, 5 young psylla nymphs and 2 hardshells per day in one study (Nelson, 1985). Campylomma overwinter as an egg and has three generations per year in Washington (DuPont and Strohm, 2020) (Figure 13–14).

adult Campylomma on a leaf
Figure 13. Campylomma adult (T.J Mullinax, T. DuPont, C. Strohm)
Graph with a red and blue line
Figure 14. Relative average occurrence of Campylomma in beat tray samples in North Central Washington orchards (DuPont, S.T., Strohm, C.). Pear psylla degree days (PDD).

European earwigs

European earwigs (Forficula auricularia) are an important predator in pear and apple orchards feeding on aphids, pear psylla, mites and insect eggs. In one study, young earwigs consumed as many as 1000 pear psylla eggs per day (Lenfant et al., 1994). Earwigs overwinter and rear their young in nests underground. They are found in the orchard canopy at night beginning in June in Central Washington (Figure 15-16).

European earwig nymph on the edge of a leaf
Figure 15. European earwig nymph (T.J Mullinax, T. DuPont, C. Strohm)
bar graph with single blue line
Figure 16. Relative average occurrence of European earwigs in traps in North Central Washington orchards (DuPont, S.T., Strohm, C). Pear psylla degree days (PDD)

Brown lacewings

Brown lacewings (Hemerobius spp.) occur sporadically in Central Washington pear orchards and are most abundant from July until late September (Horton et al., 2012) (Figure 17,19).

Green lacewings

Green lacewings (Chrysoperla carnea/plorabunda, Chrysopa nigricornis) are predators of aphids and to a lesser extent psyllids (Carroll and Hoyt, 1984). C. carnea adults are generally seen earlier in Washington orchards (first seen in April) than C. nigricornis (starting mid-May to July) (Horton et al., 2012; DuPont and Strohm, 2020) (Figure 18-19). Immatures are more common starting in May in Yakima or June in Wenatchee.

They are active until September (Horton et al., 2012; DuPont and Strohm, 2020). They are common outside of orchards on many woody and herbaceous plants.

brown adult from side on a leaf
Figure 17. Brown lacewing adult (S.T. DuPont).
green adult on a vertical branch
Figure 18. Green lacewing adult (S.T. DuPont).
graph with a red line, a blue line, and a dark blue-gray line
Figure 19. Relative average occurrence of lacewing species Crysoperla plorabunda and C. nigricornis immatures from beat tray samples and adults from traps with plant volatile lures in North Central Washington Orchards (DuPont, S.T., Strohm, C). Pear psylla degree days (PDD).


An important aspect of integrated pest management is the use of economic thresholds (ET) to make spray decisions. Control is recommended at an ET where pest densities are projected to surpass an economic injury level (EIL), the minimum pest density where the economic loss due to the pest is equal to the cost of controlling the pest (Higley and Pedigo 1996).

Research has identified the quantity of fruit damage we might expect at varying levels of pear psylla pest densities. DuPont et al. 2023 found >2% culls (US grade 3) occur with >0.4 second or third generation pear psylla nymphs per leaf, >0.6 second generation pear psylla adults per tray, or >1.1 third generation pear psylla adults per tray (Figures 20 and 21). This is similar to previous studies. Burts (1988) reports >0.3 pear psylla nymphs per leaf results in detectable fruit russet (Burts 1988). Westigard et al. (1981) report 5% fruit downgrades occur with 2 pear psylla nymphs per leaf for Bosc or 0.4 nymphs per leaf for Anjou.

injury levels for pear psylla
Linear regression of percentage of fruit downgraded to cull versus maximum psylla adults per beat tray and nymphs per leaf for second and third generation based on pooled data of 67 plots between 2018 to 2021. Solid blue lines represent the regression. Dotted grey lines represent 95% confidence intervals.

To avoid fruit damage, managers should spray before pear psylla reach injurious levels. In pear psylla management prebloom sprays are essential. During the second and third generation managers can modify spray intensity, depending on whether pear psylla are predicted to exceed economic thresholds.

To determine what economic threshold to use, managers must consider the value of losses, the economic cost of management, the delay between monitoring and management, and prospects for biocontrol. The EIL will vary depending on yield and price. To use Tables 1 and 2:

  1. Select yield and pricing which most closely align with your system.
  2. Identify the economic injury level that coincides with your scenario.
  3. Use the corresponding Treatment Threshold to determine whether pear psylla populations are likely to reach economically damaging levels if not controlled.
  4. If natural enemies are above biological control thresholds in the third generation (shown in Table 3) use the higher Treatment Thresholds. Natural enemy abundance is predicted to slow or prevent population growth of pear psylla (Table 3).
Table of information for ET for second generation
Table 1. Economic injury levels (EILs) and economic injury thresholds (ETs) for second generation pear psylla at 1300 pear psylla degree days for conventional (CONV), organic (ORG), and integrated pest management (IPM).

*The economic injury level is set to where the cost of downgrades equals the cost of management, assuming an average management cost of $808 per acre. Management costs include five critical sprays using 2022 prices and a spray application labor cost of $36 per hour. Spray applications include: kaolin at 75 PDD (dormant), kaolin+pyriproxyfen+oil at 200 PDD (bud burst), kaolin+pyriproxyfen at 350 PDD (popcorn), kaolin+spirotetromat at 900 PDD, and kaolin+spirotetramat+oil at 1200 PDD. Additional sprays at 1500 and 2600–3200 PDD should depend on economic thresholds.
Using population growth models to predict population at 1300 PDD the spray timing (1400–1750 PDD) to target generation pear psylla which coincides with maximum populations at the EIL.
1,100 lb bin.
§Free on board (FOB) prices from Pear Marketing Association for size class 90 where the high price example is the average from the 2020/21 year and low prices are the average from the 2018/19 year. 44 lb box. Revenue assumes price minus $13.50 per box packing charges.
ǁFancy estimates based on estimated downgrades at designated psylla nymph levels.

#Average of 30 beat tray samples per 10 acres.
††Average of 100 leaves per 10-acre orchard.

Table of information for ET for third generation
Table 2. Economic injury levels (EILs) and economic injury thresholds (ETs) for third generation pear psylla at 2600 pear psylla degree days (PDD)

*The economic injury level is set to where the cost of downgrades equals the cost of management assuming an average management cost of $808 per acre. Management costs include five critical sprays using 2022 prices and spray application labor cost of $36 per hour. Spray applications include: kaolin at 75 PDD (dormant), kaolin+pyriproxyfen+oil at 200 PDD (bud burst), kaolin+pyriproxyfen at 350 PDD (popcorn), kaolin+spirotetromat at 900 PDD, and kaolin+spirotetramat+oil at 1200 PDD. Additional sprays at 1500 and 2600–3200 PDD should depend on economic thresholds. Assuming a 50% reduction in pest population per spray.

Using population growth models to predict the population at 2600 PDD the spray timing (2800–3200 PDD) to target third generation pear psylla before young nymphs molt into hardshells which coincides with maximum populations at the EIL. Assumes EIL at 4000 PDD (harvest timing).

1,100 lb bin.

§Free on board (FOB) prices from Pear Marketing Association for size class 90 where the high price example is from the 2020/21 year and low prices are from the 2018/19 year. Revenue assumes price minus $13.50 per box packing charges.

ǁFancy estimates based on estimated downgrades at designated psylla nymph levels (regressions Table 2).

#Average of 30 beat tray samples per 10 acres.

††Average of 100 leaves per 10-acre orchard.

‡‡D. brevis immatures exceed 6 per 30 trays, C. verbasci immatures exceed 3 per 30 trays, or earwig populations exceed 1.5 per 30 trays or 2 per trap.

Insect Type Insects per Beat Tray Insects per 30 Beat Trays Insects per Trap
D. brevis immatures 0.2 6
C. verbasci immatures 0.1 3
European earwigs 0.05 1.5 2

Table 3: Natural densities above biological control thresholds

Honeydew Washing

Washing honeydew off fruit trees with overhead sprinklers or airblast sprayers can significantly reduce fruit marking damage (Brunner and Burts, 1981). Honeydew washing methods differ from overhead irrigation and are only used to remove honeydew. Under-tree sprinklers are recommended for general irrigation to reduce disease risk and increase irrigation efficiency. It is critical to limit honeydew washes because washing too often and for too long can cause disease issues. Time washing to target honeydew from old nymphs of the second and third generations at 1600–2400 PDD and 3500–4000 PDD, respectively. Washing is not necessary if visible honeydew is not apparent. In replicated on-farm trials, one to two washes with systems of 60–80 GPM for eight to twelve hours effectively reduces fruit marking (Strohm, DuPont data in prep). For airblast sprayer washes, use at least 800 GPA for smaller trees, and increase gallonage with tree size.

Chemical Tactics

Pre-bloom applications

Dormant/Delayed Dormant (75–100 PDD)

A particle film application (Surround CF/WP or Celite 610 at 50 lbs per acre) should be made as early as it is safe to drive a sprayer through the orchard. This spray prevents pear psylla from colonizing the orchard. Particle films reduce pear psylla adult colonization and egg lay by 80–100%, which reduces pear psylla pressure for the first generation (Hull et al., 2008; Nottingham et al., 2020; Nottingham and Beers, 2022) (Figure 5). In some years, a repeat application may be necessary during delayed dormant after a rain or if many weeks passed since the first application. Effectiveness and longevity of particle films is improved when combined with a spreader sticker. A sulfur or lime sulfur application with oil can also suppress pear rust mites and spider mites in addition to pear psylla adults.

Budburst (200 PDD)

Pear psylla adults will begin laying eggs on soft green tissues as soon as they emerge from flower buds. In some years and orchards, budburst is the earliest growers are able to spray due to wet ground. Applications just before budburst help prevent pear psylla adults from laying eggs on freshly emerging bud tissues. A second particle film (Surround CF or Celite 610 at 50 lbs per acre) applied just before budburst renews particle film residues, repels pear psylla adults and prevents egg lay. If budburst is the first spray a grower can make, a second particle film spray at popcorn may be necessary. Pyriproxyfen (Esteem 35WP) is an insect growth regulator that can sterilize pear psylla adults and has little non-target effect on natural enemies (Higbee et al., 1995; Dunley et al., 2001; Nottingham and Beers, 2022). If greater suppression is needed, pyriproxyfen can be mixed with other non-disruptive materials such as diflubenzuron (Dimilin 2L), buprofezin (Centaur WDG), Cinnerate, or azadirachtin (Aza-Direct or Neemix) (Nottingham et al., 2019; Nottingham and Sater, 2021b).

Popcorn (350 PDD)

The insect growth regulator pyriproxyfen (Esteem 35WP) sprayed at popcorn will sterilize pear psylla adults and have little negative effect on natural enemies (Dunley et al., 2001). If the 14-day window required between applications of pyriproxyfen has not been met, other selective materials such as cinnamon oil (Cinnerate), azadirachtin (Aza-Direct or Neemix), diflubenzuron (Dimilin 2L), or buprofezin (Centaur WDG) can be used instead (Nottingham and Sater, 2021a). If pear psylla adult pressure is high, a particle film (Surround CF or Celite 610) sprayed just before bloom renews the residue to repel pear psylla adults from trees.

Post-bloom conventional applications

Prior to summerform adult and young nymph emergence (900 DD)

Apply a particle film (e.g. Surround CF, Celite 610 at 50 lbs/acre) at 900 PDD to deter the emerging summerform adults from landing on trees and laying eggs. Application of spirotetramat (Ultor or Movento) at this timing will reduce survival of nymphs after they hatch (Wise et al., 2008; Beers and Greenfield, 2014; Wise et al., 2018). This material works best when two applications are made, so a second application can be made in 14 days, at approximately 1200 PDD. Spirotetramat can only be applied twice per season, and applications must be at least 14 days apart.

Prior to peak adults and eggs (1200 PDD)

At 1200 PDD (at least two weeks after the previous spray), applying a second particle film repels summerform adults from trees, and a second  spirotetramat (Ultor or Movento) suppresses newly hatched nymphs. If this is the first spirotetramat application, a second can be made in 14 days, at approximately 1500 PDD.

Prior to peak young nymphs second generation (1500 PDD)

If only one particle film (Surround WP or Celite 610 at 50 lb/acre) was applied after bloom, a second (final) application at 1500 PDD will repel the remaining summerform adults, preventing further egg lay. No more than two particle film applications should be made after bloom, as this can increase the risk of mite outbreaks. If only one application of spirotetramat (Ultor or Movento) has been made and it has been 14 or more days, a second application at this timing targets young nymphs after they hatch. If pressure is high in the summer additional sprays with more toxic materials are warranted (see treatment thresholds), the most effective time to make insecticide applications is as young nymphs are increasing toward peak and prior to hardshells (1400–1750). Use products that kill young nymphs, such as diflubenzuron (Dimilin 2L), Cinnerate, or azadirachtin (Aza-Direct or Neemix). Do not use azadirachtin products on Comice pears.

Postbloom organic applications

For organic sprays, scout and begin spraying organic insecticides weekly once young nymphs become present, and continue until just past young nymph peak, 1100–1800 PDD (2nd generation) and 2800–3500 PDD (3rd generation). Consider pear psylla and natural enemy thresholds to determine the necessity for sprays at this timing. Products to use include summer oil, Cinnerate, or neem (Aza-Direct, Neemix, or Rango) (Nottingham and Sater, 2021b). Do not use azadirachtin products on Comice. If pear psylla numbers are high, these products can be mixed to improve efficacy, but extra care should be taken to avoid marking or phytotoxicity.

Selective Codling Moth and Mite Management

For successful pear psylla integrated pest management, codling moth and mite sprays need to be compatible with pear psylla biological control. Include effective materials with few indirect effects on natural enemies.

If spider mites are found, give predators time to suppress them. In areas where pear rust mites or spider mites are becoming a problem, selective miticides such as fenbutatin (Vendex 50WP), spirodiclofen (Envidor 2SC), or cyflumetofen (Nealta) are effective and have relatively low indirect impacts on natural enemies.

Codling moth management should include mating disruption and effective use of selective pesticides. The first codling moth spray targets eggs and should be conducted at the standard (225–275 codling moth DD) or delayed first cover (375 DD) timing. This spray can include materials such as oil and methoxyfenozide (Intrepid 2F). The second spray should be conducted at 425 DD (standard timing) or 525 DD (delayed first cover timing) and can include oil in addition to larvicides such as granulovirus (Cyd-X HP) or diflubenzuron (Dimilin 2L). For second and third generations of codling moth, add 1000 DD to the previous timings, but treatment may not be necessary. For more information see Codling Moth.

Materials Available for Pear Psylla

Excerpt from the WSU Crop Protection Guide. For timings at which each pesticide can be used refer to the Crop Protection Guide.

Materials available for pear

Excerpt from the WSU Crop Protection Guide. For timings at which each pesticide can be used refer to the Crop Protection Guide.


Thank you to funding support from the Washington State Department of Agriculture Specialty Crop Block Grant program, the Washington State Tree Fruit Research Commission and the Fresh and Processed Pear Committee.


Louis Nottingham
Assistant Professor
WSU Tree Fruit Research & Extension Center
1100 N Western Ave., Wenatchee, WA 98801
540-798-2044 (cell)

Robert Orpet
Postdoctoral Research Associate
WSU Tree Fruit Research & Extension Center
1100 N Western Ave., Wenatchee, WA 98801

Tianna DuPont
Tree Fruit Extension Specialist
Washington State University
(509) 293-8758
(509) 713-5346 (cell)


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Use pesticides with care. Apply them only to plants, animals, or sites listed on the labels. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.

YOU ARE REQUIRED BY LAW TO FOLLOW THE LABEL. It is a legal document. Always read the label before using any pesticide. You, the grower, are responsible for safe pesticide use. Trade (brand) names are provided for your reference only. No discrimination is intended, and other pesticides with the same active ingredient may be suitable. No endorsement is implied.

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Cultural Tactics

Summer Pruning

Summer Pruning is the removal of vegetative shoots from trees and an important cultural control. Summer pruning improves spray penetration and light in the canopy. If timed correctly, pruning can also reduce the pear psylla population and amount of honeydew in trees. Prune between 2100–2400 PDD to remove nymphs before they molt into 3rd generation adults (Figure 20).

graph showing relative development and treatments
Figure 20. Pear psylla phenology in Washington state with spray application timings. Pre-bloom pear psylla management is mainly based on tree development. After bloom, pear psylla degree days (PDD) become more important.
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