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Using the Pear Psylla Phenology Model for “soft” (IPM) Programs

Written by Chris McCullough, Robert Orpet, Molly Sayles, and Louis Nottingham.

Summer Soft Approach

Post-bloom is when pear growers attempt to catch up on pear psylla management and begin their summer spray program. Often, this means applying a broad-spectrum insecticide like tolfenpyrad (Bexar), acetamiprid/novaluron (Cormoran), or spinetoram (Delegate WG) in an attempt to knock down developing nymphs. This is followed by summer cover sprays with products such as imidacloprid (Admire Pro), thiamethoxam (Actara), more spinetoram (Delegate WG), and other broad-spectrum insecticides and miticides. However, rather than controlling the pest, broad-spectrum sprays mainly kill natural enemies of pear psylla that are developing in the orchard, such as the Trechnites wasp and Deraeocoris bug (Fig. 1). Natural enemies that developed during bloom are quickly eliminated by post-bloom sprays. Conversely, coming out of bloom with an IPM approach that uses more selective insecticides, precise timings, and cultural control tactics can conserve natural enemies that help control psylla populations all the way to harvest. For the soft approach to succeed, remember to use codling moth mating disruption since you will no longer use the broad-spectrum materials for psylla that also control codling moth.

Figure 1: The Trechnites wasp (L) and the Deraeocoris bug (R) are two important natural enemies of the pear psylla.

After bloom, nymphs are present which are tough to control with pesticides. The best option is to wait until just before summerform adults and eggs occur, at 900 PDD. At this point, applying a particle film (Surround WP) will deter adults and egg-lay, and the selective insecticide Spirotetramat (Ultor or Movento), will suppress young nymphs as they hatch from eggs. Spirotetramat takes some time to become active in the plant, so it is necessary to spray it when eggs are present, ahead of nymphs. For growers that are uncomfortable with their nymph numbers at petal fall, it is acceptable to use a selective material like azadirachtin (AzaDirect), cinnamon oil (Cinnerate), buprofezin (Centaur WDG), pyriproxyfen (Esteem 35WP), or selective miticides like fenbutatin (Vendex 50WP) or spirodiclofen (Envidor 2SC).
For the rest of the season, if pear psylla cross the spray threshold of more than two adults per tray or one nymph per leaf, consider only using soft products like Cinnerate, azadirchtin, or diflubenzuron (Dimilin 2L).
Summer psylla generations are tough to control because spray coverage is poor due to new vegetative growth. At this point, consider honeydew washing to keep fruit clean or summer pruning to physically remove nymphs from the trees. This approach keeps the natural enemies in the orchards and the psylla under control.

More about the Psylla Phenology Model

Since insect development is related to the temperature, the use of calendar dates or other indicators may not accurately predict pest occurrence. Weather events, such as the heatwave in 2021 and the late-season snow this year, may slow down or speed up insect development. The new pear psylla phenology model in Washington State University’s Decision Aid System can help ensure the correct materials are being used for what is present in the orchard. The phenology model projects psylla development based on the number of heat units (degree days) that have accumulated and are likely to accumulate based on the forecasted weather. The phenology model is not predicting the abundance, rather it is predicting relative number of psylla that are going to be active within each life stage at that time. When the model projects a peak, it is saying that this is the time when that life stage is most abundant. For example, in Wenatchee, the model is projecting the peak of the first generation of hardshell nymph abundance to be around May 26th. From there the relative abundance of hardshell nymphs declines as they become adults or die. The pear psylla phenology model can help with five critical timings to maintaining a soft psylla program.

  1. Getting a dormant bud particle film applied before psylla activity begins, and then making another application at budbreak
  2. Applying pyriproxyfen at the lead up to peak egg abundance during the first generation
  3. Targeting the lead up to peak early instars nymphs after petalfall with spirotetramat
  4. When to use washing systems during the second generation of hardshell nymphs
  5. When to summer prune to remove the greatest number of hardshell nymphs

Remember to use the psylla phenology model as a guide only. The predicted curves are for conditions present at the weather station and do not account for pesticide effects nor biocontrol on pear psylla population dynamics. A more detailed description of the phenology based psylla program are available online and can be accessed through the links below.

Figure 2: Psylla phenology model showing the progress of the different life stages of the pear psylla across its degree days. Bloom typically is around the 400 – 600 psylla degree days.


Christopher McCullough

WSU TFREC Entomology


Funding and acknowledgments

Thank you to our funding sources for this project, the Fresh and Processed Pear Committee of the Washington Tree Fruit Research Commission and the Washington State Department of Agriculture Specialty Crop Block Grant Program.

Additional information

Phenology IPM Program

Commercial evaluations of the Phenology IPM Program

Decision Aid System

Fruit Matters articles may only be republished with prior author permission © Washington State University. Reprint articles with permission must include: Originally published by Washington State Tree Fruit Extension Fruit Matters at and a link to the original article.

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