WSU Tree Fruit

New Pear Psylla Project Initiation: Precision Management using pear psylla degree-day model

Written by Louis Nottingham, Entomology Research Assistant Professor, WSU Wenatchee, Jan 2021

Pear psylla has been a top challenge to pear growers in the PNW since it first arrived in the 1940s. Chemical insecticides have never been a fully effective option for many reasons including large, dense tree canopies, resistance, and the importance of conserving natural enemies for biological control. Currently, many growers are spending around $1500 per acre to manage psylla alone and are still not gaining acceptable control. According to a recent grower poll, about 80% of growers want to try something new to control this pest, which demonstrates the low level of success our current programs are achieving.

But what is left to try?  Perhaps a good approach is learning from past pest management success stories, like codling moth. Codling moth was an extremely challenging pest of apples and pears that required numerous broad-spectrum sprays. However, controlling this pest now is generally attainable in conventional agriculture with a handful of soft insecticide sprays, largely due to the use of precision degree-day based sprays coupled with other insecticide-free tactics (mating disruption and dwarf trees, primarily).

Basics of insect degree-day (phenology) models

Insects go through a series of “life-stages” (i.e. egg, juvenile molts, adult) that make up a generation, and there are often multiple generations that will occur within a single season. The occurrence of each of these life-stages and generations is dictated by temperature, specifically, the amount of heat units (degree-days) accumulated over a period of time. The equations used to calculate degree-days and corresponding development curves will vary among insects for various reasons. For example, some insect development is unaffected above or below specific temperature thresholds. Therefore there are different degree-day models for different insects and/or populations.

Is there a degree-day development model for pear psylla?

In 2019, Dr. Vince Jones, director of the Decision Aid System (DAS), completed a degree-day model and corresponding life stage development curves of pear psylla (Fig. 1.), with funding from Washington Tree Fruit Research Commission and the Fresh and Processed Pear Committees. Although we now have a degree-day model, the associated management recommendations (spray timings, materials, etc.) have not been determined. Fortunately, the Fresh and Processed Pear Committees and the WSDA Specialty Crop Block Grant program have agreed to fund a subsequent effort to determine how to use this model to make pest management decisions in commercial pear orchards.

This graph shows the life stages curves for pear psylla throughout the year.
Figure 1. Pear psylla life stage curves (relative abundance, y-axis) and corresponding degree-days (x-axis).


Louis Nottingham
Research Assistant Professor
WSU TFREC Wenatchee
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