Summary by Tianna DuPont, WSU Extension. April 10, 2024. Reviewed by Wendy Sue Wheeler, WSU Pesticide Resources and Education Program.
Integrated pest management (IPM) uses cultural and biological as well as insecticidal controls and employs monitoring and economic thresholds. Insect growth regulators can be used as a tool as part of a pear IPM because they can reduce psylla but have little negative impact on natural enemies.
How do insect growth regulators work?
It is important to remember how insect growth regulators work on psylla. Normally adults lay eggs. Those eggs develop into nymphs and the young nymphs develop into hardshells and adults (Figure 1 top). When we apply an insect growth regulator it sterilizes eggs so that they don’t develop into nymphs or prohibit nymphs from developing (Figure 1 bottom). Generally, insect growth regulators don’t kill adults.
When do I apply insect growth regulators for psylla?
To get the optimum effect you want to apply insect growth regulators when adults are laying large numbers of eggs. This tends to be around popcorn (when white blooms start to balloon) and popcorn plus 14 days for the overwintering generation and 900 and 1200 PDD for the second generation.
Why don’t I see dead psylla after I apply an insect growth regulator?
Growers sometimes comment that they are disappointed with insect growth regulators because after they apply them, they don’t see dead psylla. It is important to remember that insect growth regulators are not designed to kill adults. For example, in this study by Dunley the number of adults in the Esteem treatment (Figure 2 pink) is the same as the untreated. But because the insect growth regulator keeps eggs from maturing the number of nymphs stays much lower (Figure 2 green) compared to the untreated (white circles).
Notes on insect growth regulators used in pear IPM
Spirotetramat (Ultor, Movento). Time Ultor applications after petal fall, when the canopy is well developed, but leaves have not hardened off. A second application 14 days after the first is generally helpful. This material is systemic. It kills feeding nymphs and must be applied before eggs hatch. Selective on pear psylla; compatible with natural enemies.
Pyriproxyfen (Esteem 35WP) Pyriproxyfen needs to be timed with adults just before they lay eggs. In the first generation this generally coincides with budburst and a second application near 350 PDD (popcorn). It will not kill adults or prevent egg lay, but will cause adults to lay infertile eggs. Selective; compatible with pear psylla natural enemies.
Insect growth regulator efficacy
The efficacy of insect growth regulators can be variable. Esteem reduced percent hatch 80 to 97% (Higbee 1995), and nymphs per leaf to between 0.1 and 8.6 vs between 0.2 and 93.4 in control (Dunley 2000b) but had no effect in other trials (McCullough and Nottingham 2022a,b). Ultor/Movento applied 2x at petal fall, petal fall + 14 significantly reduced nymphs per leaf at 2 of 5 dates resulting in less sooty mold (Wise 2017) and significantly reduced nymphs 50 to 80% (Wise 2007).
References
Beers, E.H., and B.M. Greenfield. 2014. Pear Psylla Insecticide Test, 2013. Arthropod Management Tests 39(1).
Dunley, J. E., Greenfield, B. M., Hannig, G. T., and Bennett, L. H. 2001b. CONTROL OF PEAR PSYLLA WITH DIFLUBENZURON, ESFENVALERATE, AND PYRIPROXYFEN, 2000. Arthropod Management Tests A32.
Dunley, J.E., B.M. Greenfield, G.T. Hannig, and L.H. Bennett. 2001. Control of Pear Psylla with Diflubenzuron and Pyriproxyfen, 2000. Arthropod Management Tests 26(1):A33.
Higbee, B.S., D.R. Horton, and J.L. Krysan. 1995. Reduction of Egg Hatch in Pear Psylla (Homoptera, Psyllidae) after Contact by Adults with Insect Growth Regulators. Journal of Economic Entomology 88(5):1420–1424.
McCullough, C., and Nottingham, L. 2022. Potted Plant Assay of the Timing of Insect Growth Regulator Application for Control of Pear Psylla in Pears, 2021. Arthropod Management Tests 47.
McCullough, C., and Nottingham, L. 2022b. Field Assay of the Timing of Insect Growth Regulator Application for Control of Pear Psylla in Pears, 2021. Arthropod Management Tests 47.
Nottingham, L., and E.H. Beers. 2022. Improving Pear Pest Management with Integrated Approaches. Washington State Tree Fruit Research Commission, Wenatchee, WA.
Wise, J.C., R. Vander Poppen, and L.J. Gut. 2008. Control of Pear Psylla, 2007. Arthropod Management Tests 33:A25.
Wise, J.C., A.H. Van Woerkom, C.E. Wheeler, and L.J. Gut. 2018. Control of Pear Psylla in Pear, 2017. Arthropod Management Tests 43(1):tsy062.
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