Final Report June 2023

Principal Investigator: David James, Washington State University, Prosser

Telephone: 509-786-9280

Summary: This project aimed to determine the economic impact of the White-headed grape leaffolder (WGL) on wine grape production in eastern Washington. Monitoring of WGL populations in selected, unsprayed Red Mountain vineyard blocks during 2020 indicated that moderate to high density populations of WGL (60-80% occupation of leaves, mean 1-2 larvae/leaf) had low to negligible impact on fruit production and quality, with no economic impacts noted.
Fermenting bait traps were used to monitor adult population trends and were effective in 2020 for the second and third generations of moths, accurately identifying peak flights and when peak egglaying occurred. This in turn, allowed identification of an optimal insecticide application window in 2020 when the majority of the population was in the vulnerable non-leaf folding first instar stage. Effective, long-term control of leaffolder larvae was obtained at this time using a single application of Altacor (Chlorantraniliprole) in two Red Mountain vineyards. A single application of Leprotech (Bacillus thuringiensis var. kurstaki) in one vineyard provided inadequate control.
An insecticide trial conducted in a Red Mountain vineyard during August-October 2022, confirmed the efficacy of Altacor as a treatment for WGL providing good knockdown and persistent control over at least two months. The spinosyn insecticides, Delegate (Spinetoram) and Entrust (Spinosad) also provided good knockdown but allowed some minor re-infestation after 5-8 weeks when a new generation of WGL occurred. The Bacillus thuringiensis product, Dipel, gave slower knockdown and also allowed minor re-infestation after 5-6 weeks. Similarly, the insect growth regulator, Intrepid, provided slow knockdown (4 weeks) but was less susceptible to reinfestation. Overall, Altacor provided the best, sustained control of WGL.
Progress in this project was limited during 2021 and 2022 by significant downturns in natural populations of WGL in eastern Washington. It appears that if the first generation of adult moths emerging from overwintering pupae in May/June, is small, then populations may not build sufficiently in the following two generations to cause economic damage to vineyards.
The possibility of overwintering being a weak link in the life history of WGL was briefly investigated by looking at the survival of overwintering leaffolder pupae during thtree winters 2020-23. Experiments indicated that survival of overwintering pupae is low, particularly if the leaf shelter/cocoon in which pupae are formed is broken open, exposing pupae to the elements. There may be options for increasing the mortality of overwintering pupae by cultivating the vineyard floor.

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Pest & Disease //