Written by Robert Orpet, WSU, 5/1/2023
The Woolly Problem
Once apples were introduced to North America, a native species now known as woolly apple aphid found a great new host. Their destructiveness earned them the name “American blight” during the early 20th century. There was a lull in the importance of this pest during the second half of the 20th century, but woolly apple aphid has resurged with outbreaks in Washington during the last several decades. It feeds aboveground and belowground, induces galls where it feeds (Fig. 1A), and makes working conditions unpleasant. Underneath those white woolly fibers are purple aphids that stain your clothes.
Common rootstocks like M9 and M26 are susceptible to woolly apple aphid, but there are new resistant Geneva series rootstocks: G.41, G.213, G214, G.22, G.202, G.969, G.210, and G.890. Consult with your nursery and consider these for future plantings. Unfortunately, resistant rootstocks do not impart resistance to scions, but they could still reduce woolly apple aphids overall.
In 2017, I prevented movement of underground woolly apple aphids up to scions using tanglefoot bands, expecting that this would reduce aboveground problems (Orpet et al. 2019a). I intercepted thousands of aphids, but in the end, there were more woolly apple aphid colonies on trees with these bands! The likely explanation is I coincidentally prevented earwigs from crawling up into canopies. Earwigs are a very powerful predator of woolly apple aphid (Fig. 1C).
Woolly apple aphids get uncomfortable above 90°F and their growth rates decrease. When this happens and if hungry natural enemies like ladybugs, syrphid larvae, and lacewings are on the scene, the pest can practically disappear. I observed this during 2017 on video surveillance cameras placed in an orchard (Orpet et al. 2019b). During late June, earwigs were keeping woolly apple aphid populations in check, but did not eradicate the pest by themselves. Around the 4th of July when it got hot, ladybug larvae appeared and aphid colonies died out. We often see woolly apple aphid populations crash during the hottest part of the summer (Beers et al. 2010).
Woolly apple aphid is very difficult to manage with insecticides. Diazinon is the most effective woolly apple aphid active ingredient, but it is not a good option if you are trying to conserve beneficial insects. Ultor is probably the next most effective option, but timing is key! This systemic pesticide will provide future control but will not knock down a large existing aphid population. In 2022, my team tested some biocontrol-compatible pesticides like Cinnerate, an azadirachtin product, and EcoTec Plus with several adjuvant choices. These can be expected to reduce woolly apple aphid growth, but not knock them down like diazinon. Spray oils alone work for suppression too.
Biocontrol is key. A specialist parasitoid (Aphelinus mali) and many generalists (ladybugs, lacewings, syrphids, and earwigs) attack woolly apple aphid colonies. We have some great photos on the slideshow at the bottom of our WSU woolly apple aphid page. However, these beneficials are sensitive to some codling moth sprays. Work by Beers et al. (2016) associates Rimon 0.83EC and Delegate WG with fewer earwigs and more woolly apple aphids. In organic orchards, Spinosad is relatively harsh and can also harm biocontrol agents like earwigs.
Free Earwigs Available at Wenatchee Workshop
If you would like some free earwigs to release on your apple or pear farm to help with woolly apple aphid biocontrol and learn how to collect earwigs from places you don’t want them, a workshop is being planned in Wenatchee in mid-July. You can expect an article about this workshop in next month’s Tree Fruit Matters. For now, contact email@example.com if you would like to be kept informed on the growing project mailing list.
Dr. Robert Orpet
Washington State University
Funding and acknowledgements
Thank you Washington State Commission on Pesticide Registration and Western SARE Project #WRGR23-004 for funds supporting related research on woolly apple aphid management. Thank you Dr. Catron for reviewing this article.
Asante SK, W Danthanarayana, H Heatwole. 1991. Bionomics and population growth statistics of apterous virginoperae of woolly apple aphid, Eriosoma lanigerum, at constant temperatures. Entomologia Experimentalis et Applicata 60:261–270.
Beers EH, SD Cockfield, and LM Gontijo LM. 2010. Seasonal phenology of woolly apple aphid (Hemiptera: Aphididae) in Central Washington. Environmental Entomology 39: 286–294.
Beers, E, N Mills, P Shearer, DR Horton, ER Milickzy, KG Amarasekare, and LM Gontijo. 2016. Nontarget effects of orchard pesticides on natural enemies: Lessons from the field and laboratory. Biological Control 102: 44–52.
Orpet RJ, VP Jones, JP Reganold, DW Crowder. 2019a. Effects of restricting movement between root and canopy populations of woolly apple aphid. PLoS ONE 14(5): e0216424.
Orpet, RJ, DW Crowder, and VP Jones. 2019b. Woolly Apple Aphid Generalist Predator Feeding Behavior Assessed through Video Observation in an Apple Orchard. Journal of Insect Behavior. 32: 153–163.
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