Aug 3, 2021

Managing woolly apple aphid tips offered

When woolly apple aphid colonies make their appearance in apple foliage mid-summer, this is the time to reduce populations so they do not become a nuisance at harvest.

In its adult form, woolly apple aphids, Eriosoma lanigerum Hausmann, produce characteristic waxy filaments, which are the source of its common name (see photo). When crushed, they leave behind a sticky blood-red mess that causes consternation in workers hired to harvest apples. These pests can be found almost anywhere around the world where apples are grown and can live their entire lives on apple trees, though they can also infest pear, hawthorn, mountain ash and elm trees.

Colonies of the pest may be found feeding below ground tree roots of susceptible rootstocks, or above ground on previously wounded sites or at the base of new foliage. It is thought that the below-ground colonies do the most damage to the trees themselves over time under intense population pressure. The Malling rootstock series were bred and used specifically because they tended to be resistant to infestation by woolly apple aphids. Using resistant rootstocks is still the best strategy for minimizing below ground feeding damage from this pest.

On apple trees, woolly apple aphids overwinter as immatures, mostly on the roots, and then crawl up into the canopy to form noticeable colonies in the shoots (called the aerial phase). The key timing for suppressing populations of the pest, to prevent them from becoming a nuisance at harvest, is when they are just getting settled above ground through mid-summer.

Hoverfly larvae, lacewing larvae, earwigs and the aphid parasitoid Aphelinus mali (Haldeman) are just some of the known beneficial insects that can help with the natural suppression of this pest. Similar to what happens with mite flaring, these natural enemies of woolly apple aphids can be disrupted by the repeated use of pyrethroid or carbamate pesticides. Here we describe the best strategies for the judicious use of pesticides timed to keep this pest from becoming a nuisance at harvest.

The conventional carbamate and organophosphate nerve poisons Sevin and Diazinon are labeled for control of woolly apple aphids but have not been tested for level of effectiveness in many years. Most neonicotinoid insecticides have shown to be lethal to aerial phases of woolly apple aphids, but only Assail includes woolly apple aphids on the label. Venerate is a new Biopesticide labeled for woolly apple aphid control and has shown to be lethal to aerial phases. The Sufloximine insecticide, Closer, has shown excellent activity on aerial phases of woolly apple aphids.

The Pyrimidine Carboximide insecticide, Beleaf, is also labeled for control of woolly apple aphids. Note that for any product targeting the aerial phase of woolly apple aphids, sufficient water diluent (i.e., minimum of 100 gallons per acre) is needed to penetrate the tree canopy and achieve direct exposure to the woolly apple aphid colonies. The Tetramic Acid insecticide, Movento, controls both the aerial and root phases of woolly apple aphids because of its mobility characteristics within the tree. Movento is phloem and xylem mobile and full activity on the target pest requires time for the compound to be activated and transported throughout the tree. To get optimal plant uptake, it is important to add a spreading, penetrating adjuvant in accordance with the label instructions.

Lastly, the table below shows the speed of activity and woolly apple aphid phase for the insecticides discussed above.

– Julianna Wilson and John Wise, Michigan State University, Department of Entomology

A mid-summer aerial phase of woolly apple aphid colony. Note the characteristic waxy white filaments that make it look woolly and distinguish it from any other aphid that infests apple trees. Photo: Julianna Wilson/MSU