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Woolly apple aphid colony in the greenhouse, showing merging of colonies and heavy wool production. Photo by Dr. Elizabeth Beers, WSU Wenatchee

Woolly apple aphid colony in the greenhouse, showing merging of colonies and heavy wool production.
Photo by Dr. Elizabeth Beers, WSU Wenatchee

The woolly apple aphid has become more of a pest in the Pacific Northwest in recent years. One reason might be the shift to softer pesticides, while relatively mild winter temperatures could be another.

In February, two Washington apple shippers were suspended from exporting apples to China after dead woolly apple aphids were found on imported fruit. It appears that seven containers of fruit were involved. The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service intended to protest China’s action, since it is not officially a quarantine pest.

Dr. Mike Willett, vice president of scientific affairs for the Northwest Horticultural Council, said that, in addition, about 22 shipments of apples going into Taiwan this season had to be fumigated because of woolly apple aphids with the fruit.

“Foreign governments are taking action against woollies, and the reason they are is woolly apple aphid control has slipped substantially in recent years.”

Both producers and fruit inspectors need to be aware that there are markets where woolly apple aphid is considered a quarantine pest and that shipments with the pest could be rejected or fumigated on arrival, he added.

Honeydew

Woolly apple aphids feed on twigs or roots, and honeydew produced by the aphid can drip onto the fruit, resulting in sooty mold and downgrading of fruit. High populations can create sticky and unpleasant working conditions for harvest crews.

Dr. Elizabeth Beers, entomologist with Washington State University in Wenatchee, said conventional broad-spectrum pesticides are still effective against woolly apple aphid, but don’t fit well into an integrated pest management program. A year ago, she began a research program looking for alternative controls that are softer, more selective, and compatible with IPM.

Her goal is to look more closely at the pest and develop long-term solutions, rather than just spray recommendations. This will involve studying the role of resistant rootstocks, on which the pest is unable to become established.

Woolly apple aphid overwinters as a nymph on the roots of the apple tree or in protected places on the truck or main limbs. In severe winters, with sustained low temperatures of 10°F or below, the above ground colonies can be killed. Beers said survival of the pest on the aerial parts of the tree during mild winters might contribute to high pest pressure the following year.

The Malling-Merton rootstocks, including MM.106 and MM.111, were developed specifically for resistance to the pest. However, according to horticulturist Dr. Bruce Barritt, only about 20 percent of Washington’s apple acreage is on resistant rootstocks, and the newer, dwarfing rootstocks are highly susceptible.

Beers said woolly apple aphids often are controlled by natural enemies, such as the generalist predators lacewings, lady beetles, syrphid flies, cecidomyiid midges, and earwigs. In some circumstances, they can be controlled by the parasitic wasp Aphelinus mali, and Beers said scientists need to find out more about what those circumstances are.

Woolly apple aphid is a worldwide pest, and in some areas where A. mali has been released, it has worked very well. Sometimes, however, the woolly apple aphid populations continued to increase until by harvest it was a “giant mess,” she said, with honeydew dripping on workers, picking bags, and the fruit.

In her research, Beers found that Thiodan (endosulfan) and diazinon reduced woolly apple aphid populations. Raynox, a protective coating designed for sunburn control, provided some suppression, but Beers said tests were done with a handgun which gives very good coverage, so the product needs to be reassessed under commercial conditions.