—by Matt Milkovich
Julianna Wilson’s tree fruit entomology lab at Michigan State University studies lots of apple pests, but she focused on three of the most common when she shared the latest news on control methods during an apple session at the 2024 Great Lakes Fruit, Vegetable and Farm Market EXPO in December.
Black stem borer
The black stem borer, a kind of ambrosia beetle, bores holes in apple trees to provide space and food for its eggs. The borer is attracted to the chemical signals that emanate from injured and stressed trees, especially in young orchards. It usually bores holes in the trunk above the graft union or up in the canopy, wherever the chemical signal emanates.
The beetle lays its eggs inside the chamber it has bored, then deposits fungus spores (which it carries in “something like a pocket”) and grows a “fungus garden” to feed its offspring, Wilson said.
The old rule of thumb for managing black stem borer was to spray a pyrethroid when there are two sequential days above 68 degrees Fahrenheit, or after forsythia bloom, because the pest is mainly active in mid- to late April, she said. But management has become more complicated in the past few years, now that a second species has arrived in Michigan and emerges later in the season.
Wilson’s lab is now looking for a new rule of thumb. In recent studies of semiochemical products, they learned that formulations of SPLAT, developed by ISCA, an agricultural biocontrol company, can suppress beetle populations. Semiochemicals are the chemical substances many insects use for communication, such as mating or aggregation pheromones or plant volatiles that attract or repel insects. In this case, the successful formulations contained the plant volatiles verbenone and methyl salicylate, which can serve as an attractant for some insects and a repellent for others. Product registration is pending.
Researchers still need to determine how many applications are necessary and whether they should treat the whole orchard or just the edge, she said.
Woolly apple aphid
Woolly apple aphids are plant-sucking insects that deform trees.
Belowground, aphid colonies feed on roots and woody tissue. Aphids also crawl up the trunk and form aerial colonies throughout the growing season; those colonies then settle and feed on pruning scars, Wilson said.
In an exclusion netting study (nets were draped over the row after king bloom to prevent further pollination), they found more aerial colonies of woolly apple aphid under the nets than in untreated rows. Researchers speculate that the aphids’ natural enemies are unable to get at them under the nets, and that the microclimate might be “warm and cozy” and dry enough to allow the aphids to build up in number, she said.
In a separate study, a 2023 efficacy trial in Northwest Michigan, diazinon and Transform (sulfoxaflor) both provided good knockdown activity for woolly apple aphid.
San Jose scale
Wilson’s lab has studied various ways to control San Jose scale, which can be a high-pressure pest in orchards. Insect growth regulators such as Esteem (pyriproxyfen) and Centaur (buprofezin) can reduce crawler levels when timed correctly, Wilson said.
Mating disruption also can be an effective management tool. Trécé Inc. has submitted an application to the U.S. Environmental Protection Agency for a pheromone dispenser. Hopefully, it will be commercially available in a year or two, she said. •
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