Researchers tackle apple weevil
The pest might be incidentally controlled by Guthion, which is being phased out.
Small and black, the apple flea weevil looks a bit like its larger snout beetle relative, the plum curculio.
Photo by matt grieshop, michigan state university
A coalition of partners in four Midwestern states has applied for grant funding to respond to three new insect threats—spotted wing drosophila, brown marmorated stinkbug, and apple flea weevil.
The collaboration involves land-grant university entomologists in Indiana (Dr. Rick Foster), Illinois (Dr. Rick Weinzierl), Ohio (Dr. Jim Jasinski), and in Michigan, the biggest fruit state in the region, where entomologists Drs. Larry Gut, Rufus Isaacs, Mark Whalon, Matt Grieshop, and Anne Nielsen are all involved. Nielsen came to Michigan State University last fall in a postdoctoral position having finished her doctorate at Rutgers University working on brown marmorated stinkbug. At MSU, she is now tackling apple flea weevil as well as awaiting the arrival of the stinkbug in Michigan, which she thinks is inevitable.
Of the three insects, the apple flea weevil is the odd man out. It isn’t a new insect—it was first reported in the 1920s with occasional flare-ups over the years—but it is acting like an invasive species and is posing a major challenge for organic apple growers. Grieshop, who is the organic pest management specialist for Michigan State University, says the weevil seems largely undeterred by natural enemies and rapidly builds its numbers to epidemic levels.
Nielsen, who will work with him, agrees. The weevil has, so far, been reported infesting mainly Michigan organic apples, but with devastating effect. Of the six farms involved, all lost their crops in 2010—a fact first attributed to the four severe freezes that cut the state’s crop by 40 percent. Later, it was found that intense feeding damage as the blossoms emerged had shorn the trees of viable buds.
The damage occurred in orchards of some of the state’s best organic growers. The insect also attacked Michigan State University’s experimental organic apple orchard at the Clarksville Horticultural Experiment Station.
Grieshop believes the apple flea weevil has been controlled incidentally in conventional orchards, where they are kept in check by organophosphate insecticides such as Guthion (azinphos-methyl). “We’re losing Guthion in the very near future,” he said, noting that 2012 will be the last year it can be used in apples. “Will the reduced-risk insecticides keep them under control? We don’t have any data on that yet.
“But it’s acting a lot like the brown marmorated stinkbug. It started off slow, but last year, growers in the mid-Atlantic area were really burned. It’s better to try more things when they’re weak instead of waiting for them to get strong.”
Right now, he admits, it’s a problem for organic apple growers only. But the pattern he sees in organic orchards is not encouraging. What looks like phytotoxicity one year—a lot like sulfur damage—can the next year evolve into heavy early feeding. Before leaves emerge (even before plum curculio to which it is related), they feed on developing buds, wiping out the crop before turning to the foliage, which, in Nielsen’s words, is “ripped to shreds.”
A few years of severe foliage damage like that could lead to tree death, Grieshop said.
Nielsen’s work next year will be to evaluate the organically approved insecticides that are available and look at cultural methods and biocontrol agents to reduce populations to tolerable levels. As she did with the brown marmorated stinkbug, she needs to study its basic biology first and then find the weak spots.
The flea weevil emerges between green tip and the pink stages of apple bud development, Grieshop said. Adult beetles look much like plum curculio, but are smaller (one-sixteenth to one-eighth inch), and are black with brown legs and a snout. The hind legs are greatly enlarged, allowing the beetles to jump much like a flea. Adults feed on buds and leaves, making buds unviable and leaving shot holes
Adults lay eggs, and larvae feed as leaf miners. The larvae pupate in chambers, like bubbles, between leaf layers and emerge in June and July. They feed on leaves and then head to the soil to enter diapause in July. There is only one generation per year in Michigan.
Adult damage to buds often looks like frost injury, appearing first at the bottom of trees and moving up. Larval feeding, later in the season, looks like injury from sulfur applications or other phytotoxicity, characterized by brown patches on the leaf margins.
It appears to overwinter in vegetation under the trees and emerges early, feeding and laying eggs. Grieshop says it appears to need moist conditions and that tillage or burning off vegetation early in the year may strike the overwintering weevils, drying out their habitat and nipping them early. Mulch under trees appears to help the insect thrive.
Grieshop and the team at the Organic Pest Management Laboratory are now:
• testing insecticides, both organically approved ones like neem oil as well as pyrethrums and conventional materials
• working with entomologists in other states to survey the extent of the weevil’s range
• trying parasitic nematodes and fungi
• looking for natural enemies and
• testing cultural measures like tillage and burning surface vegetation
They ask growers and others in the apple industry to report finding orchards damaged by the pest. E-mail Grieshop at email@example.com or call the lab at (517) 432-8034.