Researchers are looking at ways to lure plum curculio into trap trees or small areas that can be treated with insecticide.
Top: Apple tree baited with the aggregation pheromone grandisoic acid. Bottom left: Plum curculio damage occurs both from feeding and from egg laying. Bottom right: Plum curculio adult.
East of the Rocky Mountains, plum curculio is one of the most difficult orchard insect pests growers have to deal with. A snout beetle a quarter-inch or more long, it attacks both pome and stone fruit and leaves behind fruit scars and white larvae.
In apples, most infected fruit falls as part of the June drop. But in cherries and peaches, the larvae are often there at harvest—a highly objectionable result.
Dr. Tracy Leskey, a research entomologist with the USDA’s Agricultural Research Service at the Appalachian Fruit Research Station in Kearneysville, West Virginia, has been trying to find effective ways to deal with plum curculio since starting on her doctorate 15 years ago.
Leskey and others in the industry working with plum curculio are developing lures because the beetles are attracted by odors, including those put out by fruit and those emitted by other plum curculio.
Currently, she is working with electroantennagrams to identify the compounds that can be used to improve the attractiveness of lures for plum curculio. In the laboratory, plum curculio antennae are hooked to an electrode to directly measure which odors elicit an electrophysiological response. These selected compounds are then targeted for more rigorous behavioral studies. Ideally, Leskey wants to identify odors that would result in directed movement of beetles to a baited trap or baited tree canopy, where they could be killed.
That latter method is already in use in some orchards in New England, she said. Referred to as “trap trees” and essentially an odor-enhanced trap crop strategy, selected perimeter-row apple trees in an orchard are baited with synthetic fruit volatiles like benzaldehyde and the “aggregation pheromone” grandisoic acid. The beetles are attracted to and aggregate within this baited canopy.
Because of the beetle’s behavior, Leskey says, this strategy works to lure them en masse to what she calls “a synthetically generated narrow hot spot.” Then the baited trap tree can be treated with an insecticide without the need to spray an entire orchard. That has been the goal of her work—use the curculio’s behavior and reduce the use of insecticides.
Plum curculio has been a particular nemesis to organic growers and is a growing problem for conventional growers. It has been controlled most effectively by organophosphate insecticides, and there aren’t many of those left in the arsenal.
Using current lures, some growers in New England bait trees to make them more attractive to the beetles. “We are in the fifth year working with a number of growers who use this strategy as part of an ‘advanced IPM’ program,” Leskey said.
The growers bait one tree every 150 feet or so in a row on the perimeter of the orchard. They monitor that tree and when curculios arrive, they can spray with something like Imidan (phosmet) or with a reduced-risk pesticide such as Avaunt (indoxacarb). “Growers will incur a lot of damage in those trees,” she said. “They must be willing to sacrifice the fruit from those trees.”
An idea for organic growers includes using the kaolin clay product Surround to push the beetles where they want them. Spraying nearby trees with the product may move plum curculios into the target trap trees.
“Surround appears to irritate the beetles,” Leskey said. “They spend a lot of time grooming the particles from themselves, and they’ll move to get away from treated surfaces.”
The plum curculio life cycle begins in woods and other sites outside the orchard, where adults emerge from hibernation and look for fruit trees. Their movement is temperature-dependent; they walk at temperatures below 60˚F, but fly at higher temperatures. They arrive at the orchard often well before bloom, and trees on the perimeter of the orchard are infested first. By petal fall, Leskey said, a full-block spray is required for control, as they will have moved well into the orchard.
After that early spray, using trap trees on the perimeter should intercept most of the later-arriving population, she said.
Other tools being developed will also help tame plum curculio, Leskey said. Researchers in Georgia and Michigan are looking at nematodes that will destroy their larvae as they pupate in the soil. Using a targeted nematode application beneath trap trees is also an idea Leskey and colleagues are pursuing to improve the overall trap tree approach.
At Michigan State, researchers found that the insect growth regulator Esteem (pyriproxifen) sends curculio adults a message that causes them to prepare for egg-laying when they should be preparing for hibernation and reduces their ability to survive over winter.
Physical removal of dropped apples eliminates larvae from the orchard before they can emerge from the dropped fruit and pupate in the soil.
These techniques can be enhanced by effective lures that congregate the beetles into smaller areas, like trap trees, and Leskey is working to make that more effective.
So far, she and her colleagues have found in the laboratory that the odors of Japanese plum cultivars are stimulating. This host tree also is highly favored by beetles foraging in the field. In fact, Formosa and Santa Rosa were 11 times more highly preferred than apple, making these cultivars ideal candidates for further study for identification of powerful attractants.
Work on the male-produced aggregation pheromone is also occurring in Leskey’s lab. “We hope to identify other male-produced compounds that will enhance the effectiveness of our pheromone lure,” she said.
If we can identify more powerful olfactory stimuli, we should be able to aggregate plum curculio adults more effectively,” said Leskey.
The ultimate goal is to manage beetles more effectively while reducing overall insecticide inputs against this pest. Leskey and colleagues are pursuing this challenge using funds received last year from the USDA’s Specialty Crop Research Initiative program.