by Courtney Llewellyn
What’s going on with plum curculio? Fruit growers definitely want to know. A snout beetle, plum curculio (PC) is pest of stone fruits such as peaches, plums and cherries, native to the region east of the Rocky Mountains. It’s one of the most serious pests of peach trees, as they lay eggs within the fruit and both adults and larvae feed on the fruit, causing brown rot of the fruit, according to the Missouri Botanical Garden. In addition, PC is considered the second most damaging pest of apples after codling moth.
The Northeast Fruit Consortium hosted a webinar with a PC research update and management tips earlier this year. Dr. Jaime Piñero, tree fruit entomology and IPM Extension professor with UMass, began by providing an overview of the 2020 and 2021 growing seasons. He noted that in 2021, overall insect pest activity was low, with the exceptions of PC and rosy apple aphid (RAA). His team looked at the levels of fruit damage in six Massachusetts apple orchards, two in New Hampshire and one in Maine.
About 5% of fruit was infested on orchard perimeters, and nearly 4% throughout whole blocks in 2020, according to Piñero. However, whole block injury was about five or six times higher in 2021. “Something happened with PC control, most likely due to timing,” he said.
Plum curculio egg laying takes place shortly after petal fall, and so in theory a petal fall insecticide spray to all trees will control multiple pests. This can be followed by one or two perimeter sprays. Of course, the performance of insecticides depends on product characteristics. Organophosphates and carbamates work primarily as contact poisons on PC adults. Piñero noted using Sevin (carbaryl) as a thinner can provide some level of protection against PC at petal fall – but its efficacy is only rated as moderate. He said Avaunt eVo works primarily by contact activity but its efficacy increases if it’s ingested.
Piñero said some “safer” options include Exirel and Verdepryn 100SL (a Group 28 insecticide which is labeled against many pests in pome and stone fruit, grapes, berries, etc.). Various non-pyrethroid insecticides work well too, with the caveat that they have two to six hours of drying time to set (but up to 24 hours for optimal plant penetration).
If orchardists are concerned about PC monitoring, they can use a trap tree to see how many pests appear. This is a single bait tree loaded with an odor lure before bloom. After the trap is set, the whole block needs a petal fall spray, and growers need to sample 25 designated fruits on the trap tree twice a week. Perimeter row trees should be sprayed again only if fresh egg-laying activity is detected.
“The amount of insecticide used in trap tree plots … was reduced by 43% compared with plots managed with the conventional approach,” Piñero said. He added that trap trees are “effective, inexpensive and expeditious.”
Piñero’s team also tested multi-cultivar grafting with apple varieties that are attractive to PC (notably, Liberty, Wickson Crab, Ginger Gold, Dabinett, Yellow Transparent and Red Astrachan) on a trap tree, but that is a long-term project (with six to eight years elapsing before results can be measured). They grafted the cultivars in test orchards in 2018 and 2019. The grafted stock saw 20% injury, compared to only about 5% for non-grafted trees in year 3 post-grafting. This experiment remains ongoing.
An alternative management program could include combining trap trees with beneficial nematodes for PC control.