Control Plum Curculios in the Organic Garden

Plum curculio is a misnomer for this garden insect pest, for these beetles will eat and damage other stone fruits like cherries and peaches, as well as apples, blueberries, and pears. Gardeners and organic farmers must control these pests early and often to prevent losing the season’s fruit harvest.

Identify the Plum Curculio

It’s easy to miss the nondescript ¼ inch brownish gray beetle that may be wreaking havoc in the garden. However, upon close examination, gardeners can observe the curved snout that is distinctive to this beetle. Gardeners may also see crescent-shaped entry holes on fruit where beetles laid their eggs.

In the spring, beetles that spent the winter in garden debris begin to stir in response to host trees and plants producing leaves and flowers. Adult beetles will eat these leaves and flowers, but the worst damage is yet to come. Beetles lay eggs in fruit, which hatch into larvae that tunnel throughout the fruit for two weeks. The damaged fruit may drop to the ground prematurely, allowing the grubs to tunnel into the soil, where they pupate for several weeks. The new adult generation emerges to feed on fruit for the rest of the summer.

Physical Controls for the Plum Curculio

In contrast to some nimble beetles that dart under leaves for cover, the plum curculio is sluggish and easy to dislodge from its host plant. Gardeners can take advantage of these traits by shaking the pests loose from fruit trees with the onset of blossoming. Lay a sheet or tarp under the infected tree, and knock the branches gently with a broomstick. Collect the beetles as they drop onto the sheet and drop them into a bucket of soapy water.

Breaking the life cycle is critical to controlling the plum curculio, especially in orchards. Simply collecting all fruit as it falls to the ground each day prevents the plum curculio larvae from burrowing into the ground to complete the metamorphosis into mature beetles.

Kaolin Clay Fruit Tree Spray

In the 1990s, researchers from the USDA began to experiment with natural minerals to deter insects in organic orchards. The result was the discovery of kaolin clay, a mineral powder marketed as the product Surround. Gardeners may imagine that a coating of this white film might slow the growth of fruit trees, but the powdery residue seems to have the opposite effect: the film functions as a protection against heat stress, allowing photosynthesis to occur with greater efficiency than it does on untreated trees.

Non-toxic kaolin clay deters plum curculios in several ways. The white color of the treated trees makes it difficult for the pest to identify its host. The clay particles attach to the beetles, irritating them. Finally, plum curculios find the powdery film on fruits unpalatable.

Gardeners or organic farmers dealing with plum curculios can apply kaolin clay weekly until harvest. Although residual film at harvest isn’t harmful, it looks unappealing, and should be washed off before fruits go to market.

Sources:

National Sustainable Agriculture Information Service

Organic Pest & Disease Control, B. Ellis, Ed., & F. Tennenbaum, Series Ed., Houghton Mifflin Company, Boston: 1997