Fig. 23. Plum Curculio adult.
Photo: Clemson University,
USDA Coop. Ext., Bugwood.org
Fig. 24. Plum Curculio oviposition injury.
Photo: Michigan State University
Plum curculio is a common pest of many fruits throughout Minnesota, including plums, cherries, apricots and other soft fruits. It can damage apples, although the damage is often only superficial. If feeding and egg-laying damage are slight, due to a low population of these weevils, there may be no need to control plum curculio in home apple plantings. However, in some orchards, damage from plum curculio leads to a large percentage of fruit that is unattractive and sometimes inedible. Late-season plum curculio feeding may also open wounds that will later be exploited by multicolored Asian lady beetles.
Plum curculio adults are mottled brownish, blackish, and grayish weevils or "snout beetles.” They have rough wing covers and a conspicuous curved beak or snout. They move into apple plantings around the time of bloom and lay eggs in apple fruitlets. "Stings” from egg-laying are the most common damage caused by plum curculio. The slit cut in the apple’s skin by the female becomes a tan patch of apple skin with a distinctive shape. These scars can be ¼” Wide or more, but are only superficial. The adults also feed on apple tissue. Early in the season, the damage is strictly superficial, not more than 1/8” deep, and typically heals over, leaving a scar in the fruit.
Plum curculio adult females lay eggs on apple fruitlets, but the larvae cannot grow and develop in the hard, expanding flesh. You will never find a live plum curculio larva in an apple at harvest, nor will you find extensive internal feeding damage caused by a curculio larva. However, plum curculio activity can cause premature fruit drop. If an apple drops early in the season, the flesh softens, and a curculio larva can finish its development. (The larvae often develop to maturity in soft fruits such as plums and cherries.)
Mature larvae leave the fruit, burrow into the soil and pupate, emerging as adults in August. These adults may feed on almost-ripe apples, causing puncture wounds. Later, they burrow into the soil or hide under leaf litter to overwinter, emerging in spring to mate, feed on immature fruit, and lay eggs.
Effective non-chemical controls for plum curculio have not yet been developed. If this insect is more than a nuisance in your planting, you can try sanitation and shaking or beating the branches, but these methods probably won’t be sufficient, and you will probably need to spray an insecticide.
You can check to see if plum curculios are present. Hold a white paper plate right under a branch, and shake or beat the branch to dislodge the plum curculios. Start looking for the adults before the flowers have opened, and check every few days. In the morning, when the insects are cold, they will fall onto the plate rather than flying away.
Host trees and shrubs near the apple planting, such as plum, cherry, or other berries may be the source of the curculio pressure. Removing these, if possible, could solve the problem. Remove fallen apples throughout the season and send out with trash or use as animal feed.
To reduce plum curculio pressure somewhat, shake and beat the trunk and branches of the tree with a padded bat or stick to dislodge the insects. Cool mornings during bloom and petal fall are the best time to dislodge plum curculios, because if they are cold, they will simply fall out of the tree, rather than flying away. The ground beneath the tree should be covered with a sheet or tarp, and the insects that fall onto it can be removed from the planting and crushed. However, this would need to be done every morning for about a month, starting a week or so before bloom, to be effective.
If you have had damage from plum curculio in previous years, spray an insecticide at petal fall, and again seven to ten days later. In many years, plum curculio adults enter the planting and begin laying eggs at about the same time as codling moths, so sprays for codling moth are likely to kill plum curculio adults. Esfenvalerate and malathion are effective against both pests. Esfenvalerate is highly toxic to bees, so do not to use it if there are any flowers open on your trees.
You could mix the insecticide with a fungicide you are using to control apple scab, or use a pre-mixed all-purpose fruit spray that does not contain carbaryl. (Although carbaryl is effective against plum curculio, it can cause all the fruit to drop if used within 30 days after full bloom.)