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Plum curculio: a troublesome pest of tree fruits

The plum curculio, Conotrachelus nenuphar (Herbst), is an important early-season pest of tree fruits. They can cause considerable damage to apple, pear, apricot, peach, plum, nectarine, cherry and other fruits.

Adult plum curculioDescription

Plum curculio adults are a type of weevil (or snout beetle), approximately 6 mm (¼ inch) in length with a mottled combination of brown, black and gray colors over the body. They have four small humps on their wing covers (elytra) and a characteristic curved snout (Figure 2).

The adults overwinter in ground debris within woodlands or along field edges and begin moving into tree fruit plantings in the spring to feed on buds, flowers and newly formed fruit. Egg-laying begins as soon as fruit set occurs and can continue for about four to six weeks.

Plum curculio egg-laying damage on cherry fruits.Symptoms

Surface feeding and egg-laying by weevils can scar or deform fruit by harvest.  The crescent-shaped scars cut into the fruit surface from egg-laying are a characteristic sign of plum curculio damage (Figure 3 and Figure 4). In apple, plum curculio larvae are unable to complete development within growing fruit because of the pressure exerted by the hard, expanding tissue. However, feeding by larvae can cause fruit to drop prematurely from trees and it is within these apples that they are able to complete development.

Larvae will generally spend two to three weeks feeding within fruit before exiting to pupate in the soil. The next generation adults emerge in late summer and also feed on fruit, but they generally do not reproduce until the following spring.

Plum curculio egg-laying damage on apple fruit.Control

There are few effective non-chemical methods for controlling plum curculio in tree fruits. However, home orchardists can help reduce future populations by promptly picking up and destroying any fallen fruit from the ground (particularly from May through June).

Adult populations can be suppressed in the spring with well-timed applications of effective insecticides immediately after petal fall. Insecticides labeled for use against plum curculio, and available for homeowners include carbaryl (Sevin), malathion and kaolin clay (Surround). Care should be taken when using carbaryl and malathion because these products can be highly toxic to pollinators and beneficial insect predators, and may promote outbreaks of secondary pests (e.g., mites). In addition, care should be taken when using carbaryl in apples because it can act as a fruit thinner when applied within two to three weeks after bloom. For organic growers, kaolin clay is a particle film that when applied to fruit and foliage helps discourage feeding and egg-laying.



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