Richard L. Miller
Julie A. Steele
Alan W. Smith
The plum curculio, Conotrachelus nenuphar (Herbst), is widely distributed east of the Rocky Mountains and is a native of America. The name is somewhat misleading because this insect attacks not only plums, but also apples, peaches, pears, cherries, quince, and other wild and cultivated fruits. It can be very destructive where no control measures are applied. Injury to all hosts results first from the spring feeding of adult beetles, then from female egg punctures in the fruit, next from the feeding of larvae within the fruit, and finally from the early fall feeding of adult beetles.
Both the adult and larval stages injure fruits. In spring, adults feed on buds, blossoms, leaves and new fruits. Feeding scars appear as shallow cavities on the fruit surface. The major injury occurs from the laying of eggs by the curculios (weevils). A small cavity is made in the fruit for the egg; then a crescent-shaped cut is made adjacent to the egg pocket. Fuzz on peaches makes it difficult to see this egg scar. The early feeding and egg-laying punctures can cause marked scarring and malformation of the fruit. Early feeding on the surface of peaches often causes severely deformed fruits known as "cat-faced" peaches.
Larvae hatching from the eggs feed inside the fruit until they are fully grown. On some fruits, few if any of the young larvae survive to maturity if the fruits continue to grow on the tree. Larval feeding in apples can cause distortion of the fruit.
The mechanical injury by adults in feeding and egg deposition can cause premature fruit drop. When the summer brood of adults appears, feeding cavities again can be found on the fruits.
The adult plum curculio is a small, hard-bodied, brownish-black snout beetle mottled with white and orange areas. It has 4 prominent black humps on its top surface. It is about 6 mm long, has a long snout, the end of which bears chewing mouthparts. The insect overwinters as an adult under debris in and around the yard or in protected places at an orchard. In spring, shortly after peaches bloom or when apples are near the pink stage, the beetles come out of hibernation and begin to fly to fruit trees to feed.
Egg-laying begins as soon as the young fruits form and continues for three or four weeks. To lay eggs, the female first cuts a small, round cavity directly under the skin of the fruit with her mouthparts. Then she turns around and lays a single tiny white egg in the cavity, just under the skin of the fruit. After this,she cuts a crescent-shaped slit nearly halfway around the cavity, creating a dead flap in the surface of the fruit.
A single female lays about 60 to 150 eggs. Within seven days, larvae emerge from the egg and begin to eat their way through the flesh of the fruit. In stone fruits, the larva works its way toward the pit, around which it feeds extensively until it is fully grown. On apples, few if any of the young larvae survive if the apple continues to grow on the tree because the egg or larva is crushed by the firm, growing tissues of the fruit. If the apple drops prematurely or is picked from the tree while the larva is still alive, or the variety is an early ripening one, the larva can complete its feeding and growth. The larva spends about 10 to 16 days feeding. On reaching completion of its development, the larva burrows out of the fruit by making an oval exit hole and enters the soil. At this stage, the larva is about 9 mm long and is a yellowish-white, legless grub with a brown head.
In the soil, the larva constructs a small cell 1 to 3 inches deep, in which it transforms into a whitish pupa and then into an adult. The time between the entrance of the larva into the soil and the appearance of the new beetle above ground is about five weeks. Summer brood adults emerge in July and August. They do not lay eggs but instead feed on fruit, showing a preference for smooth-skinned fruits such as apples and plums.
In feeding on apples, the beetle makes a small hole in the skin of the fruit and then devours all the flesh of the fruit it can reach with its snout. This feeding takes place until the adults enter hibernation, which occurs from September through early November.
|Plum curculio larva and pupa|
Mechanical control, by jarring the sluggish beetles from trees in the morning and capturing them on sheets, was an early method of control, and can still be practical today on a small scale. Natural control of the curculio results from winter mortality, attacks by birds and other predators, and from parasites. Pick up fallen fruit two to three times a week, put it in a plastic bag, tie it tightly and place it in the trash can. This will help keep larvae in fallen fruit from developing in the soil and, if done regularly, should lessen the damage done by this insect.
On apples and pears, apply an all-purpose fruit tree spray plus the insecticide phosmet (Imidan) when the petals fall from the blossoms and twice more at 7-10 day intervals for the first generation beetles. Second generation beetles only feed on the fruit skin and their damage can be tolerated. If a second generation is to be controlled, continue to apply phosmet (Imidan) or the insecticide carbaryl (Sevin) at 10 to 14 day intervals to harvest. For more detailed information on these and other fruit insects and their control, obtain a copy of Bulletin L-1, "Backyard Fruit Sprays for Insects and Diseases," from your county Extension agent. For commercial orchards, request a copy of Bulletin #506A, "Commercial Tree Fruit Spray Guide."
NOTE: Disclaimer - This publication may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author and Ohio State University Extension assume no liability resulting from the use of these recommendations.
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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