Curculio Beetle

Plum curculio, Conotrachelus nenuphar, is an injurious pest of apples, cherries, nectarines, peaches, and plums throughout the state.

The adult plum curculio is a small, hard-bodied, brownish-black snout beetle mottled with white and orange areas. It has four prominent black humps on its top surface. It is about 6 mm (1/4 inch) long and has a long snout, the end of which bears chewing mouthparts. The fully-grown larva is about 9 mm (3/8 inch) long and is a yellowish-white, legless grub with a brown head.

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 . 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.

Life Cycle :The plum curculio overwinters as an adult under yard debris or in protected parts of 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 cuts a small, round cavity directly under the skin of the fruit with her mouthparts. She then lays a single tiny white egg in the cavity, just under the skin of the fruit.

Next 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 eggs 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 growing tissues of the fruit.

If the apple drops prematurely or is picked from the tree while the larva is still alive, or if the variety is early-ripening, the larva can complete its feeding and growth. The larva spends about 10 to 16 days feeding. When fully developed, the larva burrows out of the fruit, making an oval exit hole, and enters the soil. In the soil, the larva constructs a small cell 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. 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.

Damage : 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) as described above. 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. 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.

Management : The critical period for controlling plum curculio is during the first few days of warm and humid weather following petal fall, when maximum temperatures remain approximately 70°F. Control is more difficult when feeding is greatly reduced by low temperatures and moderate rains because spray deposits are washed from fruit and foliage. Low temperatures also extend the period during which curculio is active in orchards. Temperature monitoring is important in timing sprays. A spray residue should be maintained for 308 DD base 50 following petal fall. On stone fruits, sprays should be timed for the shuck split stage and repeated if needed.

In blocks with a history of plum curculio injury, the following are important considerations:
(1) shorten interval between sprays during peak curculio activity (this may be necessary on outside rows only),
(2) increase insecticide rate during peak activity, and
(3) select the most effective insecticides without sacrificing control of other pests or interfering with the integrated pest management program. See Table 4-6 for the list of effective insecticides.

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