Pennsylvania weather is variable during May and June, frequently changing from periods of cold wind and rain to stretches of abnormally high temperatures. These climatic irregularities govern the activities of the plum curculio, Conotrachelus nenuphar, a pest injurious to pome and stone fruits throughout the state.
Adult plum curculios first appear in apple orchards during bloom. The beetle is 1/4 inch long, dark brown with whitish patches, and has four humps on its back. The adult has a long snout (one-third its body length) projecting forward and downward from its head. Most beetle activity occurs during the first warm period after petal fall, when the maximum temperature is 70°F or higher. Periods of cool, rainy weather with maximum temperatures below 70°F are not suitable for curculio activity. The plum curculio is usually more abundant on fruit trees adjacent to woods, fence rows, and trashy fields. Adults can be found in orchards for 5 to 7 weeks.
Small, pearly white eggs are laid singly in a small cavity underneath the fruit skin during the first warm periods after petal fall; eggs hatch in 7 days. Slight feeding occurs on petals, buds, and blossoms, but little injury occurs until the fruit is available. When the fruit is approximately 1/2 inch in diameter, it provides abundant food and a suitable place for egg laying. Early blooming varieties are the first to provide suitable locations for feeding and depositing eggs.
The yellowish-white larva has a brown head and is 1/4 inch long when fully grown. It burrows into the fruit's center, where it makes large irregular cavities, feeding for 16 days before maturing. Larvae then leave the fruit and enter the soil, where they transform into adult curculios that emerge during August and feed for a short time before seeking winter quarters.
Adults make both feeding punctures and egg punctures in the fruit. Feeding punctures are small, round holes extending 1/8 inch into the fruit; egg punctures are distinguished by a characteristic crescent-shaped cut that partly surrounds the sunken egg. An adult averages more than 100 feeding and/or egg punctures during its normal life. Feeding punctures cause deformed fruit and contribute to premature drop.
The critical period for controlling plum curculio is during the first few days of warm weather following petal fall, when the maximum temperature remains approximately 75°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. However, low temperatures also extend the period during which curculio is active in orchards. It is advisable to spray perennial trees or trees adjacent to woods more frequently. The length of time between sprays is as important as the insecticide combination. Kaolin clay (Surround) can be used to reduce the plum curculio infestation.
In trees with a history of plum curculio injury, the following are important considerations: (1) shorten the interval between sprays during peak curculio activity (this might be necessary on outside rows only), (2) increase the 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. On stone fruits, time sprays for the shuck split stage.