The plum curculio [Conotrachelus nenuphar (Herbst)] is a native American insect that ranges from Florida to southern Canada and west to Montana and Colorado. Although plum and peach are its preferred hosts, adults will feed and reproduce on many other fruits including apple, cherry, blueberry, huckleberry, grape, and persimmon (Snapp 1930, Bobb 1952).
The adult stage is a hard-shelled snout beetle (weevil) about 4 mm long (3/16 inch). It is usually dark brown in color with a pair of mottled gray patches near the back of the wing covers. The top surface of each wing cover has three distinct bumps that are most obvious when the insect is viewed from the side. Larvae are white legless grubs with a distinct brown or tan head capsule. Fully grown larvae reach a length of about 12 mm (1/2 inch) (Snapp 1930, 1940).
- Adult stage of the plum curculio.
- Oviposition scars in blueberries.
Life Cycle and Seasonal Distribution
Adults overwinter under debris usually in wooded areas adjacent to blueberry fields. They emerge from hibernation in early spring, usually when the daily temperatures average 10 to 15 degrees C (50-60 degrees F) for 3 or 4 consecutive days and the high temperatures are 24 degrees C (75 degrees F) for two or more consecutive days (Whitcomb 1932). These conditions usually occur about the time blueberry plants finish blooming. Nearly all weevils will have emerged from hibernation and returned to blueberry fields by the time daily temperatures reach 32 degrees C (90 degrees F).
Female weevils lay their eggs in shallow pits excavated on the surface of green berries. Usually a single egg is laid in each berry, and a crescent shaped scar develops on the surface at this oviposition site. Upon hatching, larvae burrow into the fruit where they feed on the pulp for about two weeks. Fully grown larvae leave the fruit, burrow into the soil, and pupate within an earthen chamber. Adult weevils emerge about 4 weeks later. Most of these adults enter diapause after several weeks of active feeding, but if green berries are still present, a few will mate and produce a second generation (Mampe and Neunzig 1967).
Symptoms and Damage
Feeding by adult plum curculio does not cause any significant damage to blueberry plants, however, presence of larvae in the fruit is a major concern for all growers. Crescent-shaped oviposition scars are the most reliable symptom of weevil infestation. Affected berries are usually the first to turn blue, and will often drop to the ground before uninfested fruit is ready for harvest.
Mortality of immature stages in the soil is an important factor in reducing plum curculio populations (Mampe and Neunzig 1967). Beauveria spp., a fungal pathogen that infects both larvae and pupae, is one of the biological agents responsible for this mortality. Several parasites have also been reported for the plum curculio, including an egg parasite (Anaphoidea conotracheli Giradaeu) and two larval parasites (Aliolus rufus Riley and A. curculionis Fitch). Rates of parasitization are usually very low, especially in commercial fields where insecticides are occasionally used.
Scouting, Management, and Control
Beginning at petal fall (or as soon thereafter as daily temperatures are favorable for migration), inspect several fruit clusters on each of 20-30 plants per field. Look for the characteristic oviposition scar on the back or underside of the largest berries. Although adult weevils are secretive and nocturnal, they can be found early in the morning or late in the evening by shaking the branches of a bush over a white ground cloth. Weevils disturbed in this manner will drop from the bush and feign death, folding their legs tightly against the body as they lie on the white cloth.
Two applications of a contact insecticide are usually necessary for good control of plum curculio infestations. The first spray should be applied as soon as adults begin returning to the fields, and another spray should follow the end of spring migration (after daily high temperatures reach 32 degrees C (90 degrees F)). Additional applications may be necessary for heavy infestations or during cool seasons when berry development is unusually slow.
- Bobb, M. L. 1952. Life history and control of the plum curculio in Virginia. Va. Agr. Exp. Sta. Bull. 453. 30 pp.
- Mampe, C. D. and H. H. Neunzig. 1967. The biology, parasitism, and population sampling of the plum curculio on blueberry in North Carolina. J. Econ. Entomol. 60: 807-812.
- Snapp, 0. I. 1930. Life history and habits of the plum curculio in the Georgia peach belt. USDA Tech. Bull. 188. 90 pp.
- Snapp, 0. I. 1940. Further studies of the plum curculio in the Georgia
peach belt. J. Econ. Entomol. 33: 453-456.
- Whitcomb, W. D. 1932. The relation of temperature to the activity and control of the plum curculio in apples. Mass. Agr. Exp. Sta. Bull. 285. 16 pp.
Last updated: 2 June 1997
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