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Plum curculio (PC)

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Conotrachelus nenuphar

Overview

Plum curculio  (PC) is generally considered the single most destructive insect pest in orchards. The most recognizable type of wound caused by PC is the half-moon scar, produced by ovipositing females. Prior to depositing her egg, the female first uses her mouthparts to cut a small crescent-shaped flap in the fruit skin; then, she turns around to deposit an egg. When eggs hatch, larvae tunnel into fruitlets and begin to feed. Larvae complete four instars inside the fruit in about 16 days. PC-infested fruitlets generally drop to the ground prematurely. When an egg is not viable, or a female cuts into a fruit but does not deposit an egg, the scar remains and can be seen at harvest, often making the fruit unmarketable.

Biology

  • Plum curculio (PC) is a snout-nosed beetle, aka a weevil. The adult is small, about 1/4 inch, mottled black, grey and brown. When handled it will often drop and “play dead”.
  • The larva is a whitish, legless grub, and its feeding in fruitlets causes premature drop. Larvae then crawl out of fallen fruitlets into the soil and pupate. Adults emerge from the soil after ~16 days and feed within and outside of orchards until cold weather drives them into hibernating spots.
  • Generally, commercial orchards do not have overwintering populations within their borders. Some studies, however, have shown that PCs can overwinter inside orchard blocks that are weedy in the fall.  Wild hosts (abandoned orchards, crab apples, etc.) near orchards provide habitat which allows adult PC to migrate into orchards before and after bloom.

Monitoring

  • Fruitlets should be monitored beginning at about 5 mm diameter along orchard borders to determine if new injury is occuring. If fresh oviposition scars are observed, a first cover spray should be made to the entire block. Cool, wet weather will prolong PC activity. Continue to monitor for fresh scars. If more are found, a second cover spray targeting perimeter-row trees may be needed.
  • Because PC immigration and oviposition period is affected by weather patterns after Petal Fall, insecticide coverage should be maintained until 308 DD (base 50F) from Petal Fall.
  • A monitoring system that makes use of attractive lures, termed the ‘trap tree’ approach, has been developed. It involves baiting the branches of one perimeter-row tree with a synergistic two-component lure comprised of benzaldehyde, one synthetic component of flowers and developing fruit, in association with grandisoic acid, the synthetic PC pheromone. By examining the fruit solely on the odor-baited tree for signs of fresh PC injury this monitoring technique has proven effective at determining, on a timely manner, whether perimeter-row insecticide sprays are required against PC after the whole-block petal fall spray. Lures are commercially available and last for the entire PC oviposition period. For more information about the ‘trap-tree’ approach to PC monitoring, contact Jaime Pinero at jpinero@umass.edu.

Management

  • Management of PC relies heavily on petal fall, first and second cover insecticide applications. The first insecticide application should be made to the whole orchard in order to control PC that have migrated into the inner part of the orchard.
  • Additional insecticide applications may be necessary and can be limited to the outer two rows of trees.
  • Kaolin clay (Surround WP) is an OMRI-listed material that can also be complementary to conventional management strategies. Applied in suspension in water, kaolin clay produces a dry white film layer of interlocking microscopic particles on the surface of leaves, stems, and fruit after evaporation of the water. Kaolin acts as a physical barrier preventing insects from reaching vulnerable plant tissue. It acts as a repellent by creating an unsuitable surface for feeding or egg-laying.

    Surround applications begin at Petal Fall and get reapplied weekly to maintain coverage and deter egg-laying.  

  • Do not apply insecticides until bloom is completely finished to reduce unwanted pollinator exposure to insecticides.  For information on rainfast characteristics of some insecticides, see the following article in Fruit Gower’s News: http://fruitgrowersnews.com/news/rainfast-characteristics-insecticides-f...