Content adapted from MyIPM/Bugwood Apps
Plum curculio (PC) has time and again been identified by pome fruit growers as one of the single most destructive insect pests in the orchard. There are four types of wounds caused by PC, however, oviposition is the most recognizable of these. When the female lays her eggs, she sweeps her ovipositor in an arc along the surface of the fruit. This motion causes the distinctive “half moon” scar. When an egg hatches, the larvae tunnel into fruitlets, begin to feed, and the fruitlet will generally drop to the ground prematurely. When the egg is not viable, or the female decides that she does not want to lay an egg on a particular fruit, the scar remains and can be seen at harvest, often making the fruit unmarketable.
Plum curculio is a snout nosed beetle, aka a weevil. The adult is small, about 6 mm, mottled black, grey and brown. When handled it will often “play dead” in an attempt to fool animals that might prey on it. The larvae is a whitish, legless grub, it's feeding on the inside of fruitlets is what will cause premature drop. Larvae then crawl out of the fallen fruitlets into the soil where they pupate. Adults emerge from the soil after ~16 days. They then move around, feeding here and there, until cold weather drives them into hibernating spots. Commercial orchards, generally, do not have overwintering populations within their borders. Wild hosts (abandoned orchards, crab apples, etc.) on the other hand, are a prominent source of adult curculio. These outsiders will migrate into your orchards right at petal fall, when temperature and moisture are optimal.
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 border spray should be made. Cool wet weather will prolong PC activity. Continue to monitor for fresh scars. If more are found, a second cover spray may be needed.
While there is promising work with beneficial nematodes underway, management of plum curculio relies heavily on petal fall, first and second cover insecticidal applications. The first insecticide application should be made to the whole orchard in order to clean up any PC that have migrated into the inner part of the orchard. Surround is an organically approved material that can also be complementary to conventional management strategies. Surround applications at the onset of petal fall and continue to reapply weekly to maintain coverage and deter egg laying. Do not apply insecticides until bloom is completely finished to reduce unwanted pollinator exposure. When using pre-mixes, ensure the modes of action in the material are effective against more than one of the pests you are managing. For example a petal fall spray of Voliam Flexi will be effective against both plum curculio and european apple sawfly. This material has moderate toxicity for honeybees and low toxicity to some predatory insects (not too shabby). For information on rainfast characteristics of some insecticides, see the following article in Fruit Gower’s News: http://fruitgrowersnews.com/news/rainfast-characteristics-insecticides-fruit-reviewed/