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Plum curculio

plum-curculio

Adult plum curculio

Clemson University, USDA Extension Slide Series

plum-curculio

Plum curculio larva in a 'Sapalta' plum

Thaddeus McCamant, Central Lakes College

plum-curculio

Oviposition scar on a plum

Thaddeus McCamant, Central Lakes College

The plum curculio (Conotrachelus nenuphar) is a weevil native to Minnesota that is found throughout most of the state. Plum curculio has a wide host range that includes apples, plums and cherries. Plum curculios can cause large crop losses in both cherries and plums.

Identification

Plum curculio adults are about 1/4 inch long with a mottled grayish and brown back that has several bumps on each side. They also have short, curved snouts, which are about 1/4 the entire length of the insect. The color and the bumpy back allow the curculios to blend in easily into the bark of mature trees.

Mature plum curculio larvae are a little larger than 1/4 long, which is slightly longer than adults. They are white, with no legs, and a small, brown head.

In May and June, when the plums start to grow, female curculios make distinct, crescent shaped oviposition scars on the fruit when they are laying eggs. The female inserts her egg and then cuts a small crescent-shaped flap around each egg that it lays on the fruit. In most oviposition scars, a round egg scar about the size of a pin head is visible inside the crescent.

Damage

All stone fruit with curculio larvae are unfit for consumption. Curculio larvae burrow through the developing plum or cherry fruit. Larvae that emerge before the pits harden feed through the developing seed, causing the fruit to fall to the ground. Larvae that emerge after the pits harden often stay in the developing flesh through June and into July. Fruit with curculio larvae rot during ripening.

Plum curculios are the most serious insect pest in plums in Minnesota and the most common reason for crop failure. In untreated plum trees with high curculio pressure, every plum on the tree will have at least one crescent shaped oviposition scar, resulting in a total crop loss. Tart cherries usually produce more flowers and fruit than plums, and there are rarely enough curculios to infest every fruit. In some years, tart cherries will ripen while the curculio grubs are still in the fruit, resulting in "wormy" cherries. Tart cherries with curculio larvae have rotten black spots on the fruit.

Adult plum curculios emerge in August and feed on plum and apple fruit that is starting to ripen. Adult curculios feed on the surface of the fruit, forming small, almost perfectly round holes in the plum skin. In plums, feeding injury by adult curculios can increase the incidence of brown rot.

Important biology

Plum curculios have one generation per year in Minnesota. Adults overwinter in brush near the plum trees. In the spring, the adults emerge shortly after bloom. Plum curculio adults can fly, but prefer to walk. The adults usually climb tree trunks to reach the developing fruit, rather than flying to branches. In sites with low curculio pressure, typically only one plum or cherry branch will have infested fruit, while other branches have no damage.

Cool weather slows down the development of the curculios relative to the plum or cherry fruit. In warm springs, adults will begin depositing eggs when the fruit is a half inch in diameter or less, while during a cool spring, the adults may not start laying eggs until the middle of June, after the pits start hardening. Plum curculios typically lay their eggs during warm, humid evenings.

After hatching, the larva burrows through the fruit. The curculio larvae will survive both in fruit that remains on the tree and in fruit that falls to the ground. After a month of feeding, the larvae will pupate in the soil for another month and the adult will emerge in late summer.

Management

Plum curculio populations can be reduced through a combination of cultural controls, physical removal, and insecticide sprays.

Monitoring

Start monitoring plum curculios after the shucks split in order to find when the adults are active in the fruit trees. The shuck encloses the center of the flower. After bloom, the shuck turns brown and either splits or falls off as soon as the developing fruit starts growing.

One way to check for curculios is to place a cardboard box or a piece of tarp under a branch and vigorously shake the branch. If there are curculios on the branch, they will fall onto the cardboard and play dead. Shaking branches will show that curculios are on the trees even before they start laying eggs.

A second method for monitoring is to check for oviposition scars on fruit. This method is only effective if oviposition scars are detected shortly after adults have started to lay eggs; if these scars are not discovered until after most or all of the fruit have been attacked, then it will be too late for effective management. When monitoring for plum curculios, remember that they can be abundant in one tree and absent in nearby trees. Plum curculios should be managed if any adults are detected.

Sanitation

When possible, pick up and dispose of infested fruit that falls to the ground in May and June, or infested fruit that stays on the tree in July, before the larvae crawl out of the fruit to pupate.

Physical removal

One unorthodox way to kill plum curculios without insecticides is to shake the trees when the females are actively laying eggs. Shortly after the first plums show oviposition scars, place a tarp under the tree and shake different branches of the tree. Adult curculios will fall off the tree and play dead. The adults can then be picked up and killed by hand. Unfortunately, curculios that play dead look very similar to pieces of bark that also fall on the tarp. While total control of curculios is difficult with branch shaking, enough can be killed to save most of the fruit. Shaking is most effective on warm evenings when the females are laying their eggs.

Insecticides

Curculios can also be controlled with insecticides. Sprays should be timed shortly after the first adult curculios are detected in the spring. Insecticides that kill plum curculio adults include spinosad, esfenvalerate, gamma cyhalothrin and carbaryl. Spinosad is a product often approved for organic production.


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