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Northwest Iowa Dairy Outlooks

A local discussion of current science and issues concerning dairying in northwest iowa

 Each year about this time I get questions on the pecan nut casebearer (PNC). The pecan nut casebearer is one of the most important nut infesting insect pests of pecans. Casebearer larvae tunnel into nutlets shortly after pollination, often destroying all of the nutlets in a cluster. The most effective and reliable method of control is a well-timed insecticide application(s) made in the spring to kill hatching larvae before they tunnel into the nutlets. However, insecticides should only be applied if infestations and nut load justify treatment. Typically, we will start looking for PNC in Wichita County around May 10 and spraying will progress late in the month, but it is all based on scouting for the first egg hatch.

The PNC Forecast System at http://pncforecast.tamu.edu provides information about pecan nut casebearer activity to assist pecan tree owners with management decisions. The system allows the user to predict activity of first generation pecan nut casebearer in his/her orchard using data from pheromone traps operated in that orchard and local temperatures. The System generates predicted dates when first generation eggs are expected to be found in the orchard and the date when PNC larvae first begin to feed on pecan nutlets.

The PNC Forecast System also provides information on pecan nut casebearer egg-laying activity and nut entry from selected sites across Texas and Oklahoma. These forecasts are based upon data collected by county Extension agents and entomologists with Texas AgriLife Extension and the Oklahoma Cooperative Extension Service, Master Gardeners and by volunteer pecan growers working with Extension.

First generation larvae usually have the potential to cause the greatest economic loss. For this reason, control is directed primarily at this spring generation. Often a single carefully timed insecticide application provides adequate control for first-generation casebearers. Once inside nuts, larvae are protected from insecticides.

To determine whether treatment is needed and when to apply insecticide, examine nuts carefully in spring for casebearer eggs. Most casebearer eggs are found at the tip of the nutlet, either on the top (stigma) or hidden just under the tiny leaves (sepals) at the tip of the nutlet. A good hand lens is necessary to identify casebearer eggs and determine their development (hatched, white or pink). Also, look for bud feeding just below the nut cluster to detect the presence of newly hatched larvae.

Apply insecticides within two to three days after the first eggs hatch. At this time, the first larvae begin entering nuts. Infested clusters can be flagged to monitor egg hatch.

Delaying treatment until the first nut entry occurs maximizes the insecticide’s residual activity. However, consider the time required to treat the orchard and possible weather delays so that insecticide is applied before significant nut entry occurs. A second insecticide application may be required if unhatched eggs are found after the residual period of the insecticide has passed.

The pecan nut casebearer completes two to four generations each year. Adults of the overwintering generation emerge in May and lay eggs on pecan nutlets soon after pollination. These eggs result in first generation larvae which feed on nutlets and generally cause the most damage.

Second generation larvae attack the nuts in mid-summer about six weeks after nut entry by first generation larvae. Third generation eggs are deposited on nuts from late July to early September. These larvae feed only in the shucks if the pecan shells have hardened.

Many third and, if present, fourth generation larvae do not feed, but crawl to the base of a dormant bud where they construct a tough, silken cocoon (hibernaculum) in which to spend the winter. In the spring, these immature larvae leave the cocoon and feed by tunneling into shoots. Full-grown larvae pupate in shoot tunnels or in bark crevices. Moths from these overwintering larvae lay first generation eggs on nutlets.

Insecticide sprays should be applied with caution in backyard and urban areas because of the great potential for spray drift onto nearby gardens, pets and living areas. Only products containing carbaryl, malathion, or chlorpyrifos which are labeled and packaged for homeowner use should be used for control in urban areas.

Many kinds of insects and spiders that occur naturally in pecan trees feed on the pecan nut casebearer. Some of the most important natural enemies are tiny parasitic wasps that sting and kill the larval stage. Over 25 different species of wasps are known to attack casebearer larvae and no doubt help reduce casebearer populations.

Very tiny parasitic wasps of the genus Trichogramma lay their eggs inside the casebearer egg. The wasp egg hatches and the Trichogramma completes its entire development inside the casebearer egg, turning the egg black. Trichogramma wasps occur naturally but little is known about their importance in controlling pecan pests. Trichogrammma wasps can be purchased for release in orchards but studies to date indicate that currently available species and methods of release do not provide effective control of casebearer.

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