|Pecan nut casebearer larva boring into nut
University of Missouri
Pecan nut casebearer, Acrobasis nuxvorella, overwinters as partially grown larvae in small cocoons (hibernacula) located at the junction of the bud and stem. Larvae leave the cocoons in the early spring about the time the buds open, feed briefly (about two weeks) on the exterior of opening buds and then bore into the young tender shoots, where they mature and pupate. In late May to early June, about the time that the pecan nuts are pollinated, the adult moths emerge and lay eggs on the young nuts, typically one per cluster. The eggs hatch three to nine days later. These first-generation larvae feed for a few days on the exterior of the buds, then migrate back to the nut clusters and bore into the nuts at the basal (stem) end. One larva can destroy from one to all of the nuts in the cluster (see Figure 1). Infested nuts are held together by frass (waste) and silken threads cast out by the larvae. Larvae feed inside the nuts for three to four weeks, mature and pupate in one of the last nuts attacked, and the adults emerge nine to 14 days later.
Most second-generation moths emerge in mid-July. The second-generation larvae also attack nuts, but the loss is less because an individual pecan nut casebearer typically requires only one nut for its development. Third-generation moths emerge during late August and September, and larvae feed in the nut shuck at the base of the nut, on the shuck surface and, to some extent, on the leaves.
Eggs are minute and change from white to pink as they incubate for three to nine days (an average of five days). Most are found near the flower end of the nut, on and beneath the calyx lobes. A larva has five pairs of prolegs and changes from olive-gray to gray-brown as it grows to measure one-half inch. The head is reddish-brown, and the body is sparsely covered with fine, white hairs. The larval stage lasts from 25 to 33 days. Adult moths are slate-gray with a ridge of long, dark scales on the basal end of forewings. Moths are one-third inch long, with a wingspan of four-fifths of an inch.
Scouting and control
The first generation is the most damaging. Begin scouting for pecan nut casebearer eggs/larvae when all the catkins on native trees have fallen or when the tips of the nuts turn brown after pollination. You should inspect at least 200 nut clusters. When you find that 1 percent to 3 percent of the nut clusters have been damaged, apply an insecticide. During years of heavy nut set on native trees, you can delay spraying until 5 percent of the nut clusters sustain pecan nut casebearer damage.
George S. Smith and Maureen H. O'Day
Department of Entomology, University of Missouri-Columbia
Kansas State University