Notes on Nut Insects: Butternut curculio
The adult is a snout beetle or weevil, about 6 mm long, brownish grey in colour, with a broad whitish band across the wing covers. The larvae are plump and legless, with a brown head.
Butternut curculio (Conotrachelus juglandis) is an economically significant pest of black walnut, English (Carpathian) walnut, Japanese heartnut walnut and butternut. Both the adults and larvae cause injury to the tree and to the nut crop. The adults feed on nuts, tender twig terminals and leaf petioles; the larvae burrow into the nuts, inside young shoots, leaf petioles and stems. In commercial nut orchards, injury to the flowering shoots in early spring and injury to the developing nuts during the summer usually causes premature abortion and drop. The entire nut crop can be lost due to curculio injury in severe cases. In forest plantations and orchards, larvae that feed in new stems can kill back new branches to the previous season's growth.
Period of activity
Butternut curculio overwinters as an adult in ground litter under trees or along weedy fencerows. The first eggs are laid in new twig growth in early spring. In summer, eggs are laid in crescent-shaped excavations chewed by the adult female in the nut husk near the blossom end.
Larvae become full grown in four to five weeks and emerge from infested twigs and nuts in late July and August. Mature larvae enter directly into the soil to pupate. New adults begin to emerge in late summer and may feed on shoots, terminal growth and leaf petioles until frost forces adults into the ground cover to overwinter.
Begin to monitor for curculio activity in spring, when shoot growth begins. Watch for signs of brownish to black adults, black feeding scars on new shoots and crescent egg laying scars on the nuts until early August.
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