Notes on Nut Insects
Butternut curculio
Identification
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.
Monitoring
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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