The black vine weevil (Otiorhynchus sulcatus) is one of the most serious insect pests of landscapes and nurseries. It eats over 100 different plants, particularly Rhododendron spp. (including azaleas) and yew (Taxus), but also Euonymus spp., Camellia and Japanese holly (Ilex crenata). Adults have been known to feed on lilacs (Syringa) and the larvae eat the roots of hemlock (Tsuga spp.).
Not particular about diet, black vine weevils (BVW) will also eat perennials such as hosta, wisteria, aster, astilbe, phlox, sedum and echinacea. They are related to the strawberry root weevil, which is smaller. The weevils can infest container plants as well as those in the landscape.
Cheryl Moorehead, Bugwood.org
Black vine weevils are about 3/4 of an inch long, black with gold flecks on their wings. They have six legs, a long snout, and bent antennae. Native to Europe, they were first identified in North America in the early 1800's. They are more prevalent in colder climates, less so in the south.
The insects eat in both the larval and adult stages. Larvae do the most damage; they eat the roots of plants. Adults cannot fly; they live in the soil and come out to feed at night, and will drop to the ground when caught in the beam of a flashlight or are otherwise disturbed. During the day they hide on plant stems and in leaf litter and mulch.
A female can lay 500 eggs and only females have been found in the U.S. There is one generation per year. BVW overwinters as larvae in the soil. The ½ inch long larvae are white with a brown head and C-shaped bodies.
Larvae feed on the roots of plants from early spring through fall and can do a lot of damage, because they start by feeding on the newer, tender roots and then move on to larger, mature roots. They will also girdle the main stem of the plant. Small plants can be killed outright; root damage on larger plants interferes with nutrients moving from the soil to the plant and over time the plant can decline. The most intense larval feeding is in May and June. Adults emerge in mid-June and must feed for two weeks to a month before they can lay their eggs around the host plant.
Joseph Berger, Bugwood.org
BVW are difficult to control in all life stages, because they are resistant to many insecticides. There has been success in controlling the grubs with parasitic nematodes (Steinernema kraussei and Heterorhabditis bacteriophora), which are available in garden centers and online, as well as a biocontrol, a fungus called Beauveria bassiana.
Monitoring the appearance of adult weevils and treating before egg laying begins can help control populations. Pruning plants so no foliage touches the ground, pulling mulch away from the base of the plant and using a barrier around the trunk – Tanglefoot or other sticky substances – will help keep adults from climbing plants. Diatomaceous earth sprinkled in the soil around plants will kill emerging adults.
Jim Baker, North Carolina State University, Bugwood.org
Caterpillars will also eat the foliage of many of the same plants, however the difference between BVW and caterpillar damage is the part of the leaves that are eaten. Weevils eat notches around the edges of the leaves; caterpillars eat holes all over leaf surfaces.
BVW and the smaller strawberry vine weevil sometimes enter homes in summer and fall. They are harmless, and are considered a nuisance pest like stinkbugs or lady beetles.
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