Black Vine Weevil

- Root weevil larvae damage roots sometimes killing plants -

Summary: Root weevils can be severe pests of many landscape plants. Larvae damage roots while adults feed on leaves. Control efforts should be directed at the adult weevil, leaf-feeding stage, in early summer.

Jack DeAngelis, PhD

OSU Ext. Entomologist (ret.)

What are root weevils?

Black vine weevils (Otiorhynchus sulcatus) [picture of black vine weevil] are one of numerous species of weevils (snout beetles) whose larvae feed on plant roots. Because larvae damage roots they are sometimes called root weevils. Root damage can be so severe that plants, especially young or shallow-rooted ones, can be killed. Older larvae can also girdle (remove bark) stems. In fact, root weevils are among the most damaging of woody landscape plant pests worldwide.

Adult weevils feed on plant leaves resulting in characteristic feeding notches (see photo right). There is generally only one generation per year and control efforts should be directed at newly emerged adult weevils, not larvae. When notching of new growth is first found in early summer you should begin your control program.

Common names and misspellings: strawberry weevil, obscure weevil, rootweevil, and blackvine weevil.

Rhododendron leaf with notches caused by black vine weevil. Photo by JD DeAngelis.

Least-toxic root weevil control

Proper timing is critical to successful root weevil control and the best time to initiate control is when the adult beetles start to emerge from the soil in late spring/early summer. Weevils feed for about 4 weeks after emerging from their underground pupal chambers. After feeding they begin to lay eggs for the next generation. The best time for control therefore is between when feeding notches first appear on new leaves and the start of egg laying, 4-6 weeks later. For the black vine weevil this period is early summer but the timing for other species may be slightly different. The trick is to look for feeding notches on host plants that indicate that weevils are present.

For gardeners that want a low toxicity approach to black vine weevil control you can disrupt feeding and egg laying in early summer with neem oil (see Using Neem Oil Insecticides) applied to leaves. Then, if necessary, control larvae with fall applications of insect parasitic nematodes (see below).

Use parasitic nematodes to control weevil larvae

Using nematodes to control root weevil larvae is a bit more complicated than treating adult weevils with insecticides (see Using Beneficial Nematodes) but it can be very effective. First is the problem of timing. You must apply nematodes when the soil is moist but not too cold. Second, nematodes must be applied exactly according to package instructions because they are fragile and easily destroyed. If, however, you fail to control adult weevils during the summer with insecticides then fall treatments using nematodes may be your best bet. Nematodes are non-toxic to plants and other animals.

Related Articles

Root Weevil Guide; See this guide for pictures of black vine weevil adults and larvae.You'll need Acrobat Reader or equivalent to open this pdf file.

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