Western Grape Rootworms

Western grape rootworms are leaf beetles that eat the leaves of grapes, roses, and fireweed in spring and summer. If that weren’t bad enough, their offspring are found underground, chewing  up the root system. The combined effect can be devastating.

Western grape rootworm beetle (James Lindsey) CC BY-SA 3.0

This behavior is identical to another beetle known as the grape rootworm. The grape rootworm (Fidia viticida) looks nothing like the western grape rootworm (Bromius obscurus) and they are only marginally related.

​Today, we will be looking at the western grape rootworm.

Western grape rootworm description

Grape rootworm (Judy Gallagher) CC BY 2.0

​These beetles are very small, averaging only 1/7” in length. This works out to 4 or 5 beetles standing end-to-end across a dime.

Unlike their more tan-colored cousins, western grape rootworms are usually black or reddish-brown with dull gray, yellow, or white hairs, and orangish-red antennae. Larvae are 1/4-inch long, C-shaped white grubs. These grubs have 3 pairs of prolegs, a reddish brown head, and black or brown mouthparts. Eggs are laid in clusters on old wood under loose bark.

Male western grape rootworm beetles rub their legs together to attract females, the same way crickets and grasshoppers do each summer.

Damage caused by western grape rootworms

These beetles cut slit-like holes in leaves, shredding them to tatters. The leaves then dry up and die, reducing photosynthesis and food for the plant. Adult beetles may also be found feeding on berries, petioles, and the bark of new shoots. Underground, western grape rootworm larvae feed extensively on the root system and can cause considerable damage.

​Controlling western grape rootworms

Since larvae spend most of their year 2-feet underground, control is generally only possible during spring, when larvae and adult beetles move to the surface. Monitor plants for signs of beetle feeding and handpick whenever possible. You can also use sticky barriers to capture beetles moving up and down the trunk.

If you see a tiny beetle playing ‘possum, don’t be fooled. The western grape rootworm beetle uses that trick to avoid being eaten, often falling off a leaf, stiff-legged.