Are Grubs in Vegetable Garden Soil Harmful?

Counting Grubs

• As with grubs in lawns, more than 10 in a square foot area generally means a high population of grubs exist in the garden. Multiply the number of grubs you find by the number of square feet of garden to get an estimated total. Moles, birds, raccoons and other wildlife love to eat grubs, so if they are digging up the garden regularly, the grub population is likely rather high.

Identification

• Determining what kind of grubs are living in your garden can be difficult. If the grubs are white with dark heads, they may be Japanese beetle grubs or larvae from May beetles, scarab beetles, chafers or June bugs. White grubs feed on plant roots and can cause significant damage to carrots and other root crops. Potato beetle grubs are brown, with a humped back. Use a magnifying glass to study the tiny hairs on the hind, or raster, of white grubs. A v-shape or "parentheses" pattern in the center is a Japanese beetle grub. Two parallel lines mark the May or June beetle grub, and Asiatic garden beetles have a "smile." The bumble flower beetle grub has no discernible pattern. This grub feeds on decaying matter and will not harm garden plants, though the adult beetles feed on fruit.

Damage

• A heavy infestation of grubs feeding on the plants' roots causes poor growth and reduces vegetable production. Grubs may hinder the roots' ability to pump nutrients and moisture upwards to the foliage, flowers and fruit, causing the plant to die.

Control and Prevention

• Controlling grubworms already in the vegetable garden is difficult, because pesticides can contaminate the crop. Parasitic nematodes and milky spore disease (Bacillus popilliae) can help reduce the grub population, though milky spore disease takes up to three years to become fully effective. Healthy, vigorous plants can withstand a minor infestation of grubs. Prevention is the best way to protect the garden. After harvest, remove all vegetation and till the soil to expose the grubs to predators. Till again in two weeks, then repeat in early spring, before planting time. Work granular insecticide into the soil before planting the vegetables to kill any remaining grubs in the soil.