White Grubs

Problem type: Insect

Name of problem: White grubs

Plant name(s): Primarily turfgrass and forage grass; vegetables (including potato tubers) and ornamentals may also be affected

Symptoms / Characteristics:
White grubs inhabit the soil where they feed on grass roots, causing the aerial portion of the plant to turn brown and die. Because the roots are destroyed, affected plants are poorly anchored and can be pulled effortlessly from the ground. Patches of brown or dead turf can be easily rolled up. Damage usually appears around mid-August. Rodents, skunks and other animals often tear up grub-infested turfgrass in search of the succulent white grubs, leaving behind noticeable holes in the lawn. In many cases, the digging animals create a far worse problem than the feeding grubs. Adult beetles feed on the leaves and flowers of various plants.

White grubs are June beetle larvae. They are white with reddish-brown heads and six legs towards the anterior end. These c-shaped grubs can reach a length of 4 cm at maturity. Adults are shiny, brown beetles ranging from 2 to 2.5 cm in length. June beetles have a three-year life cycle. Adults are active from mid-May to early July and are often seen at night hovering around outdoor lights. By early summer, eggs are deposited in the soil. The larvae emerge after two weeks and begin feeding on grass roots. Feeding ends in October when the larvae move deep into the soil in preparation for winter. In the spring, the larvae return to the surface and continue feeding throughout the summer. Damage is most severe during the second year of the life cycle due to the length of the larval feeding period. Again, the larvae migrate deep into the soil to overwinter. In the third year, they feed for a few weeks in the spring before pupating into adults. The new adult beetles overwinter in the soil and emerge the following spring to mate and lay eggs. The European chafer and Japanese beetle also have white grub larvae that cause the same damage and require the same control. Species identification requires an examination of spines on the tail underside.

Control / Preventions:
A properly maintained lawn will be more resistant to white grub invasion. This entails a regimented watering plan, a balanced fertilizer program, proper mowing techniques and overall good sanitation. Adult beetles are far less likely to lay eggs in lush grass with strong roots; thin, short grass is preferred. When starting a new lawn or reseeding a damaged area, choose a fescue or ryegrass variety (or mixture) that is labelled "endophytic". Endophytic grasses contain nonpathogenic fungi that deter white grubs, chinch bugs and other insects. They also have an increased tolerance for dry, infertile soils. Introducing bird feeders into the garden will help to attract starlings, blackbirds and other birds that enjoy feeding on the succulent grubs. Geranium and larkspur are known to deter white grubs.

Begin monitoring for white grubs in June. Examine the soil surface for actively feeding larvae. It may be necessary to fold back small areas of turf to reveal underlying insects. An average exceeding 10 grubs/m2 may warrant a suitable control. Burying potato pieces in and around damaged areas will attract the feeding grubs for easy collection and disposal. Predatory nematodes are now available for controlling white grubs. Thorough coverage is crucial and the soil must be kept moist, as the nematodes require water for movement.

Chemical controls are also available but should be used sparingly in order to prevent the eradication of natural white grub predators such as ground beetles, wasps and ants. Insecticides are most effective if applied when the grubs are smaller and are actively feeding near the soil surface. Late July or early August applications are usually recommended. Applying during warm, wet weather will also increase spray efficacy. Excess thatch should be removed as it acts as a barrier, preventing adequate penetration of the chemical. Always follow the manufacturer's directions and recommendations regarding application rates, timing, equipment and safety.