Grub Control in Lawns - East Idaho News
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Grub Control in Lawns

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Grub damage to lawns is a major concern to homeowners this time of year. Similar to drought stress, grub feeding damage appears as brown or tan discolored areas, thinning of grass and, occasionally, the death of large patches of grass.

There are two basic groups of insects that cause this damage to lawns, billbugs and white grubs. Although there are several species in each category, management for the species in those categories are the same.

Billbugs

Billbugs are one of the most common grub insects in Idaho lawns. They are gray, brown or black with a hard shell and curved snout. Measuring only ¼ inch long, billbugs can be seen in the spring once the soil temperature reaches 55°F.

The larvae are small, usually between 1/8 to ¼ inch long, they are white with a brown head, curved legless and are the size and shape of large grain of rice. Four common species are found in Idaho and despite differences in species, all billbugs can be managed the same way.

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Extensive damage to Kentucky bluegrass sodded lawn showing tall fescue remaining because of their endophytes. | David Shetlar, The Ohio State University, Bugwood.org

White grubs

White grubs are the larvae of several scarab beetle species. Considered the most destructive turfgrass pest found in the Pacific Northwest, the scarab beetle also eats the roots of grasses. Commonly known as the May/June beetles and masked chafers, during their larval stage they are a white grub with a characteristic C shape. They have three pairs of legs and grow up to 1¼ inch long.

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White grubs | Clemson University, USDA Cooperative Extension Slide Series, Bugwood.org

Management

Timing is everything when managing insects. An understanding of the life cycle and the most vulnerable stage of life is important to know if you want to eliminate turfgrass problems.

Bluegrass billbugs overwinter as adults, lay eggs on grass stems in May or June, and hatch in about two weeks. As the larvae mature, they move from the crown of the plant into the soil where they feed on roots. They only have one generation per year.

The Denver and hunting billbugs have a similar life cycle but may lay eggs for a longer time, and larvae may overwinter in the soil. Damage from these species can occur much earlier in the spring than the Bluegrass billbug.

The adult scarab beetles mate and lay their eggs in the soil in the early summer. Larvae hatch and become especially damaging from October through May.

Culturally, to control billbugs select grasses with elevated levels of endophytic fungi, which are resistant to billbugs. Perennial ryegrasses, fine fescues and tall fescues are resistant to billbugs. Frequent irrigation is also important to keep turfgrass healthy. A healthy plant will be able to recover when insects feed on roots.

There are two options for chemical control of billbugs, contact and systemic insecticides. Contact insecticides should only be used in the spring when you see billbugs moving on the surface of the soil. Contact insecticides are not effective on the larvae, as the larvae are generally found in the soil.

Systemic insecticides are applied to the lawn and should be watered in with ½ inch of water. Systemic insecticides move through the plant so that the plant can protect itself from insects. Apply systemic insecticides in late May or early June for best control.

Chemical control of white grubs is best when spraying in August and September when grubs are active and near the soil surface. A systemic insecticide that can move through the soil is most effective and should be watered in with ½ inch of water immediately after application.

For a list of chemicals available for control of billbugs and white grubs, refer to www.pnwhandbooks.org and search “billbug” and “white grub” in the search. Several insecticides will control both billbugs and white grubs.

Always read and follow the insecticide label, it is a legal document and gives directions for proper and legal use of the chemical.

Most insecticides are toxic to bees exposed to direct treatment or to residues on blooming weeds/flowers in the lawn. Mow your lawn prior to applying insecticides to avoid non-target action.

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