Issue 5, May 22, 2009

May Beetles

Adult May beetles are very numerous in southern Illinois. They can be a problem to ornamental trees and shrubs as well as having a delayed impact on turfgrass.

There are several species of May beetles, genus Phyllophaga, in Illinois. The adults currently numerous in southern Illinois are about one inch long, stocky, dark brown beetles. They are found throughout the state, overwintering in the soil as adults. It is common to dig them up when planting trees or shrubs in the fall or spring. Another very common species at this time of year looks similar, but is reddish-brown.

The adult beetles are strongly attracted to lights at night, as shown in the accompanying photo. It is common to see them swimming or drowned in the morning in dog water dishes. They hide in the turf thatch during the day, emerging at night to feed on the leaves of trees and shrubs.

I have noticed the adults feeding on crabapple, oak, and ash, but they attack many other trees. They feed on the leaf margin and sometimes eat all of the leaf except for the midvein. Commonly, females will be feeding while males are mating with them. If you have feeding damage on deciduous tree leaves with no apparent responsible insect, check out the tree at about 10:00 p.m. It is common for large numbers of beetles to be on the foliage at that time of night.

Mated females burrow into turf to lay their eggs. These will hatch out in 2-3 weeks into true white grubs. Although they hatch well before Japanese beetle and masked chafer white grubs, they are controlled with the same insecticide treatment at the same time. With their small size and usually modest numbers, they are unlikely to cause turf damage before treatment for these other grubs occurs in July or August. They look similar to other white grubs, being C-shaped with three pairs of legs. They are white with brown heads. All true white grubs’ raster pattern has two parallel rows of spines. Species vary in the number of spines per row, whether the rows spread apart, and whether they spread towards the front or back.

True white grubs have various life cycle lengths. The most well-known are the species known as three-year white grubs. Their larvae feed for the balance of this growing season, descend deeper into the soil for the winter, ascend to feed for a second growing season, descend for their second winter, and ascend for a third growing season. Most turf damage occurs during the second and third year of feeding. In late summer of the third growing season, they pupate in the soil, emerge in the fall as adult beetles, but stay underground until the following spring.

There are also one-year, two-year, and four-year life cycles. One of the one-year life cycle species is common in southern Illinois, emerging as adults at about this time of year. These adult beetles are tan and about one-half inch long, appearing similar to masked chafers.

True white grubs tend to be common in drier soils than Japanese beetle or masked chafer grubs, so are more likely to cause damage to non-irrigated turf areas. They also have a wider diet, feeding on mulch, other decaying plant material, and on the roots of plants other than grasses. They damage carrots, potatoes, and other root vegetables. They are commonly found in flower beds and other mulched areas where they usually cause no harm.

Feeding on tree leaves by adult May beetles can be controlled with a spray of carbaryl (Sevin), labeled pyrethroid such as cyfluthrin (Tempo) or bifenthrin (Onyx), or other labeled insecticide. Control of the larvae is normally not necessary until July. At that time, we will provide white grub control recommendations.--Phil Nixon and Ron Hines, Growmark

Phil Nixon

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