Issue 11, July 2, 2010

White Grubs

Japanese beetles and masked chafers are actively laying eggs at this time in turfgrass. The severity of this infestation is primarily governed by the adult population size and rainfall patterns.

White grubs are C-shaped, white scarab larvae with six legs, and brown heads. The ones that are the larval stages of Japanese beetle and masked chafers grow to about one inch in length. They feed on the roots of turfgrasses, causing wilting and death of large turf areas when numerous. In heavily-damaged areas, the sod can be peeled back like a carpet due to the lack of roots.

Three species of scarab are most important in Illinois turf. Japanese beetle grubs tend to require more moisture with the adults laying eggs in moister soil and the resulting larvae needing about 11 inches of rainfall and/or irrigation through the balance of the summer and fall. Northern and southern masked chafers will lay their eggs in drier turf and the white grubs will survive less moisture. Both species of masked chafers are found throughout Illinois. Japanese beetle is most numerous in the northern two-thirds of eastern Illinois, in southwestern Illinois centered in Collinsville, and in other Illinois cities. They are less numerous in many rural areas, but are increasing in number and range every year.

Adult Japanese beetles are obvious, feeding at this time on the upper leaves of various trees, shrubs, and fruit, particularly linden, crabapple, rose, birch, willow, cherry, apple, peach, grape, raspberry, currant, and blackberry. They probably prefer to feed most on smartweed. The adult beetles are three-eighths to one-half inch long, stocky, and metallic green with coppery wing covers. They will be present in large numbers into August.

Masked chafer adults are less obvious as they do not feed. They are active at night, and are attracted to lights. Masked chafers are tan June beetles, being stocky and about one-half inch long. They can be seen flying over the turf for a couple of hours after 10:00 p.m. Shining vehicle headlights across the turf is an easy way to observe this activity. Because they do not feed, they only survive for a couple of weeks around the end of June and early July.

Japanese beetles tend to be the dominant white grub in areas where they are present, partially displacing the masked chafers. We have reports of high numbers of Japanese beetles this year in the Peoria, Pekin, Chillicothe, and Rockford areas. This year, they are about one-third as numerous as they have been in the past in the Champaign area.

All three species of scarab dig into the soil to lay their eggs. As a result of that and being attracted to green grass, many more eggs are laid in moist turf. In a normal year, this concentrates the egg-laying and resulting larvae in irrigated turf as unwatered turf is usually brown and dormant and the soil is dry and hard. However, this year, as in the previous two years, we have experienced abnormally heavier and later rainfall. This makes unwatered turf attractive to egg-laying as well as irrigated turf. In these wetter years, the eggs and resulting white grubs are spread out over larger areas and turf damage is typically spotty and not widespread. There are dry areas of Illinois this year that can expect high grub populations in irrigated turf, but many areas should have less than damaging numbers.

If high grub populations are expected, preventative application of a long-lasting white grub insecticide is recommended. This includes chlorantroniliprole (Acelepryn), clothianidin (Arena), imidacloprid (Merit), or thiamethoxam (Meridian). In recent years, we have also recommended halofenozide (Mach 2) for this application. However, widespread failures of this product across the U.S. have been reported, possibly due to microbial degradation. In microbial degradation, microbes adapt to eat the pesticide. If you have had good results with Mach 2 in the past, feel free to continue using it but rotation with at least one other of the above insecticides on a yearly basis is recommended. If you have had treatment failures with Mach 2, microbial degradation may be the cause.

In most areas of the state, we recommend a wait-and-see management strategy. If you are in an area that has received adequate or more rainfall, put off applying grub insecticides until early August. At that time, scout for the white grubs by cutting through the turf with a heavy knife, pull back the sod, and look for white grubs in the root zone. Till the underlying two to three inches of soil with the knife to reveal deeper grubs.

If there are ten to twelve or more grubs per foot square, treatment will be needed to prevent damage later in the month and into the fall. With fewer grubs, treatment should not be needed unless there is a history of raccoon, skunk, armadillo, or bird damage from feeding on the grubs. These animals will damage turf while feeding on as few as one to three white grubs per foot square. Moles are more attracted to earthworms, so white grub numbers are less important to them.

If treatment is needed, apply any of the above insecticides or trichlorfon (Dylox). Trichlorfon is quicker acting than the above insecticides, killing the grubs within three days, but it is short-lived, breaking down to non-effective compounds in about five days. This short residual makes it ideal for treatment of sports fields as well as rescue treatments where damage is evident later in the season.--Phil Nixon

Phil Nixon

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