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White Grubs

August 17, 2005

The white grubs have hatched. We are finding 8 to 10 grubs per foot square in irrigated turf in the Urbana area of east-central Illinois, marginally enough grubs to cause obvious turf injury. Typically, 10 to 12 or more grubs per foot square are enough to cause turf dieback. The Urbana area had a relatively light flight of adult Japanese beetles and very few masked chafers. Japanese beetles were much more common in the Bloomington and Peoria areas, so it is likely that those and other areas that experienced high numbers of Japanese beetle and associated tree and shrub damage will have higher numbers of grubs. With the very dry conditions in much of northeastern Illinois, we expect high grub numbers in irrigated turf, especially in areas or neighborhoods where most of the turf was not irrigated or was watered irregularly, causing the beetles to concentrate in the watered, green turf.

Check for white grubs by cutting through the turf with a heavy knife and pulling up the sod: The grubs will be in the root-soil interface in moist turf and 2 or 3 inches deeper in dry soils. Tilling up the exposed soil with your knife exposes these deeper grubs. If you cut a square that is 1 foot long on each side, the number of grubs found is the number per foot square. Check several areas of the turf to get an accurate idea of the grub numbers. Usually a half dozen or so samples is sufficient. There will be more grubs where there has been more irrigation, such as greens, tees, and fairways of golf courses. In home lawns, grubs are more prevalent next to flower beds and vegetable gardens that were watered more than the lawn. Usually, grubs are more numerous in the front yards because people tend to water their front yards more than their backyards.

White grubs are C-shaped, white, and up to 1 inch long. They have legs and brown heads. Those found in Urbana on August 5 were 1/4 to 1/3 inch long, probably in the second larval instar. Most of the grubs will molt to the third and final larval instar by the end of August, remaining in that stage through the winter and much of the spring before pupating in June.

Marginally high grub numbers, 10 to 17 or so per foot square, can normally be tolerated by the turf if it is well-watered. Roots grow as fast, or faster, than the grubs can eat them. Higher grub numbers or numbers over 10 per foot square in irregularly irrigated turf should be treated to avoid injury. Injury appears as wilted, brown turf that pulls up easily due to many of the roots having been eaten off.

Trichlorfon, sold as Dylox, is very effective when applied to turf with active grubs. Watered in with at least 1/2 inch of water, it should kill about 95% of the white grubs within 3 days. Make sure that the turf is watered and that the grubs are up in the root zone because Dylox lasts only about 5 days before breaking down. If the grubs stay deeper in the soil due to dry soils for at least 5 days after application, they may not be killed by the insecticide.

Insecticidal nematodes, such as Heterorhabditis bacteriophora, are also be effective, killing about 60% of the grubs. Be sure to follow the label directions on application. If the nematodes are allowed to dry on the grass blades or thatch before being watered in, many die, and control is greatly reduced. Realize that if there are 20 grubs per foot square, 60% control reduces their numbers to about 8 per foot square, below the level where severe damage is likely.

Author: Phil Nixon


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