Issue 12, July 17, 2009

White Grubs

The consistent rainfall that we have been receiving across Illinois this year is reducing the need for white grub control efforts. In typical years with reduced rainfall from the second half of June through August, Japanese beetle adult females seek out irrigated turf to lay their eggs, laying few eggs in dormant, brownish, unwatered turfgrass. This concentrates the eggs and resulting larvae in these areas, resulting in heavy turf damage from root feeding. Also, with the northern two-thirds of Illinois experiencing a reduced adult Japanese beetle emergence, the need for grub control is reduced even further in that area of the state.

Southern and northern masked chafers are important white grub species, but also prefer to lay their eggs in moist soil. Masked chafers tend to be more tolerant of dry soil than Japanese beetle, so they are able to compete where Japanese beetles are numerous by depositing eggs and their larvae surviving in drier soil.

There are two general types of white grub insecticides based on their longevity. One group of insecticides lasts for three months or more and commonly takes three weeks or so to kill white grubs. These are generally applied preventatively in July and include halofenozide (Mach 2), imidacloprid (Merit, Allectus), clothianizine (Arena, Aloft), chlorantroniliprole (Acelepryn), and thiamethoxam (Meridian).

The other group includes trichlorfon (Dylox) which is short-lived, about five days, but kills the grubs within three days. Dylox is applied only when grubs are present due to its short longevity. All insecticides labeled for grubs are more effective on smaller, younger grubs. Water the insecticide application into the root zone with at least one-half inch of irrigation. Halofenozide and thiamethoxam are more water soluble and can be watered in with at least one-quarter inch of rain or irrigation within three days after application.

This year, it is best to wait on grub control applications until treatable grub numbers are found. You can scout for white grubs by cutting through the turf with a heavy knife and peeling it back to expose the grubs. In well-watered turf, the grubs will be primarily at the root-soil interface. They may be three to four inches deeper in dry soil. Those can easily be exposed by tilling the soil with the knife. Ten to twelve grubs per foot square is a good threshold to determine whether treatment is needed. Scout for grubs in the first half of August when the grubs have hatched, but they are not yet big enough to cause visible turf injury. Later, off-color turf with an increased percentage of brown grass blades is a sign that there may be a damaging number of white grubs and scouting that area is warranted.

Once grubs are located and the threshold has been reached or there is turf injury from them, treat with any of the above insecticides. Because Dylox kills the grubs in three days, it is preferred if the clientele is likely to do their own checking for live grubs. Although it commonly takes three weeks for the other insecticides listed above to kill the grubs, they will usually stop eating soon after the insecticide is applied. As a result, damage ceases even though the grubs are still alive.

In a high rainfall year like this one, there will be spots in turfgrass areas that will experience damage and will warrant treatment, but most areas will not. In the Illinois River Valley, such as the area around Dixon, Monmouth, and Peoria southwest to St. Louis, be particularly watchful for white grubs or consider widespread preventative insecticide application in areas where grubs have been numerous in previous years. That portion of the state tends to experience grub damage even in lower beetle years with adequate rainfall.--Phil Nixon

Phil Nixon

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