Purdue News

White grubs invade Indiana lawns

According to Timothy Gibb, Extension entomology specialist at Purdue University, beginning in August the newly hatched larvae of Japanese beetles and masked chafers will begin to feed on turfgrass roots. If the weather is dry, these larvae -- otherwise known as white grubs -- can cause serious lawn damage. The damage will be less if August brings constant rains and cool temperatures -- but when was the last time that happened?

"Grubs tend to affect the same areas from year to year," Gibb said "Therefore, if you've had problems in the past, there is a good chance that you will have problems again this year."

According to Gibb, the season for grub damage is later this year because of the cool, wet weather this spring and early summer.

A homeowner can check for white grubs during early August by digging into the top few inches of soil in numerous places. Sift the soil between your fingers and look for small white grubs, or worms, which may be one-fourth to one-half of an inch long and one-eighth of an inch in diameter.

"Grubs are very small and tough to detect," Gibb said. "If you find four to six grubs per square foot, consider applying an insecticide."

If grubs have been a perennial problem in an area, chemical controls containing Merit or Mach 2 insecticide should be applied by early August. These chemicals are ineffective if applied later in August, however, because the grubs have grown out of their susceptible stages by then.

In late August or early September, there are several other chemical insecticides available that can be used. Irrigate the turf after application, and always use pesticides according to label directions.

According to Gibb, it is important to remember, too, that you are trying to control the damage caused by white grubs -- not to rid the planet of these invaders. Often, insecticides aren't called for.

"In areas where turfgrass managers can afford to 'wait and see' what grub populations develop, they may be able to save themselves a lot of work as well as expense by using irrigation instead of chemicals," Gibb said.

By irrigating more frequently during August and September, the grub damage will not be as apparent. "Often the turfgrass will outgrow the grub feeding damage if the plants don't also have moisture stress," Gibb said.

According to Gibb, there are a number of myths about grubs among gardeners and homeowners:

• Myth : Unseasonably cold winter weather will freeze the grubs and reduce damage in late summer. Fact : White grubs contain a substance similar to ethylene glycol (the main component of automobile antifreeze) and can survive even the coldest soil temperatures.

• Myth : Unseasonably dry weather will prevent Japanese beetles and masked chafers from laying eggs, thus preventing damage later. Fact : Although Japanese beetles and masked chafers prefer to lay eggs in softer soil, there are ample areas that receive irrigation to give beetles plenty of egg-laying areas even in the driest of summers.

• Myth : The extremely wet weather this spring drowned most of the grubs. Fact : Grubs can withstand extended periods in soils that are waterlogged or submerged. "One golf course superintendent last year reported finding live white grubs as they removed dead sod from a fairway that had been submerged for 28 days," Gibb said.

More lawn and turf information, including weekly updates by Purdue turf experts, can be found on the Internet at http://www.aes.purdue.edu/agronomy/turf/turftips.html

Sources: Tim Gibb, (765) 494-4570; e-mail, Tim_Gibb@entm.purdue.edu
Writer: Steve Tally, (765) 494-9809; e-mail, tally@ecn.purdue.edu
Purdue News Service: (765) 494-2096; e-mail, purduenews@purdue.edu

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