Documents: Garden Design:

Are Grubs to Blame?
by James Schuster
June 24, 2007

Grubs are often blamed for causing the death of lawns they did not kill, said a University of Illinois Extension horticulture educator.

"Having brown areas in your lawn does not necessarily mean you have a grub problem," explained James Schuster. "It takes about 12 grubs per square foot to cause a dead area in a well-maintained lawn.

"Diseases are often the culprit behind those brown spots."

Grub eggs will start hatching in late July into early August in northern Illinois and about early to mid-July in central Illinois. For the southern third of the state, the eggs hatch in mid-June to late-June.

"Based on the last several years, though, these times may be two weeks too late," said Schuster.

Most homeowners are using one of the grub controls, either Grub X or Merit. These insecticides should usually be put down and watered in about two weeks before egg hatching.

"It takes these insecticides about two weeks to become fully activated," said Schuster. "If applied correctly, the insecticides are ready to control the insects as they hatch. Controlling grubs as they hatch is the most effective control when a chemical becomes necessary.

"Even if you are two to three weeks late, these products may give you satisfactory control even if not 'total' control. Applying in late winter to spring, as some advertisements recommend, weakens the ability of these insecticides to properly control grubs."

Schuster said that proper maintenance of your lawn can greatly reduce the need for using an insecticide to control grubs.

"If it is raining or there is a drought during egg-laying time, the grubs are spread out and there are not enough grubs developing per square foot to cause any brown dead areas in a lawn," he said. "If, however, there is a drought and you water your lawn while your neighbors do not water, then the grubs will lay a higher number of eggs in your lawn.

"You may have a grub problem later in the summer. Thus, a recommended cultural practice to control grubs is not to water your lawn during egg-laying time."

Changing watering practices to deeper but less frequent applications also reduces disease problems and their severity, he added.

"It also reduces how much fertilizer needs to be applied," he said. "You may need to mow less often, too."

Besides grubs, sod webworm may be affecting the lawn and it only takes one of them per square foot to cause dead areas. Grubs eat the grass's roots so the lawn can be easily lifted off the soil as soon as the lawn dies. Sod webworms cut the grass blades off at the crown so the lawn gets a "scalped" appearance.

"The scalped areas soon dry out and die," Schuster said. "Diseases do not cause roots to disappear right away so the lawn cannot be lifted off the soil easily right after it dies. Nor do diseases cause your lawn to look 'scalped.'

"Before treating for grubs, make sure you have correctly identified the problem."

  • New Eden
  • Kids Garden
  • Plant a Row Grow a Row