Looking ahead – white grub control

Spring is finally here! The grass is turning green and you want to keep it that way. If you have experienced white grub damage to your lawn in the past, you probably are thinking of treating for white grub this year. But let’s investigate further, so put away the insecticide, step away from the sprayer and read on.

Studies at Cornell University have found that more than 70 percent of all grub control treatments were applied needlessly because there were no grubs in the lawn.

Grub damage can be difficult to predict. Lawns that had heavy damage last year may or may not see a problem again this year. There is no method to predict if and where grubs will occur or how severe damage may be. Thus, decisions concerning white grub management are difficult; as there is no one right answer for everyone.

Grubs are white in color, with a characteristic “C” shape body when found in the soil feeding on lawn roots. Grubs are the larval stage of beetles. The most common grub species in our area is the annual white grub; as adults, are the masked chafer beetle. The adult masked chafers begin flying in late June and lay eggs in the turf during July, which hatch in 2 to 3 weeks.

The tiny white grub larvae begin to eat the grass roots, grow rapidly and are fully grown by late August or September. Damage from annual white grubs typically starts in mid-August and may continue until early October. Damage will appear as browning of the lawn.

According to Dr. Donald Lewis, ISU Entomologist, there are three approaches to white grub management.

The first approach, in places such as golf courses and some lawns, the risk of any white grub damage is so intolerable that preventive insecticides are applied to every part of the lawn, every year. When this approach is chosen, the proper time of application is between early June and Aug. 15.

The second approach is the wait-and-see approach. Watch the lawn carefully for early signs of damage (wilting, turning brown) during August to early September when grubs could be feeding. Skunks and raccoons may tear up lawns in search of grubs, even when grub numbers are relatively low. Typically a population of about 8 to 12 grubs per square foot causes lawn damage that requires control; whereas lower populations may not damage the grass. Apply a curative insecticide only where and when needed.

The third approach is to do nothing. Depending on your tolerance for damage, comfort with pesticides and willingness to spend the cash, white grub treatments are not only expensive but hard to justify from an environmental standpoint.

If you do choose to apply insecticides, read the application directions carefully before buying. Some grub treatments are preventive and must be applied before mid-August. Others are curative and work only if the grubs are present. Know which you are getting before you buy.

Did you know? Japanese beetle grubs also occur in our area, with heaviest infestations found in Webster County, and timing very similar to the annual white grub. Adult Japanese beetles are serious defoliators of many ornamental plants. A third species, the true white grub (May or June beetle), typically has a 3-year life cycle which means it could potentially damage lawns throughout the season. Both annual white grub and Japanese beetle go through their life cycle in one year.

Horticulture Questions? Contact McCormick at yvonne@iastate.edu for information or advice.

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