Monday, November 20, 2006

Lawn Challenge Course - Lesson 7

Here is Lesson 7 - "White Grub Problems in Lawns" in our Lawn Challenge Series from the University of Illinois. terry

White Grub Problems in Lawns

Whitish, "C"-shaped grubs can be a serious lawn pest.

White grubs are the most serious and destructive lawn insect pest in Illinois. While not all lawns will get grubs and the extent of grub damage varies from year to year, there are some important points to consider concerning managing grubs in lawns. 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, of which the adult is a tan chafer beetle. Eggs are laid in the soil in mid-summer, primarily on well-watered lawns in full sun, often near pavement. Damage from annual white grubs typically starts in mid August and may continue until early October. Other species may damage lawns in northern Illinois, but usually are not as common as annual white grub. Monitoring and control of these species is the same as for annual white grub. The true white grub (May or June beetle), for example, typically has a three-year life cycle, meaning it could potentially damage lawns throughout the season. Japanese beetle grubs can also occur in northern Illinois, with timing very similar to annual white grub. Adult Japanese beetles are serious defoliators of many ornamental plants.

Adult beetles, such as this Japanese beetle, lay eggs that hatch into white grubs.

Since grubs feed on the roots of lawn grasses, damage will appear as browning of the lawn. Consider that this also could be due to problems such as drought, poor soil, or diseases. However, grubs are easy to find by lifting sod in damaged areas and checking the root zone for the whitish grubs. Don't treat for grubs that don't exist! 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, they may attract skunks and raccoons.

Check the root zone of the edges of browning areas for the presence of white grubs.

Lawns showing damage from grubs may be treated with an insecticide. Insecticides available for homeowners include diazinon (25% EC [liquid] or 5% granular); trichlorfon (Dylox) (6.2% granular); bendiocarb (Intercept), halofenozide (GrubBGon, GrubEx), or imidacloprid (Merit, formerly GrubEx) for control of white grubs. Heterorhabditis bacteriophora nematode is an example of an alternative product for white grub control that is available. For all products, read and follow all label directions, then apply to damaged areas. Water the insecticide into the soil immediately. If treating a large area, stop after a portion has been treated and water the material in, then complete the rest of the lawn area needing treatment. Only treat in and around affected areas; grubs may only be in a small part of the lawn. Imidacloprid and halofenozide are suggested to be applied before grub damage appears. An example of a way to use these products would be to apply in July to irrigated lawns that are surrounded by dry lawns, especially when adult beetle flight is high in areas with a history of grub damage.

Spring treatment for annual white grub is not suggested since the grubs feed for a short period of time in spring and are reaching maturity, thus are not controlled easily. In addition, turfgrasses are actively growing at that time so usually don’t show damage.

Other insects may attack lawns in northern Illinois but severity of damage changes from season to season and also by location. Examples include sod webworm, billbug, chinch bug, and aphids. These insects differ from grubs in that they are feeding at or above the surface of the soil and usually are not as destructive.

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