Grubworms

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Grubworms are the larval stage of insects known as May or June beetles. They are white c-shaped, worm-like pests with three pairs of legs. There are hundreds of species, but only a few species can cause damage to your lawn. Grubs typically attack the roots of flowers, vegetables and ornamentals. They are easy to identify but often go unnoticed because they live underground. To identify early signs of grub infestation, look for the following:

• Grass is brown in patches, especially during hot or dry weather
• Irregular shaped areas of weakened or dying grass

If your lawn shows signs of damage, inspect the lawn for grubs before treating to avoid needless pesticide applications. To protect local waterways, Tarrant County residents are urged to use pesticides only when needed.

How do I check for grubs? 
You can check for grubs by digging small holes, three to four inches deep in several parts of the yard. Yards with four to 10 grubs or more per square foot need chemical treatment, according to Mike Merchant, urban entomologist with Texas Cooperative Extension Service. If further inspection reveals grubs, treat your lawn as soon as possible to prevent further damage.

When is the best time to treat for grubs?
The optimal time to treat is when the larvae are less than a half-inch long in late July or early August. If white grubs are present, they will be found in the top few inches of soil. Residents should dig a soil sample that is about six inches across and four inches deep. The extension service recommends treating your lawn if more than five white grubs per square foot are found. So if you find three or more in your six-inch sample, you should treat your lawn.

What methods are available to treat grubs?
There are both chemical and biological controls available for treating white grubs. Both types work best when applied while the larvae are small.

Non-chemical controls include applying beneficial nematodes. These are microscopic round worms that attack white grubs and other soil inhabiting insects. Nematodes need damp soil to travel down into soil where the grubs are feeding. Apply at least ¼-inch of water before and another ¼-inch after spraying the nematodes on the lawn. These worms are available in stores or through garden supply catalogs.  Commercial nematodes are typically mixed with water and applied to lawns using a hose-end or hand-held sprayer. Recent research shows that under good conditions commercially-available nematodes can reduce white grubs by up to 50 percent.

The most effective insecticide applications for July are imidacloprid(Bayer Advanced Lawn Season Long Grub Control or Merit) and halofenozide (Ortho Grub-B-Gone, Mach 2). These products work best when applied before white grubs reach their final life stage. Halofenozide affects the molting process and is ineffective in grub control after they enter the last growth stage.

Chemical applications made after August 1 should include only imidacloprid or diazinon. Diazinon will not be available on the market after 2002 since it has been linked to wastewater contamination.

Residents should notice a decline in grub worms two to three weeks after application. Because some insecticides are highly toxic to birds and other wildlife, always read and follow the label directions for proper application. Never use more than the recommended quantity and only purchase the amount necessary to treat the area where the grubs are active.

Irrigation is also important when insecticides are used to treat grub worms. For dry soils, apply ¼-inch to ½-inch of water the day before applying the insecticide and another ½-inch to an inch of water after applying the insecticide. Irrigate the lawn immediately after applying a liquid insecticide formula and within 24 hours of applying a granular formula. 

Avoid runoff that will wash the pesticide off the lawn and possibly into the storm drain system or sewer system, creating water quality problems. To prevent runoff, clay soils may need to be watered several times for short periods because they do not absorb water as quickly as sandy soil.

Never apply pesticides if rain is expected because this could result in runoff into local waterways.

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