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What are Grubs and How to Control Them?

By Carol Allen

Have you noticed little, white, plump “worms” in your soil? They are grubs, also known as grub worms or white grubs, and they are the larval stage of beetles. With about 24,000 different beetle species in North America, that is potentially a lot of critters in our lawns. Thankfully, only a few species are likely to be seen when we garden. In the DC Metro area, these white grubs, often referred to as “C” grubs, represent only a few of those many species. We are likely to encounter the larvae from Japanese beetles, European chafer, Asiatic garden beetle, and Oriental beetle. These are the exotic, invasive species. There are native species as well, namely masked chafers and green June beetles. Their life cycles are pretty much the same. They overwinter deep in the earth. As the soil temperatures warm up in the late winter and early spring, the grubs migrate closer to the surface. They feed on the roots of grasses and other plants during that time. Around late May, they pupate in the ground and then emerge as adults in June and July. Our plants may get a break during the late spring and early summer from the grubs, feeding damage, but the cycle starts all over again with egg laying in mid-summer. As the grubs grow, they feed on those roots again until cold weather forces them to migrate deeper into the soil in October and November. All of this may be just fine, you say, but when can we get rid of them? Their most vulnerable times are when they are closer to the surface and feeding. That would be March to early May and then again from August through October. How can you tell whether to use a chemical or biological control method? The standard is to count the number of grubs found near the surface in a square foot of soil.

If grub numbers are low, no control measures are required. Five to 10 grubs per square foot may indicate control efforts are necessary. Sampling should be done in July and August; sampling in spring would not allow you much time to implement control measures before the beetles stop feeding and pupate. There are native beetles with this life cycle. You may not wish to eliminate them from the ecosystem. If you are seeing a lot of adult Japanese or Oriental beetles, you may wish to use control measures. To deal with the adult stage, capture and flushing is often very satisfying. Beetles have the behavior of first dropping, then accelerating into flight when disturbed. You can use that behavior to your advantage. Early in the morning, when the weather is cooler and the beetles are moving slowly, hold a bowl of soapy water under the branch where there are beetles, tap sharply, and you will be rewarded with a bunch of beetles in the bowl. When you are through hunting, you can flush them down the toilet. Chemical or botanical controls can be applied to the lawn when grubs are higher in the soil profile and are feeding. Some of the biocontrol products are: Bacillus thuringiensis galleriae, Metarhizium brunneum, Paenibacillus popilliae (only for Japanese beetle control), and Heterorhabditis bacteriphora. Many other (more toxic) pesticides are available. Check the pesticide label to see if white grubs are controlled by the product and always apply according to the label directions. o

Carol Allen describes herself as a committable plant-a-holic. She has more than 25 years’ experience in the horticulture industry, with a special interest in plant pests and diseases; is a Licensed Pesticide Applicator in the state of Maryland; and is an ISACertified Arborist. She can be contacted at carolallen@erols.com. Grub image courtesy of SodSolutions.com.