Lawn grubs and dead grass

I have found patches of dead grass. When digging at the edges of these areas of dead grass I found large white worms with brown heads. What are they and what should I do about them?

You have described white grubs. These are the larvae of the May beetle or June beetle. These are the light brown to honey colored beetles which fly around porch lights, street lamps, and other lights in late May and June. They also tend to make noise as they run into window screens when attracted to windows lit by interior lamps. You described their larvae and if you remember the beetles flying around last summer, that is extra confirmation that grubs are likely the problem. However, even if you don't remember them, you probably have white grub problems based on your description. You can take a sample to your local NMSU Cooperative Extension Service office to confirm that you have collected grubs.

Another confirmation is the symptom you described, patches of dead grass. While other things can cause this symptom, the presence of the grubs at the edge, where the living and dead grass meet, makes it likely that grubs are responsible. Grubs eat the roots of the grass, causing the grass to die. With the roots removed, the grass can often be lifted from the soil like a rug. That is another confirmation.

So, what should you do? At this time of the year you should do nothing. The grubs are full grown and are not feeding much. They will soon pupate and begin the metamorphosis into June beetles. Because of their size and late pupal stage (just before pupation) they cannot be easily killed by pesticides. Besides, the damage that this generation will do has already been done. Yes, dead patches of grass are becoming apparent, but that is due to grub feeding when the grubs were smaller and more actively feeding. It is only just now appearing as the grass greens in the spring and as rising temperatures create and increased need for water which cannot be supplied by the limited root system left by the grubs.

In May and June the beetles will emerge from the soil, mate, and lay eggs. These eggs will hatch in late July and August. Once the eggs have hatched, you can treat with any of several insecticides labeled for use in managing lawn white grub problems. There is even a product which has a long residual duration in the soil so that its label recommends that you apply it in June, before the eggs hatch. However, most products should not be applied until mid-August (early August in the southern part of New Mexico). Contact your county Extension agent to determine which product you wish to use. Read the label on this and any pesticide before purchasing it to be sure it will do what you want and can be used in the type of grass, or other plants, you wish to use it. Be sure to follow the directions on the label carefully to maximize the benefits of the product. If you wish to avoid the use of chemicals, there is a biological control method which employs parasitic nematodes, small worms which kill grubs. These can be mail ordered or in some cases purchased at local garden centers. Read and follow directions for the use of nematodes if you choose to use them.

The white grub so commonly written about in garden books and publications for the Mid-West and East are the larvae of the Japanese beetle. This is currently not a problem in New Mexico. Some products which are recommended for control of the Japanese beetle white grub, are not very effective in controlling the grubs common in New Mexico, so choose only products which are for white grub/June beetle control.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!