Issue: September 4, 2004

Do I have lawn grubs?

I hope you can give us some advice on dealing with grubs in our lawns. We are the Renaissance Homeowners Association, and our groundskeeper has pointed out a number of brown spots on the neighborhood lawns that he claims are caused by grubs. Is there a way we can be certain that this is our problem and know how to deal with it? We are averse to using highly toxic chemicals if it can be avoided. I hope you can advise us about this. We will appreciate your guidance.

Robert F.
Albuquerque

To determine if grubs are indeed present, cut across the boundary between the brown and the green grass with a shovel, making an angle between two cuts. This allows you to lift the sod and look at the roots and crown (where leaves and roots meet) of the grass plant. If grubs are present, you will find some dingy-white, fat grubs in the crown or root zone of the grass. If the grubs are causing major problems, the grass may lift like a rug from a floor - its roots have been eaten by the grubs and there is nothing holding it to the soil. If you do not see any grubs present, check in a few other places. If there are still no grubs found, take a sample of grass from the boundary area of the damage (some brown, some still healthy) to your local County Extension Office where the Extension Agent can look at the sample and, if necessary, send it to the Extension Plant Pathologist in Las Cruces to determine if fungal disease is causing the problem.

Before taking in a sample, however, be sure that the brown areas are getting the same amount of water as other areas of the lawn. Do this by placing empty jars or cans on the lawn while watering. Place some in healthy areas and some in brown areas. If they catch equal amounts of water in both areas, irrigation is uniform and probably not the problem. If there is less water collected in the brown areas, adjusting the irrigation system to provide uniform coverage may be necessary (look for clogged sprinklers, turned sprinkler heads, or incorrect replacement sprinklers).

If grubs are the problem, there are insecticides labeled for controlling grubs. If used properly, these should not be a problem, but keep children and pets away from direct contact with the treated grass for a couple of days. There is an organic/biological treatment using parasitic nematodes to reduce the grub infestation. Nematodes are very small (almost microscopic) round worms that feed on insects. You can purchase the nematodes from insectaries that raise these biological control agents. The nematodes will be shipped to you in insulated, cool containers. They must be applied as soon as possible after receiving them. Early morning application to a recently watered lawn is best. After application (by watering can or sprayer) irrigate briefly to wash the nematodes from the grass blades to the crown and root zone of the grass. If you must delay a few days to wait for cooler weather or other reasons, the nematodes must be refrigerated (not frozen) until use. Do not store under refrigeration more than a few days.

Treatment is most effective in August and early September while the grubs are small and more easily managed. Later treatment is less effective since the grubs will cease feeding and migrate more deeply into the soil for the winter. In the spring, they may feed briefly but will quickly form pupae and begin their metamorphosis from grubs to june beetles. Treatment is not effective once they form the pupal stage. If this problem persists, an insecticide analog of nicotine (chemical name Imidacloprid) may be applied in late June to be present as soon as the eggs hatch. This is a chemical insecticide, but it has low toxicity to mammals (people/pets). It has a three-month residual in the soil so, if applied properly, it is waiting for the grubs when they hatch.

Whether you use biological, organic, or chemical control measures be sure to read, understand, and follow all label directions to maximize both effectiveness and safety of the treatment.

Send gardening questions to Southwest Yard and Garden, Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu or at https://www.facebook.com/DesertBloomsNM/. Please include your county Extension Agent (aces.nmsu.edu/county) and your county of residence with your question!

For more gardening information, visit the NMSU Extension Horticulture page: desertblooms@nmsu.edu.

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, office: 505-865-7340, ext. 113.