Issue: March 19, 2005

White grubs in lawn

Question:

In the last two years, the pesky grubs have all but ruined my showcase 3-in-1 fescue lawn. One turf expert and reputable friend suggested that I start the following treatment in March: Diazinon or Dursban once a month for 4 months, Milky spore once a month for 4 months, and Oftanol once a month for 4 months. He tells me that all this will take effect in two years.

Actually, I don't want to kill the whole world - I just want to kill these darn worms. What is your opinion?

I recall a release you wrote not too long ago. It recommended starting treatment in June. Isn't this a little too late? As I recall, by June my lawn was turning rather yellow and on its way to disaster. At first, I kept adding water (flood irrigating) and ammonia trying to save my lawn, until I dug on the circumference as you suggested and found these creepy-looking things. The lawn returned to normal, of course, as weather cooled off.

Willie L.
Las Cruces, NM

Answer:

I think the advice you have is overkill and improperly timed.

The June beetles lay their eggs in June and the eggs hatch into small (easily killed) larvae in August. By late autumn, the grubs have become larger and more difficult to kill. Their feeding damage occurs in the fall and a little in the early spring. The effect of the fall and spring damage becomes apparent in May or June. Treating in the spring may or may not kill them, but it won't prevent the appearance of damage already done. In the spring, after feeding only a little, the grubs burrow into the soil and form pupae. At this time, pesticides will have no impact on them. (It may damage other things.)

The proper time to treat depends on the product used. Imidacloprid (the active ingredient in several insecticide products) is safer to use than many other products. (It is still toxic, has a three month residual, and will kill the young grubs.) It should be applied in late June so that it is present when the eggs hatch and the larvae start feeding. Other products such as Oftanol¨ must be applied in early- to-mid August as the eggs are hatching. It is a good idea to monitor the effectiveness of your treatments by checking for healthy grubs in mid-or-late August. If they are lethargic and discolored, they are dying. If their dirty white color is normal, you may need to treat again.

Milky spore is not effective against the grubs we have in New Mexico, so is not recommended. (The Japanese beetle, against which milky spore works, has appeared in Albuquerque but is not yet a major problem.) Diazinon¨ and Dursban¨ are no longer on the market.

If you treat successfully this year, the benefits will be apparent next summer, but you may need to treat again.

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Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.