Widespread, heavy damage to lawns infested with white grubs last fall has heightened concern over this periodic pest. This has resulted in increased demand for preventive control and questionable sales tactics by lawn care companies.
Part of the confusion this spring originates with a relatively new insecticide for white grubs. Imidacloprid, available to professional applicators as Merit, has been on the market for several years for the control of white grubs in turfgrass. Compared to other white grub insecticides, it is relatively low in toxicity, high in cost, and long lasting. Research has shown that imidacloprid applied in April or May will persist into August and control annual white grubs 4 or more months later. The benefits of this persistence can be tremendous. For example, golf course greenskeepers have found that they can treat for black turfgrass Ataenius in May and not have to retreat for Ataenius and annual white grubs in July or August.
A new formulation of imidacloprid available this year is an impregnated fertilizer available to commercial applicators. Some lawn care companies have been aggressively marketing spring-applied grub control. A 0.25 percent imidacloprid granule is being marketed to homeowners in similar fashion. Scott's GrubEx is one such product; others may appear. While application of imidacloprid several months in advance of when it is needed for white grub control is legal (labels state that one application can be made from April to August), it may not be prudent. Although the longevity of imidacloprid is consistent, relying on it unnecessarily is not recommended. Just because you can apply it early does not mean you should.
Early application of any pesticide increases the chances of breakdown or loss leading to reduced control at a later date. Although this does not appear to be an important factor with imidacloprid, widespread use on many soil types and under varying environmental conditions may expose any vulnerability that it may have.
The application of an insecticide months before it is needed may make good profit sense, but it makes little sense ecologically. In the months-long interval between spring application and mid-summer grub emergence, a persistent insecticide will be killing other insects in the turf. Some are beneficial and many are important to maintaining a stable ecosystem and a healthy lawn. Their loss may result in impacts that we do not fully appreciate at this time. For example, there is some evidence that ants are an important predator of white grub eggs and that eliminating ants from a lawn in mid-summer may cause higher grub populations.
White grub populations vary greatly from place to place and from year to year. Preventive applications, by their nature, can lead to pesticide overuse. An IPM program that identifies and treats only when pest populations are high enough to cause intolerable damage or when conditions are likely to lead to severe damage results in lower pesticide use and less impact on the environment. Waiting until July to apply imidacloprid allows time to make a reasonable prediction as to whether or not it is needed. Abundant adult masked chafers in early July coupled with dry weather will increase the chances of high populations and severe damage. Insecticide application to areas that experienced severe damage in the past is reasonable under those circumstances. In summary, imidacloprid is one of several excellent white grub control products on the market. Its springtime use is discouraged except on golf courses and other areas where black turfgrass Ataenius is a problem. Application should be delayed until July on home lawns and other turf areas where past experience, adult flight and turf growing conditions indicate that a damaging white grub infestation is likely.
This article originally appeared in the May 3, 1996 issue, pp. 68-69.