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June 7, 2000

On June 8, USEPA is scheduled to release the final risk assessment on the insecticide chlorpyrifos, the active ingredient in Dursban, Lorsban, and several other trade names. Newspaper reports are suggesting that an agreement with the manufacturer, Dow, may eliminate over-the-counter sales of the product and severely restrict its labeled uses. Apparently, the accepted residue will be below the effective rate of the insecticide in at least some uses.

In all likelihood, any loss of labeling will result in a halt in the production of new product at a stated time in the future, with the allowed sale of stock that is in the avenues of trade. In other words, over-the-counter Dursban will likely be available for at least several months. USEPA typically only issues a stop-sale requirement if new, unusual, and severely hazardous properties are discovered about a pesticide, and there are no indications of that being the case.

One of the more important uses that commercial horticulturists have for chlorpyrifos is in the protection of trees and shrubs from borer attack. It typically lasts as an effective residue on shaded, furrowed bark for about a month and correspondingly shorter lengths of time on smooth, more exposed bark. Other insecticides provide borer protection but it would be shorter lived, resulting in more frequent applications. Other uses for chlorpyrifos in the landscape for turf grub control and chewing insect control on leaves are met as well or better by other insecticides.

In the control of structural pests, chlorpyrifos is the favorite in the Midwest for conventional termite control. There are, however, effective alternatives available. Chlorpyrifos is also formulated in a varnish base as Killmaster II. Being in a varnish base, it is very useful in commercial kitchen applications along baseboards where it is not washed away by daily mopping. In recent years, very effective baits have been developed to fill this need, at least in cockroach control. The release of the risk assessment on June 8 should answer many questions about the severity of use restrictions.

Author: Phil Nixon


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