Pesticide Problems? (pesticide)

Pesticide Problems? Maybe it's the Application

by Don Janssen, Extension Educator

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Property owners sometimes complain about the lack of effectiveness of various pesticides. Many times it isn't the pesticide that's the problem - it's the application. There can be a number of reasons for this lack of efficacy. Some of the common ones follow:

  • Lack of good penetration of foliage. This often is a problem when spraying for bagworms on junipers. The spray must penetrate the foliage and reach the bagworms toward the inside of the plant. High pressure commercial sprayers are able to get the spray to the insects but homeowner models are much more problematic. With pump-up sprayers, you may have to push the wand through the outer layer of foliage to reach insects toward the inside of the plant.

  • Not spraying where the insect is. Many insects and mites feed on the underside of leaves. If the plants are sprayed over the top, little to no pesticide reaches the pests. This problem is often seen with spider mites on broadleaf plants and cabbage worms on cabbage, broccoli and cauliflower.

  • Maturity of pest. Insects become much more difficult to control when they become adults. For example, Sevin does a good job of controlling young, early instar grasshoppers but is much less effective on adults.

  • Level of disease pressure. Most of our fungicides are better used as preventatives than as curatives. If a disease gets firmly established, it may be very difficult to bring it back under control. For example, chlorothalonil is effective in controlling early blight and Septoria leaf spot on tomato if used as a preventative. However, chlorothalonil will not control these diseases on badly infested plants.

  • Choosing the wrong product. Property owners often use a product because they have it on hand. However, products differ markedly in how well they control specific pests. Make sure the pest you wish to control is on the label. Unfortunately, even labeled products may vary in effectiveness. Check Extension recommendations for the recommended products.

  • High pH spray water. Certain pesticides are not stable in high or low pH water. Following are some examples.

    • Captan has a half-life of 3 hours at a pH of 7.0 but only 10 minutes at a pH of 8.0.
    • Carbaryl (Sevin) has a half-life of 24 days at pH 7.0, but only 1 day at pH 9.
    • Diazinon is most stable in pH 7 water, with a half life of 10 weeks; at pH 5, it is 2 weeks.
    • Malathion is stable at pH 5.0-7.0 but rapidly hydrolyzes in more acidic or alkaline conditions

(This resource was added July 2003 and appeared in the Lincoln Journal Star Newspaper Sunday edition. For information on reproducing this article or using any photographs or graphics, read the Terms of Use statement)

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