Pesticide Effectiveness
Amy Jordan, Special to the Gazette
Thursday, August 6, 2009
Pest control is an often difficult but necessary part of gardening. Identifying the problem is the first hurdle, then the gardener has to decide if the problem is severe enough to warrant control.
What control to use becomes the next question, and proper timing must be determined. After working through those dilemmas and proceeding with a control, it is frustrating to find that the problem still exists.
Ward Upham, horticulturist with K-State Research and Extension, has identified several reasons for pesticide ineffectiveness. Here are some of the common reasons.
1 Lack of good foliage penetration. 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.
2 Not spraying where the insect is. Many of our 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.
3 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.
4 Level of disease pressure. Most fungicides are better used as preventatives than as curatives. If a disease gets firmly established, it may be 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.
5 Choosing the wrong product. Homeowners 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 K-State Research and Extension recommendations for products.
6 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.
If you find that the pesticide you used did not work, reexamine your actions to decide what went wrong. Information is available on pests and pest control at the Lyon County Extension office located at 2632 W. Highway 50, or online at www.ksre.ksu.edu.
ZaneRokklyn (anonymous) says...
Amy, you forgot to mention that one of the best ways to make pesticides most effective is to reduce the need for them in the first place. Infestations can be reduced by mixing crops up together (polyculture) and by including plants with insecticidal properties (such as nightshade, a common weed which cabbage worms prefer over cabbage, even though it kills them) and insectory properties (such as dill and cilantro, which attract helpful wasps that sting caterpillars, not people). Nonwoven fabric row covers can also keep the insects, birds, and rabbits off without the need for pesticides. I have had zero insect problems in my garden this year, with some 15 varieties of vegetables and no spraying.
August 8, 2009 at 10:25 a.m. ( permalink | suggest removal )