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Martha Maletta, Home Horticultural Consultant
Dated - November 12, 2004
It’s finally time to finish garden cleanup after a much later than average first frost. Time, too, to evaluate the season’s successes and not-so-successes with an eye toward next year. If pesticides – for disease, insect or weed control – were used but with disappointing results, consider the many reasons that a product may “fail”.
Very possibly the problem was not one that could be controlled with a pesticide. Accurate diagnosis is the first and most critical step in solving a plant or garden problem. In fact, most plant problems are not due to pests or diseases but to any number of cultural and environmental causes.
The pesticide may not have been the right one for the job. For example, lawn weeds vary considerably in their susceptibility to different herbicide materials. Successful weed control depends on weed identification and careful choice of weed control product. Not all insecticides are equally effective against every pest. Bacillus thuringiensis, a biological control agent, is effective against caterpillars (larvae of moths and butterflies) but not against sawfly larvae that strongly resemble caterpillars.
The pesticide may have not been applied at the right time. In many cases insecticides must be used when the target pest is in its most vulnerable stage. For example, lawn grubs are best controlled during late summer when they are young. Scale insects are generally treated in the dormant season and when the young scales hatch, time varying with species. Some broadleaf weeds are more successfully treated in fall rather than spring. Diseases control materials are usually applied preventively, before symptoms appear.
Perhaps the pesticide was not used at the right rate. Using too little material won’t do the job; using too much may be harmful and is illegal. The application may have been inadequate. Thorough coverage of plants, including undersides of leaves, to the point of run-off is usually needed for insect and disease control. This is especially true of some of the low toxicity products like insecticidal soap and horticultural oil. Coverage can be a challenge with the equipment that many home gardeners use, especially if treating large, dense plants.
Adverse weather, such as a heavy rain, after application may necessitate retreatment. Wind can affect coverage, another reason, besides safety, to spray when it is calm. Pesticides may fail because they are old or have not been stored properly. In general, liquid products should not be exposed to extreme temperatures. Pesticides should not be mixed until just before use because many loose effectiveness rapidly once mixed with water. Mixing different pesticides together may cause problems; labels usually warm about incompatible mixtures.
Some diseases and insects have become, overtime, resistant to certain types of pesticides. The Colorado potato beetle is a notorious example of a pest that rapidly develops resistance to insecticides.
No pesticide is likely to totally eradicate a given problem, so having realistic expectations about effectiveness is important to “success”. Understanding the level of threat posed by a given problem and the level of damage a plant can sustain without real harm can help in deciding if treatment was, in fact, needed.
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