One of the best explanations of how pesticides can alter the environment to tip the scale in favor of the pest, comes from a biological control book written by DeBach (1974). Imagine a simplified system where the population of a pest is kept at equilibrium ("in balance" or at a steady level) by a single natural enemy such as a parasite or a predator. Let's say there are 200 adults of this pest of which 50% are females (100 females), and each produces 20 eggs (a total of 2,000 eggs). Now say that the beneficials normally control 90% of the pest population between the egg and pupae stage so that there are also 200 adults in the next generation (if 10% are left, then 10 x 2,000 eggs = 200 adults). The pest population is kept in equilibrium by the beneficial.
Of course, this is an oversimplification. In actuality, the pest population continues to fluctuate slightly below or above a steady number (equilibrium), but this simple version will do for our example.
Now, let's suppose a pesticide application lowered the efficiency of the beneficial by just 10%, possibly by killing some off, making them sick or temporarily repelling them from the field. If only 80% of the 2,000 pest eggs are killed off, then 400 adults survive to lay 4,000 eggs (if 20% survive, then .20 x 2,000 equals 400 adults or 200 females which lay 20 eggs each equals 4,000 eggs). So by setting back the beneficials just a little, the pest population doubles in the next generation.
You are probably thinking that the pesticide would have killed off some of the pest population to keep it from doubling in a single generation. For the sake of our simplified example, let us assume that the pesticide was put on to control some other pest and is not effective on the one in our example. This would be the case for a secondary pest which is released from its normal population controls (biological) by a pesticide application and becomes so abundant that it starts to damage the crop. Or, maybe the pest in our example is totally resistant to the chemical, so that the population does double in a single generation.
Now, if the pesticide application has a short residual period (does not last long) and is not repeated, then our beneficial might recover and control 90% of the pest population in the next generation (if 10% survive, then .10 x 4,000 eggs equals 400 adults). The pest population has now established a new equilibrium, twice as high as it used to be!
What happens if the pesticide has a long residual period or if repeated applications are made which effect the next generation? Then our beneficial may continue to kill only 80% of the pest population (if 20% survive, then .20 x 4,000 eggs equals 800 adults or 400 females which will produce 8,000 eggs). Our pest population doubles again! In just two generations, we now have four times as many pest individuals. What happens if we wipe out all the beneficials with our broad-spectrum pesticide, not just reduce their efficiency a little? Let us hope we are dealing with a pest that does not have many generations in one year, especially if we plan to continue to use this pesticide.
Can things get worse? Think about the lowly aphid. She skips the egg stage, producing up to 70 live young, all of which are females. The 70 new aphids mature and they start reproducing in as little as a week, having many generations each year. And, they may all be totally resistant to all pesticides! Now think of the size of the problem you have created!
Natural controls are effective only if you do not interfere. The proof that they are effective is that animal populations usually do not spiral up out of control. Most animals, insects or potential pests exist at a population equilibrium. Otherwise, we would be knee-deep in every type of insect!
Aphids, mites, thrips, whiteflies and leafminers are all examples of pesticide-induced secondary pests. DeBach argues that many plant bugs now considered major pests are caterpillars such as cabbage looper, beet armyworm, fall armyworm and corn earworm. They have risen to this status only because of area- or continent- wide pesticide use. He argues that they were never important pests before the wide-scale use of pesticides targeted at other insects, such as the boll weevil on cotton.
Not all costs associated with using a pesticide are reflected in the price on the jug when you purchase the product. Some associated costs include new secondary pest problems, regulatory expenses, crop residues, health-related risks (especially to the applicator), ground and surface water pollution, and land devaluation because of real or perceived hazards. Sometimes the benefits are not worth the cost, and you may go on paying for much longer than expected. The solution is to choose an environmentally friendly, selective insecticide that is "soft" on the beneficials and does not come with a lot of hidden costs.
Another option is to use alternative management techniques, if available.
It is time to give up that old 1957 battlewagon-of-a-pesticide that kills almost everything on the planet. We need most of those organisms. It is the dawn of a new millennium; time to testdrive a new model. There are lots of new selective materials available.
References:
DeBach, P. 1974. Biological Control by Natural Enemies. Cambridge
University Press.
Written by Jude
Boucher, Extension
Educator - Vegetable Crops IPM Coordinator, University of Connecticut
Originally published in YANKEE
GROWER MAY/JUNE 1999
p.14
This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
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