Do pesticides interfere with any of these natural controls ? Let's look at whether and how each of those natural controls is affected by pesticide use. See also the section on negative feedbacks for further elucidation.
1) Genetic resistance of plants to insects and diseases is probably not directly affected by pesticides except that pesticides remove pressures for plants to evolve resistance to the insects and diseases -- pressure that would be present in natural populations or in the absence of pesticides. The knowledge that pesticide controls exist also may decrease somewhat the pressure on plant breeders to emphasize pest resistance, as they know that growers can rely on pesticides for some of the control.
When thinking about how pesticides interfere with some of the other natural controls, it is important for you to know that it is estimated that less than 1% of pesticides applied actually hit the target organisms. Most reaches nontarget sectors of the agroecosystem or spreads to surrounding ecosystems. While federal and state lawa regulate drift during and immediately after application, many pesticides volatilize and become prone to drift 8 - 24 hours after application [Orion 2008].
2) There can be direct effects of pesticides on predators, competitors, and diseases of the pests. These organisms naturally work to control pest outbreaks. In fact 50-90% of pest control in agroecosystems is actually accomplished by these predators, competitors, and diseases of pests (depending on who you read).
For example, the ladybugs that eat aphids in your garden may be sensitive to the pesticide that you use to control the aphids.
As another example, apple growers using a fungicide to control apple scab, which is caused by a fungus, often noticed increased outbreaks of the disease after spraying. It turned out that the spray was toxic to earthworms. Why should that matter? Well, the fungus overwinters on fallen leaves and fallen apples, and then produces spores in the spring that re-infect the tree. Earthworms normally removed the infected leaves and apples when they fell to the ground, reducing the spores that could cause new infections, but when the worms were poisoned, this natural control ceased to be effective!
There are many documented cases like this, in which beneficial natural enemies are inadvertently harmed by pesticides, with resulting pest outbreaks; cottonboll worm, cotton aphid, spidermites, and many apple pests are but a few examples. In some cases the effect would have been difficult to predict without clear knowledge of the ecology of the pest in the system. For example, fungicides may result in insect attacks in cases where a fungus was keeping an insect under control.
(3) There can also be indirect effects of pesticides on predators of the pest, in that the food supply of the predators (that is, the pests!) declines because of pests being killed by pesticides. In this case, populations of the predators will decline as well, and so will be less able to control the pest, particularly when the pesticide treatment terminates. Time and again this is seen; a temporary suppression of the pest when the pesticide being used, but then immediately after treatment stops, pests re-bound to higher than previous levels because populations of their predators were decreased as well, and so are less able to control the pest. (See case studies from Indonesia and Bali.)
An important lesson is that predators and parasites of the target pest may suffer as great or greater mortality than the target, especially if ecological relationships and sensitivities not well understood.
4) Host availability. Most simply, epidemics in nature subside when the pests eat or kill so much of the host population that they starve themselves, essentially (or can no longer find homes). (In fact, few epidemics in nature go this far, as the other natural control agents are usually effective before this point, but in the final analysis, host availability will stop an epidemic if all else fails.) Use of pesticides keeps this from happening; under protection from pesticides, hosts are kept alive so surviving pests have a steady supply of host available. (Naturally I'm not suggesting that a farmer can afford to let a crop be eliminated to starve the pest! I'm just being comprehensive in looking at how pesticides interact with natural pest controls.)All of these reflect aspects of natural controls that maintain most natural ecosystems in a state of reasonbale stability, pest-wise; pesticides can interfere with all of these.
The following section (">>" at the bottom of the page) discusses the problem of pest resistance to pesticides, while later sections amplify the discussion of interaction between pesticides and natural controls (negative feedbacks ), discuss the problem of "secondary pests," provide some case studies , and discuss reasons for concern about pesticide use. Click "Navigate" for reminders on how to move within and among these documents.