Why should losses to pests be stable despite increased inputs of pesticides? There are many reasons, with some described briefly in this section, and more detailed focus on some of these, as well as on additional reasons, in the following four sections (click natural controls , pesticide interference , resistance , or secondary pests to jump to those now, if you wish).
1) The loss rates to pests are partly artificial, in that there have been changes in consumers' cosmetic standards (we increasingly expect blemish-free produce) and in industry standards (e.g. tolerance for insect parts in canning has decreased). Hence, something may be considered a "loss" today that wouldn't have been considered such a few decades ago.
2) Many pests have developed resistance to the pesticides. This resistance means that the chemicals s are less successful at controlling the pests, which often results in the compounds being applied at higher and higher rates.
This resistance to pesticides is genetically-based, and the development of resistance in pest populations is one of the best example we have of evolution by natural selection taking place. The phenomenon is also familiar to us in medicine, where concerns about antibiotic-resistant bacteria are increasing. Resistance on the part of agricultural pests and disease agents tends to become a significant problem within 10 - 25 years of the introduction of a new compound [Science 2001]. The US National Research Council considers pesticide resistance to be a huge problem confronting US agriculture, and suggests that it will force us to reply more on alternative methods of pest control -- the "old trick" of relying heavily on chemical pesticides just won't work. It takes many years -- and MUCH money -- to develop a new pesticide....
(See article by Palumbi on humans as an evolutionary force, listed on the supplementary reading list for this unit, for more information on this topic.) You probably remember how this would work from your introductory (or other biology classes), but just in case you don't, a brief reminder. In any population of pests there is genetic variation for a large number of characteristics, including susceptibility to pesticides. Within a population (or species), there is likely to be, by chance, some individuals who are comparatively resistant to a given pesticide. Perhaps they contain some enzyme or biochemical pathway that enables them to detoxify or in some way avoid adverse effects associated with the compound.
When the population is exposed to the pesticide, those individuals that, by chance, are better able to tolerate exposure to it will be more likely to survive and leave offspring than will those that are more susceptible. Since this resistance is a genetic characteristic, it is likely to be passed on to those offspring. In the next generation (or some other time in the future) when the population is again exposed to the pesticide, a slightly higher proportion of the individuals will possess this genetic resistance and will survive the treatment, going on to reproduce in turn. Over generations of exposure (that is, of selection for resistance), the process will repeat such that more and more of the population will be resistant to the pesticide. The pesticide has acted as a strong selective agent, causing evolution towards resistance to occur in the population. (Note: It is not the case that an individual becomes resistant as a result of exposure; the process involves a genetic change -- a change in genotype frequencies -- at the population level over generations.)
(Click on "resistance " for more about the development of genetic resistance and its implications for pest control.)
3) A third reason for stable or increased losses to pests is that the use of crop rotation has decreased. Planting the same field to the same crop year after year is very important in allowing pest populations to build up.
One clear example of this effect involves corn in the US. In the 1940's, little or no insecticide was applied to corn, and losses to insects were 3.5% of production. Since then, insecticide use on corn has increased more than 1000-fold, whereas losses due to insects (especially to the corn rootworm complex) have increased to 12%. This is largely attributed to reductions in crop rotation. When corn is rotated with another crop on which corn pests (such as corn rootworms) cannot survive, the pest population declines during the non-corn years. However, when corn is grown continuously (as it is today on 20 - 40% of US acreages depending on the year and on who you read!), populations of these pests are well fed every year and can increase immensely.
(Recent evidence suggests that insects can evolve to accommodate a regular, predictable rotation e.g. alternating years in that they can, in some cases, change their life cycle to match the rotation cycle. A farmer has to be tricky!)
4) In some cases, knowledge that pesticide protection exists promotes choices that necessitate the use of the pesticides. For example, growers plant some varieties that they know are less resistant to insects or other pests, because: (1) the variety has some desirable characteristic not found in resistant varieties of the crop and (2) they figure that they can get protection from pesticides. In this case, the grower's decision about which variety to plant is made partly knowing that pesticides are there to use.
5) The increased mobility of crops and pests (introductions into new areas) increases losses to pests and the need for pest control. In fact 40% of US insect pests are nonnative, as are 40% of our weeds and 70% of our plant pathogens. There is increased concern, in fact, about the possible use by terrorists of "biological weapons" against crops....
One example of this effect involves the introduction of crops to new areas. The commercial potato is native to Bolivia and Peru. In the US, where potatoes historically did not exist, there was an insect that fed on native Solanums (potato relatives). When the potato was introduced into the US, this beetle (now called the Colorado potato beetle) spread quickly to the introduced potato and is now the most serious insect pest of potatoes in the world. The potato had never been exposed to the beetle before, and had no resistance to it.
Another example of pest problems associated with mobility of plants related to the current global crisis in imported weeds. There are many clear examples of the problems created by exotic (nonnative) weeds in the western US; any of you from grazing country will be familiar with one or all of the following problem species: cheat grass, yellow star thistle, spotted knap weed, medusa head. These are all introduced plants that are aggressively invading western rangelands and/or crop fields. Such invasive species are one of the major and increasingly important threats to native biodiversity in the US. (I have quite a collection of papers on this topic; let me know if you are interested in learning more about it!) In 2001, invasive species were listed as the second most important threat to native species in the US (habitat loss was the leading threat).
A very recent example of pest problems exacerbated by the increased mobility of plants and their pathogens is important in our region (the PNW). This involves the resurgence of late blight disease of potato and tomato. This disease is caused by a fungus (Phytophthora infestans), and is infamous for being responsible for the great Irish potato famine of the 1840's. The disease was successfully controlled in the US (and most of the world) via a combination of plant resistance, fungicides, cultural controls, and careful screening for disease-free tubers. However, it reappeared in Europe in the early 1980's, and by the late 1980's and early 1990's was causing severe problems in North America as well. For example, in 1995, the disease was rampant in potato growing areas of the Columbia Basin in OR and WA, where it affected 66,000 hectares. The increased costs of attempting to control the problem were estimated to be $30 million! The resurgence is attributable to several factors, one of the most important of which is introduction of a new, virulent, and fungicide-resistant strain of the fungus from Mexico into other potato and potato growing areas of the world. (See June 1997 BioScience 47(6):363-371 for a detailed analysis of this threat and what it indicates for other crops.)The recent emergence of "sudden oak death," caused by an introduced fungus, Phytophthora ramorum, in some forests of northern California and southern Oregon, and in some nursery crops from these regions, is yet another example of the challenges posed by introduced species.
5) The use of reduced tillage (conservation till ) to control soil erosion in some circumstances can allow pests to build up on crop residues, instead of being plowed under, and also gives weeds a chance. This practice is partially responsible, as we already saw, for increases in herbicide use in US agriculture.
6) In addition, the reductions in crop genetic diversity (at local and landscape levels) associated with the green revolution aggravates pest problems and requires greater pesticide inputs for a given level of pest control.
Recall that the green revolution hybrid crop varieties are often more genetically uniform than were traditional crops, including uniform in responses to pests and diseases. While, historically, pest impacts were often reduced somewhat by genetic heterogeneity on the part of the crop plants (some plants within a field were more resistant than others), modern agriculture has reduced this controlling factor by planting more homogeneous varieties. (Research at OSU is examining whether planting mixtures of wheat varieties in a field, rather than planting entire fields to one variety, can reduce the losses associated with rust fungi, and the answer seems to be "Yes.")
In addition, as we saw earlier, with the advent of the green revolution, growers increasingly planted only certain crop varieties, increasing the proportion of fields that were all planted to the same variety. (That is, to plant monocultures of one crop over wide areas.) We saw the impact that this had on US corn production in the early 1970's, when there was an outbreak of southern corn leaf blight. We can think of the effect of monocultures on pests in terms of apparency -- plants are probably most apparent to pests when they are grown in monoculture. It is simply easier for pests to find the plants and to move from one to another when they are planted in monoculture over a large area, which they are in modern agriculture, as opposed to some traditional agricultural systems that used mixed species plantings -- or at least a selection of different varieties on the landscape level.
7) Another challenge -- and reason for needing to apply more and more pesticide just to stay in the same place in terms of loosed to pests -- is associated with the emergence of "secondary pests." These are creatures that were previously unimportant as pests, but become so when the system is altered by the use of pesticides. These may be insensitive to the particular pesticide being used or may develop resistance to it more rapidly than the target pest. These attributes may give such "secondary pests" a competitive advantage over the target pests, which outcompeted them in the past. Spider mites, as well as several species of scale insects and of aphids are examples of such secondary pests.
To understand more about why pesticides are so heavily used in modern agriculture, let's look at how pest control occurs in natural ecosystems (">>" at the bottom of the page). ("Navigate " for information on moving within and among these documents.)
Page maintained by Patricia Muir at Oregon State University. Last updated Oct. 23 2012.