Source: Andow, D.A. 1983. The effect of plant diversity on insect populations. p. 1-41. In Plant diversity and insect populations: Interactions among beans, weeds and insects. Ph.D. dissertation. Cornell University, Ithaca, NY.

The intercropping of plants has long been known to provide a variety of benefits compared to a monoculture system of agriculture. One of these benefits is protection against insect attack, which has been attributed to the diversity of crops grown in an area. Predator and parasite populations play key roles in preventing insect attacks. Their populations increase in diversified systems because of a more favorable microclimate and the presence of alternative prey. Diversified systems are more favorable to predators and parasites for three reasons. First, they have better spatial distribution of nectar and pollen sources, which attracts natural enemies and increases their reproductive potential. Second, diversified systems have more microhabitats to provide the requirements of individual predators and parasites. Third, diversified systems tend to have more species of herbivorous insects that provide alternative prey when other sources are scarce or at inappropriate stages of their life cycles. There were four lines of evidence that led to the conclusion that diversified systems have fewer pest problems. The first line of evidence was derived from biological control investigations during the late 1800s. Fields with low numbers of pests were located and studied to figure out why these crops were being protected against attack. Observations from historical literature and existing practices of intercropping were the second line of evidence. Japanese farm manuals from the 1600s and earlier accounts from both the Chinese and Koreans indicate that they saw low insect protection in fields that were intercropped. The third line of evidence, called the "Balance of Nature", can be seen in diversified forest ecosystems. Diversified forests are less subject to pest outbreaks. Trees in a plantation setting can become subjected to attack, but that same species in a diversified forest setting will not be attacked. The fourth line of evidence was found in undisturbed natural systems. In these systems, natural enemies regulate the population density of insect pests. Some generalizations emerge from these lines of evidence. Pest populations increased more frequently in perennial systems than annual ones and also in systems with high resource concentrations. The herbivore's host finding behavior, altered microhabitat, host quality, vegetational structures, and natural enemies also influence these pest population increases. Monophagous insects, those that feed on one plant species, have lower population numbers in diversified systems than polyphagous insects. By far, the best control occurs when the food resources of natural enemies are evenly distributed in space and time. This prevents a rise and fall pattern in both the pest and natural enemy populations, which often leads to pest attacks. Understanding of what contributes to pest insect outbreaks is slowly being developed. Three known factors are resource concentration, presence or absence of natural enemies, and level of diversification. The knowledge gained thus far is being applied in developing a biological means of pest control.

Abstract author: Carrie A. McGuire, 19 November 2000.

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