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FIGURE 4 Crop phenology and pest phenology: relationship between the phenology of soybean in the midwestern United States and three of its most common insect pests, the bean leaf beetle, C. trifurcata (Coleoptera: Chrysomelidae); the green stink bug, Acrosternum hilare (Hemiptera: Pentatomidae); and the green cloverworm, Hypena scabra (Lepidoptera: Noctuidae).

FIGURE 4 Crop phenology and pest phenology: relationship between the phenology of soybean in the midwestern United States and three of its most common insect pests, the bean leaf beetle, C. trifurcata (Coleoptera: Chrysomelidae); the green stink bug, Acrosternum hilare (Hemiptera: Pentatomidae); and the green cloverworm, Hypena scabra (Lepidoptera: Noctuidae).

example, produces 100 viable eggs, half of which yield females that in turn will lay 100 eggs and so on for 25 possible generations in one year; by the end of the year, the 25th generation would contain 1.192 X 1041 flies, which, if packed tightly together, 60,000 to a liter, would form a ball of flies 155 million km in diameter or a ball extending approximately from the earth to the sun. Obviously, such unlimited population growth does not occur in nature. Normally, populations are regulated by the combined actions of both physical (or abiotic) and biological (or biotic) factors of the environment. An understanding of the mortality factors that help regulate insect populations is one of the most active areas of research in agricultural entomology.

The set of species coexisting in an area and interacting to varying degrees form what is known as an ecological community. In a crop community, the crop plants and the weeds that persist within the crop field or grow along the borders are the primary producers. The animals within the crop community maintain dynamic trophic relationships: some feed on living plants, others on the decaying plants, and still others on animals. Those that feed on the plants are the herbivores, or primary consumers. Pests are primary consumers on the crop plants. Parasitoids and predators are the secondary consumers. Those that feed on the pests are beneficial natural enemies. Finally, decomposers and detritivores feed on decaying organic matter. All biotic components of the com munity are interconnected by "food webs." An understanding of food webs and trophic interactions in crop communities is important because it provides a basis for interpreting the nature of disturbances in crop ecosystems. Disturbances in trophic relations may lead to outbreaks of pest organisms and the need for control actions.

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