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Alfalfa fields in two climatically different regions were sampled for aphids and their natural enemies throughout 3 years (1957-1959). The fields were under an integrated control program and therefore received minimum amounts of insecticides. The four imported parasites of the spotted alfalfa aphid and the pea aphid were not yet important in the study area, so predators were mainly responsible for the high degree of naturally occurring biological control. By pooling the data of entire regions, it was possible to follow changes in populations of these mobile predators more accurately than is possible from surveys of single fields because short-distance migration resulting from harvesting practices as well as uneven distribution could be neglected. Coccinellids and hemerobiids, two oligophagous groups, were closely linked in a density-dependent fashion to the aphid populations. This is a good indication that these predators (especially the lady beetles) are able to control the aphids. The more polyphagous predators, the chrysopidae and Hemiptera, become important in the control of the aphids only when the dominant coccinellids are inactive because of diapause or when lack of aphids for reproduction prompts the beetles to leave the fields. When aphid development exceeds the lady beetles’ momentary capacity for increase, however, syrphids are able to take a large toll. Furthermore, several species having similar biologies were found to replace each other throughout the year. Finally, the phenologies of the species could be explained by their biologies (diapause, temperature thresholds, ovipostional behavior). |
