The life cycle of different insect species varies greatly, although all insects undergo the basic stages of development from egg to reproductive adult (or imago). Depending on the length of the life cycle, there is considerable variation in the number of generations per year, a phenomenon called voltinism. A uni-voltine species has one generation per year; a multivoltine species may have many generations per year. The range of variation in the Insecta is evident when one considers that the 17-year periodical cicada has one generation every 17 years, whereas whiteflies or mosquitoes may complete a generation in about 21 days. Under temperate climate conditions, generations often are discrete, but under warmer subtropical conditions they frequently overlap. The definition of temporal periodicity in an organism's developmental cycle is called phenology. The relationship between the phenology of the crop and the phenologies of its various pests is of interest in agricultural entomology. Figure 4 shows an example of such a relationship for soybean grown under conditions typical for the midwestern United States.

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