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Populations of the alfalfa caterpillar, Colias philodice eurytheme Boisduval, reach very high numbers during the more favorable summer months in California. This study was conducted to determine the important factors affecting egg production and oviposition, and their corresponding effects on population trends of this insect. The reproductive system of the female does not differ greatly from those of other Lepidoptera with two genital apertures. Mating usually occurs a few hours after emergence, and involves the passage of a spermatophore which is formed in the female from the glandular secretions of the male.Oogenesis is largely controlled by temperature although some sugar is also required for the maturation of a large number of eggs. Adult food is more important in its effect on oogenesis than on oviposition. Temperature is generally more critical than humidity, light intensity, or food in controlling the oviposition rates of Colias philodice eurysbeme, The optimum body temperature for high rates of oviposition appears to be 32°C. The upper and lower thermal limits of body temperature for oviposition are 46°C and 21°C, respectively. The optimum humidity for a high oviposition rate ranges from 26 to 45 per cent. At the higher and lower temperatures, any effect that humidity may have on oviposition is masked by the effects' of temperature.A female may lay as many as 1,172 eggs, but the average is about 715. Under field conditions, these numbers are probably seldom realized. The sex ratio usually favors the male.' In order to determine the reproductive potential of a population, the age, fat body content, and number of matings of various classes of the population must be determined by dissection.

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