Mortality of Overwintering Eggs and Larvae of the Alfalfa Weevil in Oklahoma

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The alfalfa weevil, Hypera postica (Gyllenhal), is a univoItine species whose primary period of oviposition is during fall and winter in Oklahoma, with hatching typically occuring in late winter and spring. The objectives of this study were to estimate the total number of eggs laid per generation by the alfalfa weevil, the mortality of eggs through the winter, and reductions in numbers of newly hatched larvae resulting from lethal low temperatures. The study was conducted in alfalfa fields located at Stillwater (northern) and Chickasha (southern), OK, from 1980 to 1990. Population densities of eggs and larvae of the weevil were determined at regular intervals from October to May for each generation. Subsamples of eggs were reared for viability estimates. A summation procedure was used to calculate total eggs laid per generation. Mean total egg production per generation exceeded 900 per 0.1 m2 at each location. Mortality of eggs averaged about 30% resulting from low temperatures < −15°C). In most years, hatching began before the last occurrence of temperatures below −6°C. These temperatures killed most alfalfa top growth and most larvae that were present. Combined mortality of eggs and larvae hatching during winter resulted in an average reduction of 38% in the reproductive potential for the weevil.