Phenology of Field Populations of Corn Rootworms (Coleoptera: Chrysomelidae) Relative to Calendar Date and Heat Units

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Abstract
Seasonal occurrence of corn rootworm, Diabrotica virgifera virgifera LeConte and D. barberi (Smith and Lawrence), larvae and pupae was monitored in a total of eight corn-planting dates from 1980 to 1982, while adult emergence was characterized in a total of 11 planting dates from 1979 to 1982. Delayed planting was associated with a proportionate delay in occurrence of larvae, pupae, and adults. After adjusting for the effects of corn-planting date, population phenologies were described as functions of calendar date and heat-unit (HU) accumulations. The first predictable event was peak occurrence of second instars. Seasonal occurrence of third instars, pupae, and adults appeared equally consistent and predictable with respect to calendar date and soil HU (base, 11°C). Models based on air HU (base, 16°C) were judged to have little applicability. Simple linear models for predicting the occurrence of larvae, pupae, and adults based on calendar date or soil HU are presented. The similarities in predictability between the calendar date and soil HU models indicated that relating phenology to Julian date may provide the simplest and best estimates of the seasonal occurrence of corn rootworms. Quantifying the effects of other factors that directly or indirectly affect soil temperature and development of corn rootworms (e.g., soil type, moisture, and tillage) might increase the utility of soil-HU models.