Temperature dependent models to predict regional differences in corn rootworm (Coleoptera: Chrysomelidae) phenology

Western corn rootworms, Diabrotica virgifera virgifera LeConte, only recently have become a serious corn pest in New York and other states in the Northeast. Because soil conditions, topography, and climate differ from the Midwest corn belt, questions arose as to the validity of previously published phenology models. Our study evaluated larval occurrence and adult emergence of western and northern corn rootworm, D. barberi Smith & Lawrence, in 11 fields in central New York during 1991 (2 fields), 1992 (3 fields), and 1993 (6 fields). Logistic regression models predicted cumulative occurrence/emergence from Julian date or degree-day accumulations calculated from either soil (base 11 degrees C) or air temperatures. Air temperature degree-days were calculated for minimum thresholds ranging from 1-16 degrees C. Additional accumulations were determined by varying the maximum developmental threshold from 1 degrees C above the lower threshold to 30 degrees C. Minimum thresholds in the latter series of calculations were set equal to 4, 6, 9, and 11 degrees C. Julian date was a relatively poor predictor of all immature and adult stages. However, accuracy of air temperature models was dependent on choice of thresholds. The best fitting air temperature models either used minimum developmental thresholds near 6 degrees C for immature stages and 1-2 degrees C for adult emergence or calculated degree-days using a minimum threshold of 11 degrees C and a maximum threshold of 18 degrees C. Both soil and air temperature models indicated regional differences in immature development and adult emergence. Although site-specific models gave the best prediction of western corn rootworm adult emergence, adult emergence at New York and Illinois sites could be predicted by adjusting daily air temperature degree-days by the expected accumulations from 1 January to 30 September.