Evaluation of low temperature mortality as a range-limiting factor for the southwestern corn borer (Lepidoptera: Crambidae)

Evolutionary and ecological factors were used to assess the likelihood that low winter temperatures limit the range of Diatraea grandiosella Dyar (southwestern corn borer) in the east central United States. In controlled low-temperature mortality trials, dry diapausing laboratory-reared larvae were exposed for 2 d to constant temperatures, ranging from -6.9 to - 10.0 degreesC. Mortality of larvae originating from Indiana at the northern range edge was not significantly different from mortality of larvae from Mississippi, over 500 km to the south. However, lan ae from these populations had significantly lower mortality than did those from Arizona, suggesting that selection for increased cold-hardiness may have occurred early in the species' northeastward invasion of the central United States. Temperatures inside intact maize root crowns, the overwintering location of D. grandiosella, were measured over three winters in fields near the species' northern range edge. Although air temperatures fell below -19 degreesC for up to five consecutive days, root crown temperatures only approached -8 degreesC for periods of a few hours, due primarily to an underappreciated mitigating effect of freezing soil water. Because larvae originating from the east central United States had >60% survival when exposed to -8.8 degreesC for 2 d in the laboratory trials, mortality at the range edge caused by low temperatures was unlikely to have been severe for dry larvae during the three winters during which temperatures were measured. These results suggest that in most years, low winter temperatures are probably not the most important limiting factor at the species' northern range edge in the east central United States.