Cold hardiness of larvae of the southwestern corn borer, Diatraea grandiosella

Abstract The cold‐hardening capacity of field‐collected larvae from southeast Missouri and laboratory‐reared larvae of the southwestern corn borer, Diatraea grandiosella Dyar, was examined. Supercooling points of non‐diapause and diapause larvae collected from maize plants grown in Missouri (36° 30′ N lat.) were ca. –7.0 °C. The hemolymph melting points of diapause field larvae (–0.8 °C) were significantly lower than those of non‐diapause larvae collected in July (–0.5 °C). The supercooling points of hemolymph from non‐diapause and diapause field larvae ranged randomly from –10° to –18 °C. Supercooling points of non‐diapause laboratory larvae increased from –13° to –10 °C prior to pupation, whereas those of diapause larvae increased similarly before the onset of diapause, but then decreased during diapause to ca. –17 °C. No change in supercooling points or capacity to survive in the presence of ice was observed in diapause laboratory larvae acclimated at 4 °C for 63 days. Laboratory and field larvae began to freeze at ca. –1.5 °C in the presence of ice, but survived to several degrees below their melting points. The high supercooling points of field larvae appeared to be due to the presence of an environmental ice‐nucleator. Although data for laboratory larvae indicate sufficiently low supercooling points to permit winter survival in southeastern Missouri, considerable larval mortality occurs in the field due to inoculative freezing and the presence of an ice‐nucleator.