Corn earworm (Helicoverpa zea Boddie), cold hardiness, and climate change: implications for future distributions and IPM.

Abstract

The cold hardiness of Helicoverpa zea (Boddie) pupae was assessed using three laboratory methods: supercooling point (SCP), lower lethal temperature (LLTemp), and lower lethal time (LLTime) determination. Mean SCPs for pupae ranged between -16.4°C and -19.5°C, depending on whether pupae were in diapause or had been acclimated. The LLTemp at which 50% mortality occurred (LT50) for diapausing and non-diapausing pupae was -8.8°C and -12.4°C, respectively, though the LLTemp mortality curves were not significantly different. The time until 95% mortality for non-diapausing pupae held at -10°C, -5°C, 0°C, and 5°C was 7.2, 81.6, 502.3, and 1073.4 hrs, respectively. Time until 95% mortality for diapausing pupae held at 0°C and 5°C was 2660.19 and 2796.92 hrs, respectively. Sex did not have an influence on cold hardiness. Diapause greatly enhanced cold hardiness in pupae as indicated by a significantly lower mean SCP and longer time to reach mortality at a given temperature compared to non-diapausing pupae. However, given mean SCP comparisons, acclimation of non-diapausing pupae had a cold hardening effect comparable to diapause. In-field evaluation of overwintering H. zea survival in southern Minnesota showed that temperature was a severely limiting factor in overwintering success, though likely not responsible for complete mortality. Laboratory data, coupled with the field results suggest that a small proportion of pupae may be able to survive in Minnesota. However, field observations also suggest that sufficient degree days may not be available during autumn in southern Minnesota to allow for substantial pupation before the onset of winter, thus eliminating the potential for an overwintering population. Using the cold hardiness data generated for diapausing pupae, the present and future distributions of H. zea in North America were calculated with the modeling software CLIMEX. The resulting maps depicting the current distribution of H. zea from CLIMEX did not agree with what is currently understood for H. zea overwintering distributions and overall geographic suitability; contrary to convention, cold stress is shown to not be a significant constraint to H. zea suitability for most of the U.S. Despite the discrepancies in current projections, the present study corroborated Diffenbaugh et al. (2008) in illustrating a northern expansion of suitability for H. zea, under future climate change. The implications of potential northern expansion in the geographic range of H. zea are discussed within the context of future Integrated Pest Management (IPM) needs for sweet corn, as well as other vegetable and field crops throughout North America.