COLD-HARDINESS AND OVERWINTERING STRATEGY OF THE PINK MAIZE STALK BORER, SESAMIA NONAGRIOIDES LEF (LEPIDOPTERA, NOCTUIDAE)

The cold-hardening capacity of larvae of the pink maize stalk borer, Sesamia nonagrioides Lef., was examined. Supercooling points (SCPs) of field collected diapausing larvae from south-east and south-west France and non-diapausing and diapausing laboratory-reared larvae did not differ and ranged between -5 and -8 degrees C. Thus, this insect possesses sufficient supercooling ability to avoid freezing over its normal environmental temperature ranges. Despite this, we found that Sesamia presents paradoxical cold reactions. Mortality of cold acclimated diapausing larvae after short-term exposure to temperatures above the SCP is high, supporting the view that Sesamia is cold-sensitive. On the other hand, Sesamia could survive freezing for at least 24 h to temperatures close to the SCP. This ability does not seem to be related to haemolymph trehalose, the sole cold-accumulated compound detected by gas chromatography and mass spectrometry. Despite the mildness of the winter 1990-1991, only 5% of the field population survive and pupate in April. The main part of the population died from November to January, the period during which larvae were mainly located in the part of the corn stem above the ground and experienced air temperatures. After January, all surviving larvae were exclusively located in the root, 10 cm below the soil, where they experienced milder temperatures than air. They exhibited a constant low rate of mortality, possibly independent of the cold. In their current distribution area, survival of overwintering larvae of S. nonagrioides is only related to the microclimate of the overwintering site and freezing tolerance capacity seems to be irrelevant. This study allows us to propose a non-pollutant pest control method based on the ''behavioral strategy'' of this insect.