Changes of supercooling point and cold tolerance in diapausing pupae of sugar beet moth, Scrobipalpa ocellatella (Lepidoptera; Gelechiidae)

Sugar beet moth (SBM) Scrobipalpa ocellatella (Boyd) is one of the important pests of sugar beet that causes quantitative and qualitative yield loss late in the season. Pest damage is occurred in the central buds and the root of the sugar beet plant. The last instar larvae enter the soil and pupate for overwintering at harvest time of sugar beet in Karaj, Iran in November. The younger larvae having suitable habitat and enough food for development may have chance to pupate in soil gradually whenever weather permit. In order to study the overwintering potential of the pest, pupae were collected monthly from November 2015 to April 2016. Seasonal changes in cold hardiness indices such as supercooling point and lower lethal temperature of the beet moth were studied. The mean supercooling points of field collected pupae varied from –13.2 ± 0.33 to –18.2 ± 0.23 °C. The highest cold hardiness of diapausing pupae was found in November, December and January, and the lowest in April. The cold hardiness of diapausing pupae was higher than that of nondiapausing pupae at -10,
-15 and -20 °C. Lethal temperature for 50% of population (LT50) changed from -13.99 ºC in November to -5.5 ºC in April. In spite of the fact that the supercooling point of the diapause pupae did not exceed -15 °C in the late-winter and April, the pupae could not withstand temperatures below zero under a supercooling point. However, the findings show that the ability of pupae to tolerate in the diapause stage is more than the capacity of cold and frost environment, and therefore, temperatures below zero in winter cannot be considered as a significant risk for the survival of sugar beet moth.