Effect of long term and rapid cold hardening on the cold torpor temperature of an aphid

The effect of long-term (seasonal) acclimation and rapid cold hardening is investigated on the cold torpor temperature (CTmin) of adult grain aphids, Sitobion avenae, reared at 20 or 10 degrees C for more than 6 months before experimentation. Rapid cold hardening is induced by exposing aphids reared at 20 to 0 degrees C for 3 h and aphids reared at 10 to 0 degrees C for 30 min (acclimation regimes previously found to induce maximum rapid cold hardening). The effect of cooling aphids from the same rearing regimes from 10 to -10 degrees C at 1, 0.5 and 0.1 degrees C min(-1) is also investigated. In the 20 degrees C acclimated population, rapid cold hardening and cooling at 0.1 degrees C min(-1) both produce a significant decrease in CTmin from 1.5 +/- 0.3 to -0.9 +/- 0.3 and -1.3 +/- 0.3 degrees C, respectively. Rapid cold hardening also results in a significant reduction in CTmin of the population reared at 10 degrees C from 0.8 +/- 0.1 to -0.9 +/- 0.2 degrees C. However, none of the cooling regimes tested reduces the CTmin of the winter-acclimated (10 degrees C) population. The present study demonstrates that rapid cold-hardening induced during the cooling phase of natural diurnal temperature cycles could lower the movement threshold of S. avenae, allowing insects to move and continue feeding at lower temperatures than would otherwise be possible.