Geographical variation in egg cold hardiness:: a study on the adaptation strategies of the migratory locust Locusta migratoria L.

1. For many species of insect, cold hardiness is an important trait that enables a population to develop in the next season and to extend its range. To elucidate the role of cold hardiness of the migratory locust Locusta migratoria L. in its outbreak and distribution areas, egg cold hardiness was examined in locusts derived from four locations from latitude 18degrees23'N to latitude 41degrees10'N in eastern China. 2. The supercooling points of eggs from different geographic populations did not differ significantly for the first development stage, with an average +/- SE of -24.5 +/- 0.51degreesC, or for the second stage, -22.06 +/- 0.68 degreesC, however there was a significant difference for the embryonic development phase among the four geographical populations. The egg supercooling point increased gradually from neonatal egg to old egg; eggs prior to hatching always had a much higher supercooling point. 3. Comparisons of the cold hardiness of four populations were carried out by validating the close correlation between latitude and the effects of cold on hatching, low lethal temperature (Ltemp(50)), and low lethal time (Ltime(50)). There were significant differences among the four populations; the northern population was more cold hardy than the southern population, and the two mid-latitude populations were intermediately cold hardy. 4. The cold hardiness of all populations was enhanced to various degrees by short-term cold acclimation at 0 degreesC and 5 degreesC. For most populations, a 2-day acclimation period seemed to be optimal.