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1. The capacity to adapt to low temperature is critical for the persistence of insect populations in heterogeneous environments. Locusts show remarkable phenotypic plasticity, termed phase change, in response to local population density.

2. In this study, the hypothesis that population density, as a social factor, affects the cold hardiness of progeny is validated in the migratory locust, Locusta migratoria, which shows remarkable density-dependent phase changes between gregarious and solitarious phases.

3. We demonstrated that eggs of gregarious and solitarious migratory locusts significantly differ not only in size and weight, but also in cold hardiness. Eggs of solitarious locusts are more resistant to cold stress compared with those of gregarious locusts, with longer 50% lethal time at different low temperatures and under different acclimation treatments of cooling rates or cold exposure time, lower upper limit of chill injury zone, and lower sum of injurious temperature resulting from temperature-time regression. The lowest cooling rate (0.05 degrees C min(-1)) yields the highest survival of cold for eggs.

4. A custom-made microarray covering 9154 unigenes of the migratory locust demonstrated quite different gene expression profiles in the two phases in response to normal or low temperature. Under cold stress, the gregarious-phase eggs have higher transcriptional levels of heatshock proteins, DOPA decarboxylases and tyrosine hydroxylase, whereas the solitarious-phase eggs exhibit stimulated lipid metabolism and carboxylic acid transport.

5. Hybridization between the two phases showed that the cold hardiness of eggs from the hybrid with solitarious females is significantly higher than that of the hybrid with gregarious females, and the cold hardiness of eggs from each reciprocal hybrid is close to their maternal origins. These results indicate that the cold hardiness of progeny is affected by the parental phase status.