THE INFLUENCE OF HEAT ON SOME APHIDS

The thermal death-points of five species of aphids removed from their host plants lay between 38 and 41°C., when tested for 1 hr. at 60% r.h . Many aphids alive after 1 hr. at high temperatures died within the next day; no Myzus persicae recovered and reproduced after 1 hr. above 37.5°C. Third and fourth instars and adult apterae withstood heat better than first and second instars and alatae. More aphids died at 90% r.h . than at 60% r.h , and more at 60% than at 30% r.h . Aphids kept at 15% r.h . for 4 hr. before being heated showed a higher mortality than those kept at 95% r.h . Aphids on plants withstood temperatures higher than their thermal death-point off the plant. Presumably aphids can continue to cool themselves by evaporation while feeding; also lower temperatures on the surface of transpiring plant tissues will aid survival.