Studies on the aphid, Cavariella aegopodii Scop

SUMMARY The important aphid pest of carrots, Cavariella aegopodii Scop., overwintered in the egg stage on all the common willows ( Salix spp.) found in the English Midlands, and showed no host preference between willows on the same site. The eggs were found more frequently on Salix alba and S. fragilis , however, probably because these trees, being larger and generally growing in less sheltered conditions than other species of willow, provide obvious targets for the migrating aphid. Neither eggs nor fundatrices were found on Salix babylonica . The spring migration from willow to carrot started in May, reached its peak about the beginning of June and was over before July. April‐sown carrots were the most severely attacked and high, damaging aphid populations sometimes developed on these within 6–7 weeks from the onset of immigration. The number of aphids on carrots declined in July when winged forms developed among the second and third alienicolae generations and dispersed. Some of these winged alienicolae flew to later‐sown carrots, where they did little colonizing but might have furthered the spread of motley dwarf virus. Natural enemies did not check aphid multiplication: years of highest aphid numbers were those with low rainfall over the first half of the spring migration period. Seasons when motley dwarf virus disease was most troublesome were those in which C. aegopodii apterae survived on overwintered carrot crops.