SUMMARYIn 1959 and 1961, when the willow‐carrot aphid,Cavariella aegopodiae, infested carrots at Rothamsted and Woburn early and severely, crops were infected with motley dwarf virus in late May or early June and yielded about 6 tons of roots per acre. In 1962, when invasion was late and sparse, the crops remained almost uninfected and yielded 24–25 tons/acre. In 1960, when aphids invaded early but multiplied slowly, about 85% of carrots on unsprayed plots became infected in July and August, and the yield was 9.2 tons/acre. Spraying three times with Metasystox starting at an early stage of growth affected yield little in 1961, or in 1962, but increased yield by about 3 tonslacre in 1960.Field‐plots experimentally infected in 1962 by aphids fed on infected plants in the glasshouse, lost II tons/acre from infection in early June, 8 tons/acre from infection in late June and 6 tons/acre from infection in July. Plots cultured with virus‐free aphids in early June yielded as much as control plots. Experimental infection did not affect yield in 1959 and 1961, when the crop became naturally infected before treatment.The yields in different years were linearly related to the log. mean weekly numbers ofC. aegopodiaecaught on sticky traps near the sites, and the regression accounted for much of the variance in yield. The residuals of the log. mean weekly trap‐counts were negatively related to residual weekly rainfall in inches; I in. of rain above average approximately halved the increase in aphids. This may explain the failure of early invading aphids to become numerous at Woburn in 1960, when an inch of rain fell in three consecutive weeks in June.Treating seed or seedlings with systemic insecticide did not prevent young plants from becoming infected when infective aphids were cultured on them 10–14 days after treatment.Aphids taken from willow in the spring did not transmit motley dwarf to healthy carrots, but did so after they had fed on infected carrots. Aphids from wild umbellifers often transmitted motley dwarf to heal