Previous work in Britain has shown that insecticides, even systemic ones, fail to protect brassica plants in the seed-bed from virus diseases transmitted by aphids, and an explanation was sought through experiments carried out at Rothamsted. Observations on the effect on apterae and alates of Myzus persicae (Sulz.) of contact with leaves detached from cauliflower plants previously treated with a 0·2 per cent. DDT emulsion spray showed that exposure for up to 30 min. was insufficient to kill the majority of those tested, although many were temporarily incapacitated; alatae of Brevicoryne brassicae (L.) were even less affected.

In cage experiments using infected and healthy cauliflower seedlings, apterae of M. persicae were able to transmit cauliflower mosaic virus (ClMV) and cabbage black ring spot virus (CBRSV) from infected to healthy seedlings when no spray was used, but failed to do so when the infected plants were sprayed with DDT; when alates were used in similar tests using only CBRSV, only low rates of transmission were obtained. In similar experiments using turnip seedlings and ClMV, transmission by apterae of M. persicae was high on untreated plants when the infector plant was itself untreated, but distinctly lower when it was sprayed with DDT; alates of M. persicae were found to transmit ClMV from a DDT-treated infector to untreated plants but not to a treated plant. Virus spread was thus reduced but not prevented by the use of DDT sprays. It is considered that these results lend support to the hypothesis that increase of virus within a seed-bed is preponderantly the result of the arrival, in succession, of many individual viruliferous alates from outside.