DID APHIDS COME TO SUFFOLK FROM THE CONTINENT IN 1982? G. D. HEATHCOTE
It is generally accepted that some strong-flying insects can migrate over long distances, and some butterflies and moths found in England such as the clouded yellow butterfly (Colias croceus) and the silver-Y moth (Plusia gamma) rarely or never overwinter here, but come from the European mainland. There is strong circumstantial evidence suggesting also that aphids and other small arthropods (e.g. leafhoppers, whitefly and mites) can occasionally fly or be blown by the wind for tens or even hundreds of kilometres, and some are carriers of plant diseases. This long-distance dispersal of vector species may account for the unexpected appearance of diseases in crops, and cause significant loss to growers, although the diseases do not usually spread far (Thresh, 1983). I believe that at the end of July, 1982, large numbers of aphids came to Suffolk and other parts of eastern England from the Continent, and probably from the Netherlands. In this instance they did little harm. It is, perhaps, rather ironic that in a recent article in the Transactions (Heathcote, 1980) I set out to prove that the exceptionally large numbers of aphids Aying in East Anglia at the end of July 1979 did not come from the Continent! The peach-potato aphid, Myzus persicae (Sulz.), is of great agricultural importance as it can carry virus diseases affecting widely-grown arable crops, including virus yellows of sugar beet and potato leafroll. It is not a particularly abundant aphid compared with some species attacking cereals (the cause of the 'aphid plague' in 1979), and it forms only about 2 or 3% of the aphids caught during the year in the large suction trap of the Rothamsted Insect Survey which operates continuously at Broom's Barn Experimental Station near Bury St. Edmunds. However, on 30th July, 1982, it formed 45% of the aphids in the trap, compared with an average of 5% of the catch during the previous 10 days; this was a catch of 156 M. persicae compared with an average of 4 per day during the previous 10 days. Neighbouring suction traps at Wye, not far from the coast of Kent, and at Writtle, little further inland in Essex, also caught large numbers of M. persicae on the 30th and 31st of July, but they were relatively few in the catch of a trap at Rothamsted Experimental Station in Hertfordshire in central England (see Table). The sudden rise and rapid decline in the numbers of this aphid in South-East England are shown by density distribution maps (Figs. 1-4) prepared by Mr. Ian Woiwod using the Rothamsted VAXII-750 Computer and data from all 23 suction traps of the Rothamsted Insect Survey (Taylor & French, 1974). The shading becomes darker with increase in density using a logarithmic scale (0.1 intervals). Examples of the technique are given by Taylor (1977) except that the Kansas Geological Survey's 'Surface II' program is used here rather than 'Symap'. To try to explain this sudden appearance of M. persicae Ifirstexamined the local weather records to look for any unusual features. The previous week Trans. Suffolk Nat. Soc. 20