WERE WE INVADED BY GREENFLY? G . D . HEATHCOTE
Exceptionally large numbers of ladybirds or hover flies are reported on the coast of East Anglia occasionally (de Worms, 1962; Heathcote, 1972; Johnson, 1960) but aphids ('greenfly'), which form their prey, less often make 'news' in this way. However, in the summer of 1979 the radio, television and national newspapers gave considerable time or space to what they called 'the green invasion' or the 'aphid plague'. Exceptionally large numbers of aphids were in the air over Suffolk at the end of July. Workers at Birds Eye frozen food factories were laid off and it was even necessary to halt work at Felixstowe docks for a time because of them. Queues of cars formed at garages where drivers wanted their windscreens cleared and pedestrians covered their faces with scarves or walked in the sunshine with umbrellas up in an attempt to protect their faces from these irritating, but otherwise harmless insects. It was generally assumed that the aphids came from the Continent, and holiday makers on the beaches who were troubled by them while winds were blowing from the sea quite reasonably thought so, but were they really of Continental origin? A study of the meteorological conditions at the time was made by Mr. J. Cockrane of the Meteorological Office ADAS Cambridge (Cockrane, 1980). H e was able to show that aphids were brought from cereal crops inland to the coastal region by light westerly winds and that they were carried back to the coast by the developing onshore sea breezes during the afternoon when the abundance of holiday-makers enjoying the hot, dry weather coincided with the abundance of aphids. The winds were mainly westerly during July 1979 and aphids were extremely nuhierous inland as well as on the coast. The theory of a mass migration from Europe is untenable. Most of the aphids involved in this mass flight were the rose-grain aphid (.Metopolophium dirhodum) which feeds on the underside of the leaves of cereals and other grasses, unlike the better-known pest of cereals, the grain aphid (Sitobion avenae), which attacks the ears. Both these species may, but do not necessarily attack cereal crops at the same time (Heathcote, 1970). Some indication of the numbers of rose-grain aphids Aying at the end of July in Suffolk can be gained from the catches of aphids in traps at Broom's Barn Experimental Station at Barrow, near Bury St. Edmunds. One of these is a suction trap which samples 2,800 m 3 (100,000 cu ft.) of air every hour at 12.1 m (40 ft.). It is one of a series of such traps operating as part of the Rothamsted Insect Survey, and it is used primarily to monitor the times of flight of aphids throughout the year. During the whole of 1978 the trap at Broom's Barn caught a total of 29,100 aphids. In the 24 hours from 26-27 July the trap caught about 51,000 aphids, of which 85 per cent were M. dirhodum, 7 per cent were other aphids which attack cereals, 3 per cent were the black bean aphid (Aphis fabae) and only 0.2 per cent were peach-potato aphids (Myzus persicae) which are the aphids responsible for most of the spread of yellows of sugar beet and other virus diseases and are therefore of particular Trans. Suffolk Nat. Soc. 18 part 2.