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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.


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interest to the scientists of Broom's Barn. DĂźring the period 26 July to 1 August the Broom's Barn suction trap caught a total of about 115,000 M. dirhodum, more than any other suction trap in England. The largest catch of M. dirhodum during this period by a similar trap on the European mainland was of less than 10,000, by a trap at Arras in the Pas de Calais region of France. Some interesting calculations on the mass of aphids in the air over southeastern England were made by Professor Glen Schaefer and his colleagues (1979) of the Cranfield Institute of Technology who has specialized in tracking migrating insects using radar. He is able to track individual aphids up to 500 m, and has shown that aphids can ascend altitudes of 1.5 m (1 mile) in strong convection currents. He is able to watch aphids at low levels even at nights using a gated television system which detects the insects passing through a high-powered infrared beam. Shaefer estimated that there were about 1 trillion (10' 2 ) aphids in the airspace over south-eastern England when the winds were light at the end of July, 1979. As each aphid weighs about 0.2 mg, this represents about 1,000 tonnes of aphids. This figure is perhaps more believable if one realizes that there were up to 200 rose-grain aphids per tiller (shoot) of the cereals at that time, and there may be 500 tillers per m 2 , so that the total aphid population may be up to 1,000 million per hectare (400 million per acre). Furthermore, cereals cover about 20 per cent of the land area in south-east England. If we accept that they were a local population, why were more aphids Aying in eastern England than ever before recorded since the Rothamsted Insect Survey (using large suction traps) began in the region in 1965? Undoubtedly, the 'aphid plague' was due directly or indirectly to several weather factors. Most M. dirhodum overwinter as eggs on roses (holocyclic individuals) although a few probably overwinter in the active State on cereals and other grasses (anholocyclic individuals). Exceptionally large numbers of eggs were Iaid on rose twigs in 1978 (pers. comm. Dr. AlanM. Dewar) and so there were larger numbers than usual migrating to cereals in 1979. Few aphids will have survived the severe winter of 1978/79 on plants in the open. However, the cold will have also killed many of the insects which prey on aphids, such as ladybirds, and once the weather became favourable the surviving aphids multiplied faster than their natural enemies. When writing of the 'aphid plague' the correspondent of a reputable London newspaper claimed that '. . . aphids are impregnated with sperms and this supply lasts over several subsequent generations'. This is not true, but aphids can reproduce without mating (parthenogenesis), adult females giving birth to many live young. Dr. G. J. W. Dean (1973) found that under Iaboratory conditions when there are no natural enemies, the Optimum temperature and unlimited food, the population of M. dirhodum can multiply each dayby about x 1.25 (i.e. by about five timeseach week). The first two M. dirhodum of the season at Broom's Barn in the suction trap were caught on June 17, and if there had been the theoretical x 5 increase each week a catch of 31,250 might have been expected after 6 weeks. In fact the catch on 27 July was of about 38,600 M. dirhodum, which suggests that the aphids were reaching their fĂźll potential rate of increase, but of course the numbers of

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aphids caught in a suction trap on any one day depend upon many factors, such as wind speed, and 27 July was a calm, warm day, ideal for low-level a p h i d flight. T h e r e are many other complications to the story, for example, n o t all t h e young aphids produced on cereals develop wings, but undoubtedly cereal aphids multiplied extremely rapidly at the end of July. T h e n u m b e r s of aphids Aying in eastera England declined as rapidly as they h a d increased, partly because the cereals were ripening and being harvested so t h a t t h e aphids' food supply was decreasing, but also because they were being killed by their insect enemies. Adult hoverflies, ladybirds and lacewings w e r e caught in the B r o o m ' s Barn suction trap in large numbers from midA u g u s t . They were caught when few aphids remained on crops, but this was to be expected as the life cycles of these predators are not synchronised with t h o s e of their prey and they develop relatively slowly. Adult hoverflies do not eat aphids, and it is the larvae of all three groups of insects which mainly affect t h e a p h i d population. O n e final c o m m e n t arising f r o m the press reports on the 'aphid plague'. O n e c o r r e s p o n d e n t wrote: 'Annoying people is just a side-line for the aphids, w h o a r e busy making honey by the bucketful. They feed on the leaf sap and any surplus shoots out from tubes on their backs'. What the writer called ' t u b e s o n their backs', technically called cornicles, in reality secrete a wax as a p r o t e c t i o n against their insect enemies, as already described in this journal ( H e a t h c o t e , 1969), and what we call 'honeydew', which can be collected by b e e s , is mainly sugary water which formed part of the sap from plants on which t h e aphids were feeding. T h e honeydew has passed through the aphid's b o d y w i t h o u t being absorbed and it is excreted through the anus. When crops are heavily infested with aphids the loss of sap can be appreciable and local cereal crops would have yielded more without the 'aphid plague'. Unfortunately, we cannot blame our Continental neighbours for producing these aphids as they were ' h o m e grown'.

References C o c k r a n e , J. (1980). Some meteorological aspects of the numbers and distrib u t i o n of the rose-grain aphid, Metopolophium dirhodum (Wik.), over s o u t h east England in July 1979. PI. Path. (in the press). D e a n , G . J. W . (1973). Bionomics of aphids reared on cereals and some G r a m i n e a e . Ann. appl. Biol. 73, 127. H e a t h c o t e , G . D . (1969). T h e defence of aphids against predators and parasites. Trans. Suffolk Nat. Soc. 15,55. H e a t h c o t e , G . D . (1970). T h e abundance of grass aphids in eastern England as shown by sticky trap catches. PI. Path. 19, 87. H e a t h c o t e , G . D . (1972). Coccinellid beetles on the east coast. Trans. Suffolk Nat. Soc. 15, 515. J o h n s o n , C . G . (1960). Syrphid (Dipt.) migration on the Norfolk coast in A u g u s t . Ent. mon. Mag. 96, 196. S c h a e f e r , G . , B e n t , G . & C a n n o n , R. (1979). T h e green invasion. New Scientist, 9 A u g u s t . 440. Trans. Suffolk

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Worms, Baron de (1962). A ladybird plague on the east coast. Trans. Suffolk Nat. Soc. 12, 55. Dr. G. D. Heathcote. 2 St Mary's Square, Bury St. Edmunds.

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aphids caught in a suction trap on any one day depend upon many factors, such as wind speed, and 27 July was a calm, warm day, ideal for low-level aphid flight. There are many other complications to the story, for example, not all the young aphids produced on cereals develop wings, but undoubtedly cereal aphids multiplied extremely rapidly at the end of July. T h e numbers of aphids Aying in eastern England declined as rapidly as they had increased, partly because the cereals were ripening and being harvested so that the aphids' food supply was decreasing, but also because they were being killed by their insect enemies. Adult hoverflies, ladybirds and lacewings were caught in the Broom's Barn suction trap in large numbers from midAugust. They were caught when few aphids remained on crops, but this was to be expected as the life cycles of these predators are not synchronised with those of their prey and they develop relatively slowly. Adult hoverflies do not eat aphids, and it is the larvae of all three groups of insects which mainly affect the aphid population. O n e final comment arising from the press reports on the 'aphid plague". O n e correspondent wrote: 'Annoying people is just a side-line for the aphids, who are busy making honey by the bucketful. They feed on the leaf sap and any surplus shoots out from tubes on their backs'. What the writer called 'tubes on their backs', technically called cornicles, in reality secrete a wax as a protection against their insect enemies, as already described in this journal ( H e a t h c o t e , 1969), and what we call 'honeydew', which can be collected by bees, is mainly sugary water which formed part of the sap from plants on which the aphids were feeding. The honeydew has passed through the aphid's body without being absorbed and it is excreted through the anus. When crops are heavily infested with aphids the loss of sap can be appreciable and local cereal crops would have yielded more without the 'aphid plague'. Unfortunately, we cannot blame our Continental neighbours for producing these aphids as they were 'home grown'.

References C o c k r a n e , J. (1980). Some meteorological aspects of the numbers and distribution of the rose-grain aphid, Metopolophium dirhodum (Wik.), over south east England in July 1979. PI. Path. (in the press). D e a n , G . J. W. (1973). Bionomics of aphids reared on cereals and some G r a m i n e a e . Ann. appl. Biol. 73, 127. H e a t h c o t e , G . D . (1969). The defence of aphids against predators and parasites. Trans. Suffolk Nat. Soc. 15, 55. H e a t h c o t e , G . D. (1970). The abundance of grass aphids in eastern England as shown by sticky trap catches. PI. Path. 19, 87. H e a t h c o t e , G . D . (1972). Coccinellid beetles on the east coast. Trans. Suffolk Nat. Soc. 15, 515. J o h n s o n , C. G . (1960). Syrphid (Dipt.) migration on the Norfolk coast in August. Ent. mon. Mag. 96, 196. Schaefer, G . , Bent, G. & Cannon, R. (1979). The green invasion. New Scientist, 9 August. 440. Trans. Suffolk Nat. Soc. 18 part 2.


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Worms, Baron de (1962). A ladybird plague on the east coast. Trans. Suffolk Nat. Soc. 12, 55. Dr. G. D. Heathcote. 2 St Mary's Square, Bury St. Edmunds.

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Were we invaded by greenfly?  

Were we invaded by greenfly?