Did aphids come to Suffolk from the Continent in 1982? by Suffolk Naturalists' Society

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

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Suffolk Natural History, Vol. 20

had been generally dry, with only 0.5 mm of rain recorded at Broom's Barn, which would have favoured aphid flight, and the air temperatures had also been favourable (daily maximum air temperature averaging 20.8°C and the daily minimum averaging 11.3°C—which is little below the flight threshold for aphids of about 15°C). But it was the record from a pressure-tube anemograph which measures wind direction and speed fluctuations continuously at the same height as the suction trap (12.2 m) which proved to be of the greatest interest. On 28th July the wind veered from the North to East-North-East at about 6 am, and blew strongly during the day. Throughout the 29th and 30th July the wind blew steadily from the same direction. By day the wind was of about 10 knots, although there were gusts of more than 15 knots, but it was less strong during the night.

Numbers of Myzus persicae caught in suction traps in England and in the Netherlands in Late July, 1982 Date (July)

Rothamsted (Herts.) Broom's Barn (Suffolk) Writtle (Essex) Wye (Kent) Zoutkamp (Nth Holland) Nagele (central) Colijnsplaat (Sth)

0 2 6 0 2 96 4

0 0 4 0 3 162 24

0 1 2 0 6 272 30

5 1 6 16 9 482 258

23 38 156 96 62 300 216 162 61 22 1252 1979 22 358

No major source of M. persicae was known to the East of Broom's Barn, or in any other direction within a radius of more than 25 km. Potato crops were thought to be the most likely source of large numbers, but several were examined and few aphids found, although in one crop on the Fens there were, on average, nearly 10 per lower or middle Compound leaf, about 9% of them winged. Oil-seed rape crops had already been harvested and the aphids were few (less than 1 per plant) on sugar beet. Some weed species supported populations of M. persicae, e.g. an average of 2 per plant were found on samples of fat hen (Chenopodium album) and of 1.5 per plant on samples of small nettle (Urtica urens), but aphids on weeds are unlikely to have accounted for a sudden increase in the aerial population. The wind direction near the ground is not necessarily the same as at higher altitudes, and the next step was to contact Mr. J. Cochrane of the Meterological Office, who has a special interest in the problems of agricultural entomology. He reported that on 29th July there had been a mainly East wind across East Anglia, but East-North-East over the Netherlands and the German coast. On 30th July the winds were stronger, 25 to 30 knots, East

Trans. Suffolk Nat. Soc. 20

DID APHIDS COME TO SUFFOLK FROM THE CONTINENT IN

1982?

across the North Sea. Temperatures in the Rhine area had been 20-25Â°C by 9 am, good conditions for aphid flight, with a lift from thermals. Aphids could have left Germany at 9 am and reached Broom's Barn by 3 pm. It takes about 6 h to cross the North Sea at 800-1000 m, and aphids could have survived at even greater altitudes. Aphids have been trapped at up to 1500 m and found to be alive and still able to reproduce (Taylor, 1960). Although aphids normally settle out of the air about sunset, as the light intensity and temperatures fall, they can continue to fly in warm, low-level jet-streams, and survive very low temperatures (or even supercool) for a short period. There are no suction traps operating continuously in Germany and I have been unable to obtain any information on the aphid Situation in the Rhine area during this period, but there were three suction traps operating in the Netherlands, at Z o u t k a m p in the North, Nagele in central Holland, and at Colijnplaat in the South. Details of the catches from these traps were supplied by Miss Jacomijn Prinsen of the Research Institute for Plant Protection at Wageningen. The table shows the numbers of M. persicae trapped at the end of July. As with the traps in England, the Dutch traps are emptied daily early in the morning and the dates refer to catches made during the previous day. The numbers of M. persicae trapped at the end of July in the centre and South of the Netherlands were clearly higher than those in England, and higher than they have ever been recorded in East Anglia. Miss Prinsen's colleagues assured her that these aphids did not originate on potatoes, sugar-beet or lettuce, but arrived in the Netherlands in mid-July from elsewhere. Dr W . Heijbroek of the Instituut voor Rationele Suikerproductile at Bergen-op-Zoom reported that very many M. persicae developed on potatoes f r o m the second half of July, first in the South of the country and later also in some Northern areas. The aphids were far more numerous on potatoes than on sugar beet (possibly because sugar beet were wilting and therefore not attractive), but south of Limburg (extreme South-East) the population reached an average of 26 per sugar-beet plant, and in NoordHolland (North-West) 9 per plant. If peach-potatoe aphids were carried in an air mass at high altitude to East Anglia at the end of July, as trajectories drawn by Mr. Cochrane (Fig. 5) show to be at least possible, there is no evidence that they caused appreciable damage to crops. Mature plants are generally resistant to infestation or infection with viruses, and slow to develop symptoms if they are infected. For example, sugar beet at that stage of growth take about 6 weeks to develop symptoms of yellowing viruses, and may never do so. Counts of infected plants m a d e by British Sugar fieldmen of the Cantley (Norwich), Bury St. E d m u n d s and Ipswich sugar factories showed there to be no sudden increase. They estimated that 1.2% of plants were infected at the end of July, 3.9% at the end of August and 5.0% of plants infected with yellows at the end of September. I believe that the M. persicae I trapped at the end of July 1982 came from potato crops or sugar beet in the Netherlands, but I cannot prove it. I thank Dr. L. R. Taylor of Rothamsted Experimental Station and my Dutch colleagues for suction trap data.

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D1D APHIDS COME TO SUFFOLK FROM THE CONTINENT IN

1982?

References Heathcote, G. D. (1980). Were we invaded by greenfly? Trans. Suffolk Nat. Soc. 18, 144. Taylor, L. R. (1960). Mortalityandviabilityofinsectmigrantshighintheair. Nature, Lond. 186, 410. Taylor, L. R. (1977). Migration and the spatial dynamics of an aphid, Myzus persicae. J. Anim. Ecol. 46, 411. Taylor, L. R. & French, R. A. (1974). Rothamsted Insect Survey. Rep. Rothamsted exp. Stnfor 1973, 2, 240. Thresh, J. M. (1983). The long-range dispersal of plant viruses by arthropod vectors. Phil. Trans. R. Soc. Lond. B. 302,497. Dr. G. D. Heathcote, 2 St. Mary's Square, Bury St. Edmunds.

A batch of bird fleas On 6th April, 1983, a bright, sunny day, Eleanor, my wife was in our garden in Bury St. Edmunds when her eye was caught by an unusual shimmering effect in front of a disused nesting box fixed to an old apple tree. It was caused by living fleas caught in a spider's web an inch or so from the entrance hole. Light was being reflected from the fleas as they struggled to escape. I was able to collect more than 40 fleas on this occasion, and more the next day. The nesting box had come from another site and not been used for more than one year. The identity of its last occupant to make a nest there is not known unfortunately. The fleas were identified by Mr. Ian Simmons at the Ipswich museum as Ceratophyllus gallinae (Schrank), a common bird flea. Clearly they had been roused by the warmer weather and, finding nothing in their nest box, were attempting to leave for pastures new. The Suffolk Biological Records Centre at Ipswich Museum is gathering data on all forms of wildlife throughout the county, including fleas. Mr. Simmons would be grateful for fleas from any birds or mammals. They should be sent to the museum accompanied by relevant data. Disused nests would also be appreciated, as these often harbour fleas. They are best sent in plastic bags to prevent fleas and any other inhabitants escaping. Dr. G. D. Heathcote, Bury St. Edmunds.

Trans. Suffolk Nat. Soc. 20