WHY W E R E THERE SO MANY GRAIN APHIDS AMONG THE MOTHS IN MY LIGHT TRAP? G . D . HEATHCOTE
Research projects, whether undertaken by professional or amateur entomologists, tend to expose more problems than they solve. This is perhaps the charm of research, but it can be very frustrating, especially for the professional; financial restraints always prevent him or her from following up many potentially interesting lines of work because they offer no financial reward for their employers. This article presents some unexpected facts and minor mysteries regarding insect behaviour arising from one such incomplete study. It provides a little in the way of explanation, and shows the danger of assuming that closely related insects must behave in similar ways. We still have a lot to learn about common insects. Many tungsten-lamp traps (Williams, 1948) are operated in England as part of the Rothamsted Insect Survey (Taylor et al., 1981) to determine the numbers of some migratory moths and pest species. The insects caught by these traps (which include many species other than moths) are killed and examined later. Tungsten-lamp traps catch many fewer insects than the mercury vapour lamp traps used by most amateur lepidopterists, who release most of the insects they trap unharmed. I operated one of the Rothamsted light traps at Broom's Barn Experimental Station, Higham, Bury St. Edmunds, from 1968 to 1984. (The details of the moths caught are kept by the Biological Records Centre at Ipswich.) Similar traps were operated at Fiatford Mill (Field Studies Council), at Ipswich in parkland (Miss V. A. Short), although not throughout the whole of this period, and at Santon Downham (Forestry Commission Research Branch). Trap sites in neighbouring counties included Methwold Fen and Monks Wood (Institute of Terrestrial Ecology). There were, in all, about 80 traps throughout England each year, all l-4m from the ground Over short grass and operating from sunset to sunrise. Broadbent (1947) established that aphids are caught throughout the night by Rothamsted light traps in Britain, but he did not establish which species. Most aphids fly between 09.00 and 21.00 h GMT but, using small, low-level suction traps, Eastop (1951) found that up to 12% of the total catch might be Aying between 18.00 and 07.00 h. I wanted to know if the Rothamsted light traps could provide useful information on the time of flight and numbers of aphids which are pests of crops in addition to the valuable data on moths. I therefore removed and identified all the aphids from the Broom's Barn trap each day from 1968 to 1977. In addition, in certain years, I examined the aphids caught in similar traps at Rothamsted Experimental Station, Harpenden, Herts., which is about 80km from Broom's Barn but at much the same latitude. Unfortunately, due to the bulk of insect material involved, it is normally impossible to retain most of the insects caught by the light traps from the Rothamsted Insect Survey; the aphids caught are seldom identified or retained.
Trans. Suffolk Nat. Soc. 23
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GRAIN A P H I D S A M O N G THE MOTHS IN MY LIGHT TRAP
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The numbers of aphids caught by other kinds of trap near the light traps were also being recorded in the period 1968 to 1977, both at Broom's Barn and at Rothamsted. There were yellow, cylindrical sticky traps (Heathcote, 1971) at l-5m and large suction traps (Taylor, 1973) at 12.2m throughout the period, together with a much smaller suction trap at l-2m in 1976 and 1977. However, the catches from these traps made during the day and the night were not kept separate and direct comparisons cannot therefore be made with the light traps. As expected, the numbers of aphids caught by the light traps were generally small, at Broom's Barn ranging from an annual total of 41 (1974) to 1222 (1976), and averaging only 392. Three light traps at Rothamsted caught on average a total of 1,816,553 and 76 each year from 1968 to 1971. The catch of aphids in other parts of Britain is probably low also. Dr. John A ' B r o o k had a trap at Aberystwyth (Welsh Plant Breeding Station) from 1970 to 1972 (A'Brook, 1973) and caught on average only 39 aphids each year. It is therefore clear that too few aphids are caught by light traps to be of use in any prediction scheme aimed to advise farmers on the need to apply insecticides or other protective measures to crops. This was the end of the study from any farm advisory point of view, but a closer look at the numbers of the various aphid species caught in the light traps proved interesting. The yearly total of the more numerous and economically important species trapped at Broom's Barn are shown in Table 1. On average, over the 10-year period, the Grain aphid, Sitobium (Macrosiphum) avenae (F.) formed 72% of the total catch, the Black bean aphid, Aphisfabae Scop., formed 6 % , the Bird-cherry aphid, Rhopalosiphumpadi (L.) (which has cereals and other grasses as its secondary host plants) formed 5 % , the Rose-grain aphid, Metopolophium dirhodum (Walker) 3 % , the Peach-potato aphid, Myzus persicae (Sulz.) and Tuberculoides annulatus (Hartig.) (small aphids which live on oaks throughout the year) each formed 2 % . All other aphid species formed 1% or less of the total. I t i s n o t surprising that many cereal aphids were caught as the trap was in a m a j o r cerealgrowing area, nor that the Black-bean aphid should be relatively numerous as crops of field beans were grown nearby and the winter host plant, spindle, Eunoymiis europaeus L., is abundant, especially around Newmarket. However, it was surprising that the Grain aphid formed a much larger proportion of the catch from the light trap than from the suction or sticky traps, which catch most aphids by day. Table 2 shows the numbers of several species expressed as a percentage of the total catch during two years. The proportion of the catch formed by the different species was remarkably consistent at Broom's Barn and Rothamsted. Grain aphids formed more than half the catch of the light traps but less than one third of the catch of the suction or sticky traps. Conversely, the Cavariella species (the Willow-carrot aphid and its near relatives) were relatively very numerous in the sticky trap catches, few in the suction trap and very few in the light trap catches. These differences are probably due to differences in the reactions to light and colour shown by these species. The sticky traps were painted a bright, golden yellow, which strongly attracts some aphids (such as the aphids which attack sugar beet, and which are
Trans. Suffolk Nat. Soc. 23
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GRAIN A P H I D S AMONG T H E MOTHS IN MY LIGHT TRAP
21
familiar to me) but does not attract Grain aphids (Heathcote, et al., 1969). Grain aphids are more attracted to white or UV light than to yellow, and Frost and Pepper (1957) caught phenomenal numbers of Grain aphids in Pennsylvania in a light trap although no cereals were grown in their trap area. The Situation is very complex. Rautapää (1980) showed that the light and colour preference of cereal aphids changes as they grow older, the positive reaction to U V radiation decreasing and the number moving towards yellow or green increasing. Table 2.
Some aphid species caught by suction, sticky and light traps at Broom's Barn and at Rothamsted, as a percentage ofthe total catch ofthat type of trap, 1968 to 1969 Suction trap (12-2m) BB* RES*
Grain aphid Bird-cherry aphid Oat-apple aphid Rose-grain aphid Black bean aphid Peach-potato aphid Carrot aphids
31 16 6 13 4 1 4
25 16 12 9 4 1 4
Sticky trap (T5m) BB RES
Light trap (l-4m) BB RES**
11 3 1 11 4 1 41
59 10 1 5 10 2 2
13 12 3 18 9 2 20
57 12 1 5 9 1 1
*BB = Broom's Barn, RES = Rothamsted Experimental Station. ** mean catch of 'Barnfield' and 'Allotments' traps at Rothamsted.
Not only may the reactions of aphids to colour and light change as they get older, but different generations may react differently throughout the season. A'Brook (1973) gave evidence to show that the height at which some species fly and their response to colour may vary with the season. He suggested that aphids could be divided into three categories. One group, mostly tree feeders (including T. annulatus), always show greatest density at a high level throughout the season (i.e. are caught more by atrap at 12m than one at Im). The second group (including M. persicae and R. padi) always have the greatest density at the lower level, whilst a third group (iacluding S. avenae) change each atumn from a maximum density at a low level to high. It is perhaps unfortunate for my story that the Grain aphid is included in the group whose habits change. I think that most of the aphids caught in the light traps had taken off during the day and had been carried into the upper air, Coming down during the night as the temperature dropped. Night temperatures in southern England are generally below the flight threshold for aphids, and dim light prevents take-off and repopulation of the air (Halgren & Taylor, 1968). Johnson (1951) showed that in southern England there is a general tendency for aerial populations of aphids to build-up during the day and decline at night so that the air is virtually free of aphids in the early morning. Unfortunately another complication creeps in here, and yet again the Grain aphid is different from the aphids which are more familiar to me. Some aphids require füll sunlight
Trans. Suffolk Nat. Soc. 23
Suffolk Natural History, Vol. 23
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before they will take off, but Taylor (1969) found that Grain aphids took flight in light equivalent to civil twilight. It is therefore possible that the light from the tungsten-lamp traps stimulated some Grain aphids to fly, but the traps were over short grass and there would have been few winged Grain aphids on plants in the immediate area of the traps. With many factors affecting the behaviour of some aphid species, and several types of insect trap functioning in different ways, it is hard, or even impossible to relate the catch of one type of trap with another. No consistent relationship between the numbers of aphids caught by the light trap and the suction or sticky traps was established. Large catches in the light trap might be on the day before, on the same day, or on the day after large catches by the other traps. A Single example may be sufficient to illustrate this. The numbers of Grain aphids caught during 8 days in late June 1976 are shown in Table 3. During this period the day temperatures were high, but at night sometimes feil below the general take-off threshold for aphid flight, approximately 15-0°C. All the traps were emptied daily at 09-00 h. However, daily catches from the suction traps cover 19 h of one day and 9 h of the following day, whereas the light traps operated from sunset on the first day to sunrise on the next. Table 3.
Numbers of Grain aphids caught in a light trap and in suction traps at Broom's Barn, from 22 to 29 June, 1976
Day Light trap Suction trap (12-2m) Suction trap (l-2m) Daily max. temp. (°C) Daily min. temp. (°C)
1
2
3
4
5
6
7
8
0 224
49 576
200 1384
5 484
774 952
1 2432
21 1116
0 196
8
48
21
32
165
52
69
7
25-7 14-3
30-6 15-5
29-4 15-2
31-3 17-0
33-3 17-5
33-0 17-2
31-8 15-7
28-0 12-2
What then have we learnt from this study? It has shown that Rothamsted light traps catch few aphids and they cannot be used by agricultural entomologists in their study of aphids which attack crops. However, the light traps catch more Grain aphids than other species. The reasons for this remain obscure, but are thought to include differences in the reactions to light and colour and in the light intensity required by different species of aphids for take-off. Suction traps, which do not rely on attraction to light or colour, are clearly more useful than light or coloured traps when attempting to estimate the numbers of Aying aphids in the air. Dr. G. D. Heathcote, 2, St. Mary's Square, Bury St. Edmunds
Trans. Suffolk Nat. Soc. 23
GRAIN APHIDS AMONG THE MOTHS IN MY LIGHT TRAP
23
Acknowledgements I thank Mrs. Joan Nicklen, Miss Maureen Dupuch, Dr. L. R. Taylor and other members of the Rothamsted Insect Survey for many years of friendly co-operation. References A ' B r o o k , J. (1973). Observations on different methods of aphid trapping. Ann. appl. Biol., 74, 263. Broadbent, L. (1947). An analysis ofcaptures of Aphididae (Hemiptera) i n a light trap. Trans. R. ent. Soc. Lond., 98, 475. Eastop, V. F. (1951). Diurnal V a r i a t i o n in the aerial density of Aphididae. Proc. R. ent. Soc. Lond. (A), 26, 129. Frost, S. W. & Pepper, J. O. (1957). Aphids attracted to light traps. Ann. ent. Soc. Arn., 50, 581. Halgren, L. A. & Taylor, L. R. (1968). Factors affecting flight responses of alienicoles of Aphis fabae Scop. and Schisaphis graminum Rondani (Homoptera: Aphididae). J. anim. Ecol., 37, 583. Heathcote, G . D. (1971). Abundance of aphids and some insects that prey upon them in Suffolk, as shown by catches on sticky traps. Trans. Suffolk Nat. Soc., 15,437. Heathcote, G. D . , Palmer, J. M. P. & Taylor, L. R. (1969). Sampling for aphids by traps and by crop inspection. Ann. appl. Biol., 63, 155. Johnson, C. G. (1951). The study of wind-borne-insect populations in relation to terrestrial ecology, flight periodicity and the estimation of aerial populations. Sei. Prog., 153, 41. Rautapää, J. (1980). Light reactions of cereal apl.ids (Homoptera: Aphididae). Anne. ent. Fenn., 4 6 , 1 . Taylor, L. R. (1969). Thresholds for take-off by grain aphids. Rep. Rothamsted exp. Stn. for 1968, (Part 1), 209. Taylor, L. R. (1973). Monitoring for migrant insect pests. Outlook on Agriculture, 7, 109. Taylor, L. R . , French, R. A . , Woiwod, I. P., Dupuch, M. J. & Nicklen, J. (1981). Synoptic monitoring for migrant insect pests in Great Britain and Western E u r o p e . 1. Establishing expected values for species content, population stability and phenology of aphids and moths. Rep. Rothamsted exp. Stn. for 1980, (Part 2), 41. Williams, C. B. (1948). T h e Rothamsted light trap. Proc. R. ent. Soc. Lond. (A), 2 3 , 8 0 .
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