A defence mechanism of aphids: can we turn it against them?

A DEFENCE MECHANISM OF APHIDS: CAN WE TURN THIS AGAINST THEM? G. D.

HEATHCOTE

Most m e m b e r s of our Society study the structure and life histories of plants and animals simply for the pleasure it gives them in so doing. However, detailed knowledge of wildlife can sometimes be of direct use to us, enabling us to decrease the damage caused by pests or weeds by using 'Biological control'. Applied biologists can sometimes tip the balance of nature in our favour and m a k e the use of conventional pesticides by farmers and horticulturalists unnecessary, or at least decrease their use. Many attempts have been m a d e to limit the damage caused by aphids ('greenfly' and 'blackfly') using such methods. T h e r e follows a brief account of one such study, involving many scientists (and many scientific publication) over more than 10 years. It shows the inevitable ups and downs of such work.

Attempts are being made to use our knowledge of the secretions from the cornicles ( = siphunculi) of aphids. It was once thought that these tubular structures which lie one either side of the abdomen of most aphids gave out 'honeydew', the sweet, sticky substance which covers the leaves of aphidinfested plants. Honeydew is only the unwanted fraction of plant sap which has passed through the aphid's body and is passed out through the anus. T h e cornicles release a waxy substance containing complex chemicals such as primary triglycerides, myristic acid etc., which rapidly hardens into a waxy plate when in contact with a hard surface. This can give some protection against predators and parasites, as described in an earlier article (Heathcote, 1969).

Wax is not normally seen on the cornicles, but the photograph shows blobs of wax on t h e cornicles of a Peach-potato Aphid, Myzus persicae (Sulz.) killed by t h e carbamate insecticide, aldicarb ('Temik'). Aldicarb is a very effective 'systemic' insecticide, one which can be applied to the soil in granular f o r m and is absorbed by the roots and carried to all parts of plants with the sap. An aphid feeding on an aldicarb-treated plant is likely to ingest sufficient of the poison to cause its death. However, death is far from instantaneous. In some laboratory experiments I made at B r o o m ' s Barn Experimental Station at Barrow (unpublished) I found no appreciable difference in behaviour between aphids on aldicarb-treated and untreated sugar-beet plants for one hour or more, except that at least 1 in 5 of the aphids on the treated plants produced large drops of cornicle wax. Many of these aphids died with their sucking mouthparts ('stylets') still embedded in the plant tissues.

In the 1970s it was discovered that the cornicle wax of aphids contains an insect-behaviour controlling chemical, a ' p h e r o m o n e ' , which is a natural alarm signal for most species (Edwards et al., 1973). It contains a compound known as trans-Beta farnesene, which can be synthesized by chemists. This raised the question as to whether the artificial alarm signal could be used to unsettle aphids and prevent them f r o m feeding, or even prevent t h e m from

Trans. Suffolk Nat. Soc. 24

36

Suffolk Natural History, Vol. 24

Balls of wax on the cornicles of a Peach-potato Aphid killed by aldicarb. ( P h o t o g r a p h B r o o m ' s B a r n Experimental Station.)

landing on treated crop plants? Sadly, this was not so. The chemical breaks down very rapidly, by oxidation, giving plants only a few hours protection. In addition, aphids were found to act less and less strongly to the p h e r o m o n e in the same way as birds become familiar with a scarecrow which does them no harm. H o w e v e r , aphids were disturbed initially, and moved about the plants more than usual.

It soon became clear that the alarm p h e r o m o n e could never replace systemic insecticides in the farmers' armoury against aphids. T h e r e remained the possibility of using the p h e r o m o n e with another type of insecticide, the 'contact' type (the most familiar of which is probably D D T ) , which remains on a treated surface for a long time but does not penetrate plant tissue or protect any new growth. Contact insecticides are usually much less toxic to man than systemic ones. Aphids are killed by sprays of contact insecticides or pick u p sufficient on their feet (tarsi) when walking on a treated surface. T h e crop protection theory which was suggested was a simple one, i.e. an alarm p h e r o m o n e would cause aphids to move around more on treated crop plants and pick u p sufficient insecticide to destroy them more quickly.

Encouraging results were obtained from experiments done with a mixture of insecticide and alarm p h e r o m o n e . For example, Griffiths and Pickett (1980) used the synthetic p h e r o m o n e alone or in combination with permethrin ( ' A m b u s h ' , a good contact insecticide but with no systemic action) against Peach-potato Aphids on Chinese Cabbage. The insecticide alone decreased the aphid population by 38%, the p h e r o m o n e alone by 2 1 % , and the combined spray by 9 2 % . Any spray will decrease the aphid population to

Trans. Suffolk Nat. Soc. 24

A D E F E N C E MECHANISM OF A P H I D S

37

some extent (gardeners will know that soapy water helps keep the roses free from greenfly) and in these experiments the numbers of green aphids fell by 12% when sprayed with water. The kill of Potato Aphids, Macrosiphum euphorbiae (Thos.), by permethrin was also increased by the addition of the alarm pheromone. Although this particular approach to aphid control has not been abandoned it does not seem likely to be used commercially in the near future.

One other approach is being tried. It involves the alarm pheromone and one of the aphid's natural enemies. Several fungi attack aphids, and some have been used in biological control against them. Unfortunately fungi require very special conditions if they are to spread rapidly through an insect population and, because of this, most attempts to control aphids in the field with fungi have failed. The fungi found attacking aphids in Suffolk are mostly Entomophthora's (E. thraxteriana, E. aphidis, E. fresenii and E. planchoniana) (Byford & Reeve, 1969), but attempts to use them have failed (Robert, Rabasse & Scheltes, 1973). The resting spores of these fungi require a long period at high temperature and high humidity (a combination of factors seldom found during an English summer) if they are to germinate, and the spores are killed by the ultra-violet light in sunlight. A large population of infected aphids is required to produce sufficient inoculum to spread the fungi widely (Roberts & Yendol, 1971).

Greater success has been achieved in the glasshouse, where temperature and humidity can be controlled to suit the fungus. Most work has been done with the fungus Verticillium lecanii, using it to control aphids attacking Chrysanthemum indicum. The kill can be improved by the addition of the synthetic alarm pheromone, even against the Cotton Aphid, Aphis gossypii Glover, which does not respond well to the alarm pheromone (Pickett etal., 1986).

It will never be possible to completely eliminate the pests attacking our crops - they are only pests because they are able to take full advantage of our methods of food production, but the work described above shows that there are many possible ways of limiting the damage they do. The battle continues.

References Byford, W. J. & Reeve, G. J. (1969). Entomophthora species attacking aphids in England, 1962-66. Trans. Br. mycol. Soc., 52, 342.

Edwards, L. J. et al., (1973). Trans-B-farnesene, alarm pheromone of the Green Peach Aphid, Myzuspersicae (Sulzer). Nature, London., 241,126.

Griffiths, D. C. & Pickett, J. A. (1980). A potential application of aphid alarm pheromones. Ent. exp. appl. 27, 199.

Heathcote, G. D. (1969). The defence of aphids against predators and parasites. Trans. Suffolk Nat. Soc., 15, 55.

Pickett, J. A . et al., (1986). Compounds influencing behaviour of invertebrates. Rothamsted Experimental Station Report for 1985, 105.

Robert, Y., Rabasse, J. M. & Scheltes, P. (1973). Facteurs de limitation des populations d'Aphis fabae Scop, dans l'Ouest de la France.

Trans. Suffolk Nat. Soc. 24

38

Suffolk Natural History, Vol. 24

1. Epizootiologie des maladies a Entomophthales sur feverole de printemps. Entomophaga, 18, 61.

Roberts, D . W. & Yendol, W. G . (1971). Use of fungi for microbial control of insects. In: Microbial control of insects and mites. Ed. Burges, H . D . & Hussey, N. W. 125.

Dr. G . D . H e a t h c o t e 2, St. Mary's Square, Bury St. E d m u n d s , Suffolk, IP33 2AJ

Trans. Suffolk Nat. Soc. 24