The Scent of Fear – the aphid alarm pheromone

We are all familiar with the effects of epinephrine (adrenaline) and norepinephrine (noradrenaline) on us when placed in a position of stress, such as public speaking or even worse danger.  We flush, shake, our heart rate accelerates and many of us we begin to sweat profusely, thus visibly advertising our distress; sometimes embarrassingly so

 

if we have an antiperspirant  fail and happen to be wearing a dark shirt.

http://www.silkyskin.co.uk/blog/wp-content/uploads/2012/07/man_giving_speech.jpg

Those seeing these symptoms may feel a degree of sympathy for the victim, but do not usually flee the scene, although they may sometimes feel tempted to do so.

The case with aphids is very different.   Aphids, when perceiving a threat to their neighbours by a predator or parasite, flee the scene rapidly, by flight, if winged, on foot if not, or even by leaping from their host-plant to the ground below.  The pea aphid, Acyrthosiphon pisum walks away or drops from their plant (Clegg & Barlow, 1982) as does the rose-grain aphid, Metopolophium dirhodum (Larsen, 1988).  This may seem a risky move since it seems that about 10% of all aphids that fall from their host plant don’t manage to get back (Sunderland et al., 1986), but for a clonal organism the risk is obviously worth it.

So how does this communal fright and flight response come about?  Most aphids have a pair of siphunculi or cornicles at the rear of their abdomen.  These vary in size and shape, in some aphids being long and slender, in others short and stubby and in yet others reduced to a shallow indentation (pore) or in a few species, totally absent.

The role of aphid siphunculi has been debated since the days of Linneaus and Reaumur who considered them to be the source of honeydew (Hottes, 1928).  Hottes himself in a comprehensive review of the various theories put forward for the function of the siphunculi, dismissed the defence theory of Busgen (1891) and plumped for an excretory role, although he did suggest that volatile substances were produced by the siphunculi in addition to the waxy visible drops.    By the middle of the last century it was generally accepted that the siphunculi were involved in defence, but in a purely physical way, in that the waxy exudate was used to deter or disable the attacking predators or parasites (e.g. Dixon, 1958; Edwards, 1966).  At about the same time, the chemical composition of the visible exudate was confirmed as being primarily triglycerides with myristic acid being the major fatty acid present (Strong, 1966).

Aphis nerii siphuncular exudate.  http://springfieldmn.blogspot.co.uk/2014/08/aphid-cornicles.html

 

 

Hawthorn-parsley aphid Dysaphis apiifolia producing sipuncular exudates whilst under attack by a parasitic wasp.  Many thanks to Tom Pope for permission to use this clip.

In 1968 an alarm pheromone was identified and isolated from the cotton stainer, Dysdercus intermedius (Calam & Youdeowei, 1968) so it was not surprising that attention should be focused on aphids, many of which show a similar group dispersive behaviour when a predator approaches them.   The aphid alarm pheromone (E)-β-farnesene was, however, not formally identified until  1972 (Bowers et al, 1971), although Maria Dahl had demonstrated the  previous year that a solution made from crushed aphids would cause an alarm response in other aphids of the same and different species (Dahl, 1971). Unsurprisingly, as during the 1970s and 1980s scientists from the USA were notorious for only citing papers written in English, Bowers et al. (1972), failed to cite her in their paper, instead citing two other American authors (Kislow & Edwards, 1972).

This discovery resulted in a flurry of papers from around the world as insect physiologists vied to be the first to isolate alarm pheromone from different aphid species (e.g. Weintjens et al., 1973; Montgomery & Nault, 1977; Wohlers, 1980).  There were also more ecological studies such as that examining the way alarm pheromone in ant-attended aphids enhances the relationship between them and their ant farmers (Nault et al., 1976) thus acting as a synomone (Nordlund & Lewis, 1976).  As time has gone on the interest in aphid alarm pheromone has remained unabated with new twists and surprises being discovered.  For example, as well as stimulating the escape response, the alarm pheromone also stimulates those surviving pea aphids to produce winged offspring thus facilitating future long-distance dispersal away from the predators (Kunert et al., 2005).  Aphid alarm pheromone can also act to help natural enemies find their aphid prey (e.g. Micha & Wyss, 1996), in this case acting as a kairomone.

The use of sex pheromones in integrated pest management is well established (Witzgall et al., 2010) and works very effectively in most cases. More recently, researchers at Rothamsted Research have courted controversy by trialing GM wheat that has been engineered to produce aphid alarm pheromone.  Many entomologists, including me, although finding the concept (Yu et al., 2012) interesting, doubt that it will work in a field situation.  I can certainly see a role for using alarm pheromones as an alternative to conventional chemical control of insect pests and it will be interesting to see if it will prove as effective as using sex pheromones.

 

References

Bowers, W. S., Nault, L. R., Webb, R. E. & Dutky, S. R. (1972). Aphid alarm pheromone: isolation, identification, synthesis. Science 177, 1121-1122.

Busgen, M. (1891)  Der Honigtau. Biologische Studien an Pflanzen und Pflanzenläusen.  Jenaische Zeitschrift für Naturwissenschaft, 25, 339-428

Calam, D.H. & Youdeowei, A. (1968) Identification and functions of secretion from the posterior scent gland of fifth instar larva of the bug Dysdercus intermedius. Journal of Insect Physiology, 14, 1147-1158

Clegg, J.M. & Barlow, C.A. (1982) Escape behaviour of the pea aphid, Acyrthosiphon pisum (Harris) in response to alarm pheromone and vibration. Canadian Journal of Zoology, 60, 2245-2252.

Dahl, M. L. (1971). Über einen Schreckstoft bei Aphiden. Deutsche Entomologische Zeitschrift 18, 121-128.

Dixon, A. F. G. (1958). The escape responses shown by certain aphids to the presence of the coccinellid Adalia decempunctata (L.). Transactions of the Royal Entomological Society London,110, 319-334.

Dixon, A. F. G. (1958). The protective function of the siphunculi of the nettle aphid, Microlophium evansi (Theob.). Entomologist’s Monthly Magazine, 94, 8.

Edwards, J.S. (1966) Defence by smear: supercooling in the cornicle wax of aphids.  Nature,  211, 73-74.

Hottes, F. C. (1928). Concerning the structure, function, and origin of the cornicles of the family Aphididae. Proceedings of the Biological Society of Washington 41, 71-84.

Kislow, C.J. & Edwards, L.J. (1972)  Repellent odour in aphids.  Nature, 235, 108-109.

Kunert, G., Otto, S., Rose, U.S.R., Gershenzon, J., & Weisser, W.W. (2005) Alarm pheromone mediates production of winged dispersal morphs in aphids. Ecology Letters, 8, 596-603.

Larsen, K.S. (1988) Responses of different age classes of the rose-grain aphid, Metopolophium dirhodum (Wlk.) to attack by a simulated predator.  Journal of Applied Entomology, 105, 455-459.

Montgomery, M. E. & Nault, L. R. (1977). Comparative response of aphids to the alarm pheromone, (E)-B-farnesene. Entomologia experimentalis et applicata 22, 236-242.

Micha, S.G. & Wyss, U. (1996)  Aphid alarm pheromone (E)-B-farnesene: a host finding kairomone for the aphid primary parasitoid Aphidius uzbekistanicus (Hymenoptera: Aphidinae).  Chemoecology, 7, 132-139

Nault, L. R., Montgomery, M. E. & Bowers, W. S. (1976). Ant-aphid associations: role of aphid alarm pheromone. Science 192, 1349-1351.

Nordlund, D. A. & Lewis, W. J. (1976). Terminology of chemical releasing stimuli in intraspecific and interspecific interactions. Journal of Chemical Ecology 2, 211-220.

Strong, F.E. (1966)  Observations on aphid cornicle secretions.  Annals of the Entomological Society of America, 60, 668-673.

Sunderland, K.D., Fraser, A.M., & Dixon, A.F.G. (1986) Field and laboratory studies on money spiders (Linyphiids) as predators of cereal aphids. Journal of Applied Ecology, 23, 433-447.

Wientjens, W. H., Lakwijk, C. J. M., & Van Der Marel, T. (1973). Alarm pheromone of grain aphids. Experientia, 29, 658-660.

Wohlers, P. (1980). Die fluchtreaktion der erbsenlaus Acyrthosiphon pisum Aausgelöst durch alarmpheromon und zusätzliche reize. Entomologia experimentalis et applicata 27, 156-168.

Witzgall, P., Kirsch, P. & Cork, A. (2010) Sex pheromones and their impact on pest management. Journal of Chemical Ecology, 36, 80-100.

Yu, X.D, Pickett, J., Ma, Y.Z., Bruce, T., Napier, J., Jones, H.D. & Xia, L.Q. (2012)  Metabolic engineering of plant-derived (E)-β-farnesene synthase genes for a novel type of aphid-resistant genetically modified crop plants.  Journal of Integrative Plant Biology, 54, 282-299.

 

Post Script

A brief guide to mones

An allomone is any chemical substance produced and released by an individual of one species that affects the behaviour of a member of another species to the benefit of the originator but not the receiver e.g. the ability of some plants to release aphid alarm pheromen and thus deter aphids form landing on them.

An apneumone is any substance produced by nonliving material that benefits a recipient species but is detrimental to a different species associated with the non-living material

A kairomone is a semiochemical, emitted by an organism, which mediates interspecific interactions in a way that benefits an individual of another species which receives it, without benefiting the emitter.  For a detailed critique of the term kairomone see Ruther et al. (2002).

A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behaviour of the receiving individual e.g. alarm pheromones, food trail pheromones and sex pheromones.

A synomone is a substance produced by an individual of one species that benefits both the producer and the recipient which is of a different species.  An example is the release of chemical elicitors by plants that attract entomophagous insects when they are attacked by herbivores.

 

Not all aphids taste the same

One morning sometime in the late 1970s, when I was doing my PhD at the University of East Anglia, I walked into the lab to find one of my fellow PhD students sitting in front of a row of Petri dishes filled with different species of aphids.  Curious, I asked him what he was doing.  His reply was that he was tasting the aphids to see why his ladybirds made the choices they did.  This was a guy whose hobby was collecting and identifying Chamaemymiid flies, so I was not entirely surprised, although I did point out that it was unlikely his taste receptors and those of the two-spot ladybird (Adalia bipunctata) had a lot in common.

That said, his premise that aphids don’t all taste the same was of course perfectly correct.  Aphid predators, in particular ladybirds, seem to have quite strong preferences for different aphid species and these preferences are strongly correlated with larval development and subsequent fecundity as adults (Kalushkov, 1998; Kalushkov & Hodek, 2004).  What is perhaps not as well-known is that certain aphids, like many lepidopteran larvae, are extremely good at sequestering potentially toxic chemicals from their host plants.  Over a century ago, Johnson (1907) noted that highly coloured and woolly (in this case meaning waxy) aphids were not eaten as readily as smoother, greener aphids.  Half a century later Hodek (1956, 1957), showed that larvae of the seven spot ladybird (Coccinella septempunctata) were unable to complete their development if fed a diet of the elder aphid Aphis sambuci and that young adult ladybirds died if fed on the same diet.  Note that A. sambuci although not brightly coloured has waxy plaques on its abdomen.

Aphis sambuci  http://aramel.free.fr/INSECTES10-4′.shtml

The mealy plum aphid, Hyalopterus pruni, although bright green, is also waxy, and when attacked by larvae of the ten spot ladybird, Adalia decempunctata, is released as oon as the ladybird larva comes into contact with the aphid’s haemolymph (insect blood) (Dixon, 1958).

Hyalopterus pruni, the mealy plum aphid http://influentialpoints.com/Gallery/Hyalopterus_pruni_Mealy_Plum_Aphid.htm

The vetch aphid Megoura viciae, on the other hand, is rather a handsome dark green aphid, with startlingly red eyes.  If however, a larva of the ten-spot ladybird is foolish enough to eat

Megoura viciae, the vetch aphid  http://influentialpoints.com/Gallery/Megoura_aphids.htm

one, it will, after about two minutes, be violently sick.  Those are the lucky ones; those that don’t regurgitate the aphid are very likely to die a few days later.  The two spot aphid, Adalia bipuncatata is also killed if it is unlucky enough to eat M. viciae (Blackman, 1967).  It appears that the vetch aphid is full of all sorts of interesting and potentially fatal chemicals (Dixon et al., 1965). Truly a toxic treat.  Megoura viciae is not the only toxic aphid out there.  Aphis craccivora, the cowpea aphid, is a beautiful mahogany brown aphid, but despite its attractive

Aphis craccivora, the cowpea aphid

appearance, it is extremely toxic to the eleven spot ladybird Semiadalia undecimnotata, although the seven spot ladybird finds it perfectly acceptable (Hodek, 1970).

A similar effect is seen with the cabbage aphid, Brevicoryne brassicae, which is extremely good at sequestering glucosinolates, especially sinigrin, from its Brassica host plants.  Glucosinolates are the compounds that give cabbages and related plants, such as Brussels sprouts, their distinctive flavour.  In high dosages they can cause liver damage in young mammals, one of the reasons why children are so reluctant to eat cabbage, despite their parent’s urgings.  Brevicoryne brassicae is so good at sequestering  glucosinolates that larvae of the two spot ladybird die when fed on aphids from sinigrin rich cabbages (Kazana et al., 2007). The seven spot ladybird however, although not entirely happy when fed on a diet of sinigrin- rich cabbage aphids is able to survive, develop and reproduce successfully (Pratt et al., 2008).  It obviously has a much better detoxification system than that of the two spot ladybird which for an aphidophagous predator seems singularly specialist.

Brevicoryne brassicae – the cabbage aphid; a colony being approached by a hungry seven spot ladybird (Photo Corin Pratt & Tom Pope).

Another aphid that sequesters plant derived toxins is the Stinkvine aphid, Acyrthosiphon nipponicus, which accumulates an iridoid glycoside, paederoside from it’s host plant the stinkvine Paederia foetidae (syn = scandens) (Nishida & Fukami, 1989). This brightly coloured aphid does not have to merge into the background; its chemical defence is so strong that

Acythrosiphon nipponicus – Stinkvine or Skunk-vine aphid http://homepage3.nifty.com/MICHI_A/akigase/AKIGASEKOUEN_hannsimoku2-1.htm

even that voracious predator the Harlequin ladybird, Harmonia axyridis, turns tail when dabbed with the aphid’s siphuncular fluid, or if it is unlucky enough to bite into the aphid, drops the aphid, regurgitates and rapidly leaves the leaf on which the aphid colony is feeding.

And finally, this striking, although rather noxious yellow aphid, the oleander aphid, Aphis nerii, which very honestly advertises that it is indeed a mouthful to be avoided.

Aphis neri – the oleander aphid http://www.zenthroughalens.com/2012/01/aphis-nerii-and-i.html

Aphis nerii is packed full ofcardiac glycosides which it sequesters from its host plant(Rothschild et al., 1970) and provides a powerful defence against potential predators, not just ladybirds.

So bear in mind, that although aphids may seem to be soft-bodied, small and defenceless, many of them are extremely well defended chemically as well as behaviourally.  This suite of complex defence mechanisms may go some way to  explain the ability of aphids to keep at least one step ahead of their natural enemies.

References

Blackman, R.L. (1967) The effects of different aphid foods on Adalia bipunctata L. and Coccinella septempunctata.  Annals of Applied Biology, 59: 207-219  http://onlinelibrary.wiley.com/doi/10.1111/j.1744-7348.1967.tb04429.x/abstract

Dixon, A.F.G. (1958) The escape responses shown by certain aphid to the presence of the coccinellid Adalia decmpunctata.  Transactions of the Royal Entomological Society London, 110: 319-334  http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2311.1958.tb00786.x/abstract

Dixon, A.F.G., Martin-Smith, M. & Subramanian, G. (1965) Constituents of Megoura viciae Buckton. Journal of the Chemical Society, 296: 1562-1564.

Hodek, I. (1956) The influence of Aphis sambuci L. as prey of the ladybird beetle Coccinella septempunctata L. Acta Societatis Zoologicae Bohemoslovacae, 20: 62-74 (in Czech)

Hodek, I. (1957) The influence of Aphis sambuci L. as prey of the ladybird beetle Coccinella septempunctata L. II. Acta Societatis Entomoligicae Cechosloveniae, 54: 10-17 (in Czech)

Hodek, I. (1970)  Coccinellids and the modern pest management. BioScience, 20:543-552

Johnson, R.H. (1907) Economic notes on aphids and coccinellids.  Southwestern Entomologist: 12: 107-118

Kalushkov, P. (1998). Ten aphid species (Sternorrhyncha: Aphididae) as prey for Adalia bipunctata (Coleoptera: Coccinellidae). European Journal of Entomology 95: 343-349. http://www.eje.cz/scripts/viewabstract.php?abstract=412

Kalushkov, P. &Hodek, I. (2004). The effects of thirteen species of aphids on some life history parameters of the ladybird Coccinella septempunctata. Biocontrol 49: 21-32. http://link.springer.com/article/10.1023%2FB%3ABICO.0000009385.90333.b4

Kazana, E., Pope, T.W., Tibbles, L., Bridges, M., Picket, J.A., Bones, A.M. & Rossiter, J.T. (2007) The cabbage aphid: a walking mustard oil bomb. Proceedings of the Royal Society B., 274: 2271-2277. http://rspb.royalsocietypublishing.org/content/274/1623/2271.full

Nishida, R. & Fukami, H. (1989) Host plant iridoid-based chemical defense of an aphid, Acyrthosiphon nipponicus, against ladybird beetles.  Journal of Chemical Ecology, 15: 1837-1845  http://link.springer.com/article/10.1007/BF01012270

Pratt, C., Pope, T.W., Powell, G. & Rossiter, J.T. (2008)  Accumulation of glucosinolates by the cabbage aphid Brevicoryne brassciae as a defence against two Coccinellid species.  Journal of Chemical Ecology, 34: 323-329 http://link.springer.com/article/10.1007/s10886-007-9421-z

Rothschild, M., von Euw, J. & Reichstein, T. (1970)  Cardiac glycosides in the oleander aphid, Aphis nerii.  Journal of Insect Physiology, 16: 1141-1145