Tag Archives: galls

Not all aphid galls are the same

A galling experience – what on earth is an aphid-induced phytotoxemia?

Scientists, actually let me correct that, all members of specialist groups, be they plumbers or astrophysicists, love their jargon.  Insect-induced phytotoxemias is a great example. What entomologists and plant physiologists mean by this term is plant damage caused by an insect.  The visible damage that insects can cause to plants ranges from discolouration, lesions, and malformation of stems and leaves. As the title of this post suggests I am going to discuss galls.  Many insects produce galls, some of which can be spectacular such as Robin’s pin cushion gall caused by the wasp, Diplolepis rosae, but being a staunch aphidologist I am going to concentrate on various leaf deformities caused by aphids.

Robin’s pin cushion gall, caused by Diplolepis rosae.


Aphids are true bugs, they are characterised by the possession of piercing and sucking mouthparts, the stylets, think of a hypodermic needle, being the piercing part of the mouthparts.

Aphid mouthparts, showing the passage of the stylets to the phloem (Dixon, 1973).

It was originally thought that the various leaf deformities resulting from aphid feeding was a direct result of the mechanical damage caused by the stylet entering the leaf and rupturing cell walls or possibly by the transmission of a disease. A series of elegant experiments by Kenneth Smith in the 1920s showed however, that insect salivary gland extracts were needed to cause the damage (Smith, 1920, 1926).  Puncturing leaves with needles did not produce the same symptoms.  The leaf rolls, leaf curls and pseudo-galls caused by aphids vary between species even when the aphids are closely related or their host plants are.  As an example of the latter, the bird cherry-oat aphid, Rhopalosiphum padi, causes what I would describe as a leaf roll, i.e. the leaves curl in from the edges towards the mid-rib, to make something that resembles a sausage.

Leaf roll pseudo-galls on bird cherry, Prunus padus, caused by the bird cherry oat aphid, Rhopalosiphum padi.

On the other hand, the cherry blackfly, Myzus cerasi, that has Prunus avium as its primary host, causes what I describe as leaf curls (think ringlets and curls in human hair terms), in that the leaf rolls up from the tip down towards the stalk (petiole).

Leaf curl on Prunus avium caused by the Chery black fly, Myzus cerasi

Similarly, there are two closely related aphid species, Dysaphis devecta and D. plantaginea, both feed on apple leaves, but D. devecta prefers to feed on the smaller veins while D. plantaginea prefers to feed on the mid-rib. The former causes a leaf-roll, the latter a leaf curl.

Dysaphis galls http://influentialpoints.com/Gallery/Dysaphis_devecta_species_group_rosy_leaf-curling_apple_aphids.htm

As well as leaf rolls and leaf curls, some aphids are able to induce leaf folds.  The poplar-buttercup gall aphid, Thecabius affinis being a good example.

Leaf fold on poplar caused by Thecabius affinis Poplar-buttercup gall aphid. Photo from the excellent Influential Points web site. http://influentialpoints.com/Gallery/Thecabius_affinis_Poplar-buttercup_gall_aphid.htm

You might think that it is the aphid feeding site that causes the characteristic roll, curl or fold, but if groups of D. devecta or D. plantaginea are caged on the stem of an apple seedling, young leaves several centimetres away will develop leaf rolls characteristic of each species suggesting that they are caused by specific substances in the saliva of each aphid (Forrest & Dixon, 1975).  Aphid saliva is known to contain a huge range of proteins from amino acids to digestive enzymes (Miles, 1999) so it is highly likely that different aphid species have evolved different suites of enzymes that enable them exploit their respective host plants more efficiently.  Entomologists who work on plant galls suspect that there is something in the saliva that makes the plant’s hormones trigger the gall formation, but they freely admit that they are still just guessing.  Leaf rolls and curls are pretty tame when you come to look at the galls some aphids can induce.  Aphids from the family Pemphigidae cause structural deformations that totally enclose them and their offspring.

Petiole galls caused by (left) Pemphigus spyrothecae (photo Graham Calow, http://warehouse1.indicia.org.uk/upload/med-p1771un6n510nt146ugosslt1hip5.jpg) and (right) Pemhigus bursarius gall (Photo Graham Calow http://www.naturespot.org.uk/species/pemphigus-bursarius)

Pemphigus populitransversus, the Cabbage root aphid or poplar petiole aphid (Photo Ryan Gott Ryan Gott‏ @Entemnein)

Not all enclosed galls are on petioles, the witch-hazel cone gall aphid (Hormaphis hamamelidis causes very distinctive galls on the leaves of its host plant.

Cone galls on witch hazel caused by Hormapahis hamamelidis http://www.inaturalist.org/photos/377819

So what is it with insect galls?  Are they of any use?  Peter Price and colleagues (Price et al., 1987) very succinctly summarised the four hypotheses that address the adaptive value of insect galls; a) No adaptive value (Bequaert, 1924), b) adaptive value for the plant (Mani, 1964), c) adaptive value for plant and herbivore (mutual benefit) (Cockerell, 1890) and d) adaptive value for the insect.  This last hypothesis is further subdivided into nutritional improvements, micro-environmental improvements and natural enemy protection (Price et al., 1987).

Becquaert’s non-adaptive hypothesis is and was easily and quickly dismissed (Price et al., 1987), so I will move swiftly on to the plant-protection hypothesis which Price et al., dismiss almost as swiftly.  In essence if galls are not associated with enhanced growth and survival of the galled plant then there is no protection offered.  In fact, galling insects have been used as biological control agents against weeds (e.g. Holloway & Huffaker, 1953; Gayton & Miller, 2012) which to put it mildly, does not suggest any benefits accruing from being galled.  That said, you could argue (weakly) and assuming that the plant is in control of producing the gall, that by confining the insect to a particular part of the plant it is “contained” and can be dealt with if it is causing too much damage by for example premature leaf abscission (Williams & Whitham, 1986).

The mutual benefit hypothesis is also easily dismissed as there is no evidence that galls improve the fitness of a plant as galling insects are parasites of the plant.  You might argue that fig wasps and figs mutually benefit each other, but in this case I think we are looking at special case pleading as the fig wasp are pollinators (Janzen, 1979).

So that takes us on to the adaptive value for insects hypothesis which makes a lot more sense as it is the insect (in this case the aphid), that has made the investment in what you might justifiably term, mutagenic saliva (Miles, 1999).

There is overwhelming evidence so support the nutrition hypothesis that galled leaves and galls are nutritionally superior to ungalled leaves (Llewellyn, 1982); e.g. acting as nitrogen sinks (Paclt & Hässler, 1967; Koyama et al., 2004), enhancing development and fecundity for succeeding generations of aphids (e.g. Leather & Dixon, 1981) and providing better nutrition for non-galling aphids and other insects (e.g. Forrest, 1971; Koyama et al., 2004; Diamond et al., 2008).   I also found a description of an aphid, Aphis commensalis, the waxy buckthorn aphid, which lives in the vacated galls of the psyllid Trichochermes walker, but whether this is for protection or nutritional reasons is not clear (Stroyan, 1952). 

The microenvironment hypothesis which suggests that the galls provide protection from extremes in temperature and humidity was hard to support with published data when Price et al. (1987) reviewed the topic. They mainly relied on personal observations that suggested that this might be true.  I found only two references in my search (Miller et al, 2009) that supported this hypothesis, albeit one of which is for gall wasps.  I have so far only been able to find one reference that suggest galls benefit aphids, in this case protecting them from very high temperatures (Martinez, 2009).

The natural enemy protection hypothesis has been tested almost as much as the nutrition hypothesis and in general terms seems to be a non-starter as gall forming insects seem to be especially attractive to parasitoids; see Price et al., (1987) for a host of references.  Aphids, however, may be a different case, free-living aphids have many parasitoid species attacking them, but those aphids that induce closed galls are singularly parasitoid free, at least in North America (Price et al., 1987). Although this may have been from lack of looking, as parasitoids have been identified from galls of the aphid Pemphigus matsumarai in Japan (Takada et al., 2010).  Closed galls are not always entirely closed as some need holes to allow honeydew to escape and migrants to leave (Stone & Schonrogge, 2003) which can act as entry points for natural enemies, but cleverly, the aphids have soldier aphids to guard against such insect invaders.

Sometimes the potential predator can be a vertebrate.  The aphid Slavum wertheimae forms closed galls on wild pistachio trees, and are, as with many other closed gall formers, not attacked by parasitoids (Inbar et al., 2004).  Wild pistachios are, however, attractive food sources to mammalian herbivores and gall aphids being confined to a leaf, unlike free living aphids could be inadvertently eaten. The galls however, contain higher levels of terpenes than surrounding leaves and fruits and emit high levels of volatiles that deter feeding by goats and other generalist herbivores thus protecting their inhabitants (Rostás et al., 2013). Not only that, but to make sure that any likely vertebrate herbivores avoid their gall homes, they make them brightly coloured (Inbar et al., 2010).   Aphids really are great at manipulating plants.

Cauliflower gall on wild pistachio, caused by Slavum wertheimae (Rostás et al., 2013).

Leaf rolls and curls on the other hand are more open structures, and in my experience, aphids that form leaf rolls or curls, are very vulnerable once a predator finds them crowded together in huge numbers.  Gall-dwelling aphids, including those that live in rolls and curls, tend, however, to be very waxy, and this may deter the less voracious predators.  I tend to support the nutritional benefit hypothesis in that with host alternating aphids, the enhanced nutrition enables rapid growth and development and is a way of building up numbers quickly, and hopefully the aphids are able to migrate to a new host, before the natural enemies find them.

Real life drama, Rhopalosiphum padi on Prunus padus at Harper Adams University May-June 2017.  In this instance the aphids won, and the plant was covered in hungry ladybird larvae eating mainly each other and the few aphids that had not managed to reach adulthood.

One thing that struck me while researching this article was that all the aphids producing galls, rolls or curls were host-alternating species. A fairly easily tested hypothesis for someone with the time to review the biology of about 5000 aphids, is that only host alternating aphids go in for galls.  This could be a retirement job J.

There are, depending on which estimate you agree with, somewhere between 8 000 000 to 30 000 000 insect species (Erwin, 1982; Stork, 1993; Mora et al., 2011), but even the highest estimate suggests that only 211 000 of these are galling species (Espirito-Santos & Fernandes, 2007).  And a final thought, if galls are so great why don’t all aphids and other phloem and xylem feeding insects go in for them?


Becquaert, J. (1924) Galls that secret honeydew.  A contribution to the problem as to whether galls are altruistic adaptations.  Bulletin of the Brooklyn Entomological Society, 19, 101-124.

Cockerell, T.D.A. (1890) Galls. Nature, 41, 344.

Diamond, S.E., Blair, C.P. & Abrahamson, W.G. (2008) Testing the nutrition hypothesis for the adaptive nature of insect galls: does a non-adapted herbivore perform better in galls?  Ecological Entomology, 33, 385-393.

Dixon, A.F.G. (1973) Biology of Aphids, Edward Arnold, London

Erwin, T.L. (1982) Tropical forests: their richness in Coleoptera and other arthropod species. The Coleopterists Bulletin, 36, 74-75.

Espirito-Santos, M.M.  & Fernandes, G.W. (2007) How many species of gall-inducing insects are there on Earth, and where are they?  Annals of the Entomological Society of America, 100, 95-99.

Forrest, J.M.S. (1971) The growth of Aphis fabae as an indicator of the nutritional advantage of galling to the apple aphid Dysaphis devecta. Entomologia experimentalis et applicata, 14, 477-483.

Forrest, J.M.S. & Dixon, A.F.G. (1975) The induction of leaf-roll galls by the apple aphid Dysaphis devecta and D. plantagineaAnnals of Applied Biology, 81, 281-288.

Gayton, D. & Miller, V. (2012) Impact of biological control on two knapweed species in British Columbia. Journal of Ecosystems & Management, 13, 1-14.

Holloway, J.K. & Huffaker, C.B. (1953) Establishment of a root borer and a gall fly for control of klamath weed.  Journal of Economic Entomology, 46, 65-67.

Inbar, M., Wink, M. & Wool, D. (2004) The evolution of host plant manipulation by insects: molecular and ecological evidence from gall-forming aphids on PistaciaMolecular Phylogenetics & Evolution, 32, 504-511.

Inbar, M., Izhaki, I., Koplovich, A., Lupo, I., Silanikove, N., Glasser, T., Gerchman, Y., Perevolotsky, A., & Lev-Yadun, S. (2010) Why do many galls have conspicuous colors?  A new hypothesis. Arthropod-Plant Interactions, 4, 1-6.

Janzen, D.H. (1979) How to be a fig. Annual Review of Ecology & Systematics, 10, 13-51.

Koyama, Y., Yao, I. & Akimoto, S.I. (2004) Aphid galls accumulate high concentrations of amino acids: a support for the nutrition hypothesis for gall formation.  Entomologia experimentalis et applicata, 113, 35-44.

Leather, S.R. & Dixon, A.F.G. (1981) Growth, survival and reproduction of the bird-cherry aphid, Rhopalosiphum padi, on it’s primary host. Annals of Applied Biology, 99, 115-118.

Llewellyn, M. (1982) The energy economy of fluid-feeding insects.  Pp 243-251, Proceedings of the 5th International Symposium on Insect-Plant Relationships, Wageningen, Pudoc, Wageningen.

Mani, M.S. (1964) The Ecology of Plant Galls. W Junk, The Hague.

Martinez, J.J.I. (2009) Temperature protection in galls induced by the aphid Baizongia pistaciae (Hemiptera: Pemphigidae).  Entomologia Generalis, 32, 93-96.

Miles, P.W. (1999) Aphid saliva.  Biological Reviews, 74, 41-85.

Miller, D.G., Ivey, C.T. & Shedd, J.D. (2009) Support for the microenvironment hypothesis for adaptive value of gall induction in the California gall wasp, Andricus quercuscalifornicus. Entomologia experientalis et aplicata, 132, 126-133.

Mora, C., Tittensor, D.P., Adl, S., Simpson, A.G.B., & Worm, B. (2011) How many species are there on earth and in the ocean? PloS Biology, 9(8):, e1001127.doi:10.1371/journal.pbio.1001127.

Paclt, J. & Hässler, J. (1967) Concentrations of nitrogen in some plant galls. Phyton, 12, 173-176.

Price, P.W., Fernandes, G.W. & Waring, G.L. (1987) Adaptive nature of insect galls.  Environmental Entomology, 16, 15-24.

Rostás, M., Maag, D., Ikegami, M. & Inbar, M. (2013) Gall volatiles defend aphids against a browsing mammal.  BMC Evolutionary Biology, 13:193.

Smith, K.M. (1920) Investigations of the nature and cause of the damage to plant tissue resulting from the feeding of capsid bugs.  Annals of Applied Biology,7, 40-55.

Smith, K.M. (1926) A comparative study of the feeding methods of certain Hemiptera and of the resulting effects upon the plant tissue, with special reference to the potato plantAnnals of Applied Biology, 13, 109-139.

Stone, G.N. & Schönrogge, K. (2003) The adaptive significance of insect gall morphology. Trends in Ecology & Evolution, 18, 512-522.

Stork, N.E. (1993) How many species are there? Biodiversity & Conservation, 2, 215-232.

Stroyan, H.L.G. (1952) Three new species of British aphid.  Proceedings of the Royal Entomological Society B, 21, 117-130.

Takada, H., Kamijo, K. & Torikura, H. (2010) An aphidiine parasitoid Monoctonia vesicarii (Hymenoptera: Braconidae) and three chalcidoid hyperparasitoids of Pemphigus matsumurai (Homoptera: Aphididae) forming leaf galls on Populus maximowiczii in Japan.  Entomological Science, 13, 205-215.

Williams, A.G. & Whitham, T.G. (1986) Premature leaf abscission: an induced plant defense against aphids. Ecology, 67, 1619-1627.



Filed under Aphidology, Aphids

Tinker, Tailor, Soldier Aphid!

As a follow-up to my earlier article about biting aphids https://simonleather.wordpress.com/2013/01/25/not-all-aphids-are-vegans/, I thought it would be fun to draw people’s attention to a little-known aspect of aphid biology and ecology and yet another reason for why I love aphids so much.  Tree leaves are only really good food sources for aphids in spring and autumn, when the levels of soluble nitrogen are at their highest levels in the phloem sap (Dixon, 1973).

seasonal changes

To get around this, a number of aphid species modify the leaf tissues by forming galls.  The galls, which are induced by chemicals in the saliva of the aphid, contain cells which are higher in nitrogen than the surrounding tissues and ensure that the nutritional quality of the leaves remains high throughout the year.

gall aphids

Those tree-feeding aphids, such as the sycamore aphid, Drepanosiphum platanoidis, which are unable to form galls, enter summer aestivation (reduce their metabolic rates) as an adaptation to the low nitrogen levels found in summer leaves.  Amongst aphids, the Pemphigine aphids are renowned for their gall forming activities, many of which are formed along the leaf petiole.  The position of the gall along the petiole is very important, the closer the gall is to the base of a leaf, the better the quality of the food that the gall inhabitants experience.  There is thus a premium to be gained by either being first to form a gall, or, if in second place, to usurp the aphid stem mother that got there first and push her further up the leaf petiole.   We thus see two strategies.  In the first strategy, newly emerged first instar nymphs, (such as Pemphigus betae) fight each other  for the best position on the petiole (Whitham, 1979). These fights are serious affairs, and incredibly, can last for up to two days.

figting aphids 2

In the second strategy, as seen in a related species, Epipemphigus niisimae, where a gall has already been formed by an earlier arrival, the first instar nymphs fight to gain possession of this premium dwelling place.  These fights are even more vicious and protracted and can actually result in the death of the loser (Aoki & Makino, 1982).

Fighting aphids

As a further development, a number of gall-forming aphids, such as Ceratoglypnia styracicola and Hamamelistes cristafolaie, have a specially adapted nymphal morph, with thicker and broader front legs, whose function is to defend gall usurpation by other aphid stem mothers, so-called soldier aphids (Akimoto et al, 1996; Aoki & Kurosu, 2010).  These stay outside the gall and repel possible invaders.  It should thus not come as a surprise to find that in some species, the soldiers don’t just fight other aphids, but actually defend their siblings against predators, even being able to kill lacewing larvae (Kutsukake et al., 2004).

Soldier aphids

Truly a force to be reckoned with and of course why I love aphids so much.

Akimoto, S., Ozaki, K. &Matsumoto, Y. (1996). Production of first-instar defenders by the Hormaphidid gall-forming aphid Hamamelsites cristafoliae living anholocyclically on Betula maximowicziana. Japanese Journal of Entomology 64, 879-888.

Aoki, S. &Makino, S. (1982). Gall usurpation and lethal fighting among fundatrices of the aphid Epipemphigus niisimae. Kontyu 50, 365-376.

Aoki, S. & Kurosu, U. (2010) A review of the biology of Cerataphidini (Hemiptera, Aphididae, Hormaphidinae), focusing mainly on their life cycles, gall formation, and soldiers. Psyche, 2010, http://connection.ebscohost.com/c/articles/60639059/

Dixon, A.F.G. (1973)  Biologyof Aphids, Edward Arnold, London

Kutsukake, M., Shibao, H., Nikoh, N., Morioka M., Tamura, T., Hoshino, T., Ohgiya, S, & Fukatsu, T. (2004) Venomous protease of aphid soldier for colony defense,  Proceedings of the National Academy of Sciences USA, 101,11338-11343. http://www.pnas.org/content/101/31/11338.full.pdf+html

Whitham, T. G. (1979). Territorial behaviour of Pemphigus gall aphids. Nature 279, 324-325.



Filed under Aphidology, Aphids

Not all aphids are vegans

Typically we think of aphids as plant pests, existing solely on a diet of plant sap that they obtain by tapping into the phloem tubes of plants, using their specially adapted piercing mouthparts (stylets). These when viewed externally,  when the aphid is feeding on the plant, look fairly straightforward as in this beautiful photograph of the Oleander aphid, Aphis nerii, from Lisa Sell’s great blog site Zen Through a Lens http://www.zenthroughalens.com/

aphis nerii Stylet


The stylets can, however, actually take a rather convoluted route to their destination .

aphid mouthparts

Adapted from Roger Blackman (1974).

This is, however, not the whole story.  Some years ago when I was a PhD student, I was looking after a friend’s pea aphid (Acythrosiphon pisum) culture, whilst he was on holiday.  One day, whilst adding new pea plants to the cages, I became aware of a sharp stinging sensation on my wrist.  Looking down I was somewhat surprised to find that I was being probed by an aphid.  After my initial shock I decided that the aphid had been confused by the pea plant scent on my hands and decided that I was a suitable host plant to investigate.  The incident was not repeated although the story has become a feature of my lectures on aphid biology.  Many years later, one of my PhD students who was working on the Asian ladybird, Harmonia axyridis,  which she used to feed on pea aphids, came and asked me if I had ever heard of aphids eating each other as she had noticed that some of the juvenile pea aphids appeared to be feeding on some of the adults.

Pea aphid attack

My response was in the negative, but I said I would do a bit of research on the subject for her.  To my surprise, I found a short note (Banks et al., 1968), where the authors described not only incidents of aphids acting as cannibals and feeding on each other,  but also of aphids acting as predators and feeding on the eggs of ladybirds and lacewings, a reverse of the normal situation.   They also described an even more interesting report of what might have been biological control of  a plant pest by another plant pest – in this case hop feeding mites were apparently being eaten by aphids, unfortunately not names so I was unable to follow it up.  One of the authors also reported seeing many incidents of cannibalism by the aphid Megoura viciae, during some of his experiments.   They also noted that there was a report of an aphid species that was capable of causing swellings and rashes on people in what is now Taiwan.  I tracked the paper down (Takahashi, 1930) and had it translated by a PhD student and the paper was indeed entitled An Aphid that Bites People.  Apparently the gall forming aphid, Ceratoglyphnia (Astegopteryx) styracicola, (pictured below alongside the gall)

Ceratoglyphnia styracicolaCeratoglyphnia styracicola gall


is well-known to attack people who stand or sit underneath the snowbell tree, Styrax suberifolium, which incidentally has rather nice flowers.


The result of being ‘bitten’ by the aphid is a red swelling that disappears after an hour or so, but leaves a very itchy rash that can persist for two to three days.  In fact this phenomenon is so common that people avoid passing underneath infested trees.

So those of you who thought that only your house plants were in danger from an aphid attack better watch out!

Aoki, S. & Kurosu, U. (2010) A review of the biology of Cerataphidini (Hemiptera, Aphididae, Hormaphidinae), focusing mainly on their life cycles, gall formation, and soldiers. Psychehttp://www.hindawi.com/journals/psyche/2010/380351/

Banks, C.J., MaCaulay, E.D.M. & Holman, J.  (1968), Cannibalism and predation by aphids.  Nature, 218, 491 http://www.nature.com/nature/journal/v218/n5140/abs/218491a0.html

Blackman. R. L. (1974)  Aphids, Ginn & Co.

Takahashi, R. (1930) An aphid that bites people.  Transactions of the Natural History Society of Formosa, 20, 43-44.


Post script


And new in 2015, here is an aphid that sucks the blood (haemolymph) from ant larva!  Aren’t aphids wonderful?


Filed under Aphidology, Aphids