As I wrote a little while ago, thanks to the Covid-19 lockdown I have been roaming the countryside around my rural retreat a lot more than I normally do. One of the things that I have noticed is that there seem to be a lot more of the spectacular galls caused by the gall wasp, Diplolepis rosae, than I can remember seeing in previous years. I have absolutely no empirical evidence for this observation, so it could all be down to shifting baselines (Jones et al., 2020). Nevertheless, to me they have been much more noticeable this year, perhaps confounding the insect apocalypse narrative (Leather, 2018), but then again, perhaps not.
I am, when it comes to galls, more of an expert on those caused by aphids, than on those induced by Hymenoptera, although, as it happens I am a co-author on an oak gall wasp paper (Walker et al., 2002). Of course, that does not make me an expert. One the rose bushes in our front garden always has at least one of Robin’s spectacular pincushions clamouring for attention. The others rarely have one and I have idly wondered about the host preferences of the wasp and the suitability of different roses for the development of the larvae.
Our front garden Diplolepis rosae gall
Having noticed that despite the relative abundance of the galls in the hedgerow roses, some bushes were totally gall-free while others supported several, sometimes in close proximity to each other, my thoughts immediately turned to possible student project.
Multiple infestations of Diplolepis rosae on hedgerow roses in the environs of Sutton, Staffordshire
A preliminary bit of research with the aid of Google Scholar quickly disabused me of that idea, but, as is the way with the internet, soon had me delving deep into the past in search of the history of this fascinating manifestation of insect activity.
My first discovery was that is also known as the rose bedeguar gall, something that had, until now, totally passed me by. According to my favourite dictionary (Gordh & Headrich, 2001),
“Bedeguar, Bedegar or Bedequar’ comes from a French word, bédégar, and is ultimately from the Persian, bād-āwar, meaning ‘wind-brought’”.
I’m guessing this meant that the first people to see them had no idea what caused them.
Not just complex galls
My next discovery was that D. rosae and the parasitoids that share its gall is a fairly well studied system (Randolph, 2005; Urban, 2018), although I am sure that it would still be possible to come up with some sort of project. Diploelpis rosae is a member of the order Hymenoptera, so related to the larger and much more obvious bees, wasps and ants. It belongs to a family of wasps, the Cynipidae, commonly known as gall wasps. Considering how small they are, 3.8 mm (Urban, 2018), the galls they make are spectacularly huge as are those that their relatives on oak form. Interestingly* more than 80% of gall wasps are associated with oaks, with most of the rest forming galls on members of the rose family (Shorthouse, 1973). That in itself is, at least to me, an interesting fact; why such a restricted host range? It is univoltine (one generation a year), overwinters as mature larva or pre-pupa in the galls, and emerges as adults in the spring when it seeks out suitable egg-laying sites. It is mainly parthenogenetic, although males are occasionally found (Callan, 1940; Stille, 1984). Despite being tiny, each wasp has the potential to lay about 500 eggs (Stille & Dävring, 1980). The adults are not very adventurous, usually laying eggs in the developing buds or flowers of the bush they emerged on, or on another close by (Bronner, 1985: Urban, 2018). That said, there must be some sort of host preference and selection going on, as in Sweden and the Czech Republic most galls are found on Rosa canina (Stille, 1984; Urban 2018). They also seem to favour younger bushes, or those that produce long vigorous stems (Stille, 1984). The potentially high fecundity is presumably an adaptation to the high rates of parasitism that the larvae of D. rosae can experience, up to 70% in some cases (Stille, 1984). In fact, so varied and numerous are the parasites, that many of the early papers about D. rosae pay more attention to the other inhabitants of the galls than they do the architects (Osten Sacken, 1870; Blair, 1945; Bugbee, 1951). Females that lay a lot of eggs in the same developing bud produce bigger galls and a greater proportion of the larvae survive (Stille, 1984).
Survival of D. rosae in relation to gall weight (after Stille, 1984)
It also appears that the closer to the ground the galls are, the lower the parasitism rate (Laszló, 2001).
The ideal strategy would then be for female D. rosae to lay big galls as low down on the plants as possible, but from personal observation this is not always the case , so as is often the case with the “Mother knows best” hypothesis there is something we humans are missing that the insects aren’t (Awmack & Leather, 2002).
Those darned taxonomists!
So, as so much is already known about this tiny wasp and its spectacular gall, I thought I would do a little bit of entomological archaeology and trace the entomological history of this little insect. Now I think taxonomists are wonderful people and have a huge respect for the very often unacknowledged work that they do, and am a great supporter of the campaign to make sure that people cite them in their papers (Packer et al., 2018), but they are a contentious bunch :-). I mention this because inputting Diplolepis rosae into Google Scholar didn’t get me very far back in time, 1951 to be precise (Bugbee, 1951). This paper justifies my good-natured jibe at the argumentative nature of taxonomists as he explains his renaming of what was then Rhodites rosae, by citing a 1917 paper (Rohwer & Fagan, 1917) who argued that the French entomologist Étienne Geoffroy (1725-1810) who raised the Genus Diplolepis in 1762, should take precedence over Theodor Hartig’s (1805-1880) 1840 Genus Rhodites. This pointed me back to the 1940s and the discovery that the mossy rose gall was then known as Rhodites rosae and I quote “I have recently published an epitome of my own experience e in rearing from galls of R. rosae” (Blair, 1945). A bit more delving and I found that in France in the 1930s and in the USA in the 1920s, it was still known as R. rosae (Weld, 1926; da Silva Tavares, 1930). Wending my back via citations I arrived in 1903 to find it listed as Cynips rosae (Ashmead, 1903).
Historical insights – Monsieur Wirey was ahead of his time
Armed with this knowledge, my journey back into the history of D. roase was much simplified and introduced me to a gem of a book, Insect Architecture by James Rennie (1787-1867) (Rennie, 1851). Here I found an interesting account of galls in general but a detailed exposition of the Bedeguar gall of rose as he described it, including this rather nice drawing.
Professor Rennie presents some hypotheses on the formation of insect galls in general;
“Many of the processes which we have detailed bear some resemblance to our own operations of building with materials cemented together; but we shall now turn our attention to a class of insect-architects, and who cannot, so far as we know, be matched in prospective skill by any of the higher orders of animals. We refer to the numerous family which have received the name of gall-flies,
- Wirey says, the gall tubercle is produced by irritation, in the same way as an inflamed tumor in an animal body, by the swelling of the cellular tissue and the flow of liquid matter, which changes the organization, and alters the natural external form. This seems to be the received doctrine at present in France. “
As you can see from the above he has little time for the French explanation (typical English exceptionalism) and puts forward his own idea that the galls are formed because the egg laying process blocks the vessels of the plant and the fluid that would normally flow unimpeded blows up the tissue surrounding the egg like a balloon. Of course he was wrong and M. Wirey was correct :-). Considering that he had no access to the sophisticated techniques we have he pretty much hit the nail on the head.
That aside, his book introduced me to an entomologist I had never heard of, Priscilla Wakefield (1751-1832), yet another overlooked and forgotten female scientist.
Although my plans for lots of great MSc projects were reduced somewhat I have had a lot of educational fun and I think that there are still some things that could usefully be looked at, long term recording across multiple sites which I hope the British Plant Gall Society is doing would be interesting. On my walks I noticed a lot of variability in size and phenology of gall formation. At the end of August I was coming across small very fresh looking galls at the same time as I was seeing larger more advanced galls.
A very fresh looking Bedeguar gall, August 26th 2020, Sutton
As far as I can tell, the timing of gall formation and its effect on final size of the galls has not been looked at in detail; do early galls enter winter larger than later formed galls, or is it entirely due to the number of eggs laid? Given the huge number of other inhabitants of the galls, at least fourteen different species (Laszló, 2001, there is probably a viable project in looking at the timing of invasion by the different gall parasites and the outcome this may or may not have on the final composition of the gall fauna.
Feel free to suggest additional projects in the comments.
Ashmead, W.H. (1903) Classification of the gall-wasps and the parasitic cynipoids, or the superfamily Cynipoidea. IV, Psyche, 10, 210-216.
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Blair, K.G. (1945) Notes on the economy of the rose‐galls formed by Rhodites (Hymenoptera, Cynipidae). Proceedings of the Royal Entomological Society, Series A, 20, 26-31.
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Bugbee, R. E. (1951). New and described parasites of the genus Eurytoma illiger from rose galls caused by species of the cynipid genus Diplolepis Geoffrey Hymenoptera: Eurytomidae). Annals of the Entomological Society of America, 44, 213–261
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