all pictures © robin williams

revised 7-7-2021

Gall insects

causers, inquilines & parasitoids
in oak and rose bedeguar galls

Many plants and trees in Britain have galls on their leaves, roots, stems, twigs or trunks. Galls are growths on a parent plant that provide a food store for the causer. Hymenopteran galls are formed from the cells of the parent plant as a result of a reaction to the hatching of eggs laid in the plant tissue by a parasitic wasp, or ‘causer’. Others result from the action by other insects, fungi or bacteria. The first time I looked into the extraordinary life and population of an oak-gall, I was hooked by the numbers involved, the exotic colours of many of the insects and their extraordinary life-histories. For instance, I found that one common gall had the potential for containing over 30 different insects directly involved in the gall and its food-store. How come? There are three main types of these insects.

Gall-causers, with a single distinct species for each specific gall. Her eggs hatch after injection into the tissue and the subsequent gall is always to the same design. Each gall is a food store for the young causer, formed by multiplication and growth of the plant cells to a precise pattern which does not vary for that species of insect. There are many shapes and sizes of gall, from soft pimples on leaves to large, hard-skinned galls formed on buds or even on woody trunks.

Inquilines are insects which do not cause the gall, but lay their eggs in its tissue, so that its young may benefit from the food within the gall. They do not set out to conflict with or hurt the causer larva or pupa, but may do so by starving the other inhabitant through consuming its vital food store.

Parasitoids prey on the causer, inquilines or even other parasitoids – as in ‘small fleas live on bigger fleas’. These insects are parasitic wasps, often brilliantly coloured, ranging from just over 1mm in length to over 6 in extreme cases, though many are between 3 and 4mm long. They lay their eggs either in the tissue of the prey larva, or on its surface. In both cases, the end result is that their eggs hatch and eat the living larva, eventually killing it. 

Insects other than these may be found emerging, but they are so-called ‘followers’. These are insects that use the old galls as shelters or even lay their eggs in the outer crevices of a weathered skin. They are not classed as gall insects if they do not feed on the gall food store, or its associated insects. Learning about galls is a fascinating, time-consuming, but rewarding business. The British Plant Gall Society (BPGS) is devoted to the study of plant galls in Britain and is a good place to look for guidance and help.

Photography of the various inhabitants is demanding, as they are so small, but endlessly worthwhile. The sculpture, colours and shapes are extraordinary. It is amazing to watch them emerging from the galls, drilling though the outer skin and bursting into the light. Some chalcids are brilliantly coloured - metallic golds, reds and greens - all for only a brief period in the light when it is able to attracting a mate. The majority of their time is spent in pitch blackness inside the gall. Rearing insects from galls depends on their type. Hard galls, such as the Oak marble-gall, are relatively easy. The perfect nursery for these is an old transparent film container; keep the gall in it and store in a cool, dry place, such as a garden shed. Eventually you will see insects pouring out and it is time to check the variety of creatures emerging. Some of the tiny soft galls found on the underside of leaves are much harder to keep, as they tend to go mouldy and rot when removed from their parent plant, but strict hygiene and care will eventually yield results. Another method is to enclose the gall in fine gauze while on the tree, keeping careful check for emergences every day.


A common oak-gall first got me interested in the insects involved in its biology. I put one of these in an old film pot and kept an eye on it. A few days later insects started to emerge through tiny holes drilled in the hard skin and I was treated to flashes of the most brilliant metallic colouring. This had me hooked. The eventual end-product was a two volume book on Oak-galls in Britain, after many years of work. That particular gall, the Oak marble-gall, turned out to have  33 different potential inhabitants, exciting in its own right, but it is also an example of an even stranger facet of insect biology.

Oak-galls causers have a most extraordinary story which is almost impossible to believe when you first come across it. These cynipid wasps exist in two separate states, but both have the same DNA, while looking and behaving entirely differently. Let me explain. The insects have two generations; one of conventional males and females and a second where the females are asexual, or agamic – producing fertile eggs without the benefit of a male. This on its own is extraordinary, but the end results differ completely. See Andricus kollari below. The first picture is of the agamic female, the second of the sexual female. It is quite amazing how anyone realised they were even related.

The agamic generation, which produces the Marble-gall, is a bulky wasp, hunched, around 5mm long and a rich golden-brown in colour. In contrast, the sexual wasp is around 1.5mm long, black and slender. The agamic Marble-gall is round and hard, usually around 10mm in diameter, set on the end of twigs on Pedunculate or Sessile oaks. The sexual gall is under 2mm long and soft-skinned, set in the buds of Turkey oaks. Yet their DNA is identical.



Some years ago, the British Plant gall Society decided to take an in-depth look at The Rose bedeguar, or Robin’s pincushion gall (Dipolepis rosae), to try and understand more about its inhabitants and their biology. The scientific name of the gall is taken from the cynipid causer. At the time it was common and, like the Oak marble-gall, easy to rear out in the same type of container. Numbers have declined considerably since, which may well be part of a normal cycle where current numbers of parasitoids influence the population of causers. The first result of our study was the production of a volume of desk research, designed to dig out as much of the existing information as possible – as far as we could see this had never been brought together in one entity before. Simon Randolph produced a quite excellent book containing a mass of information which fulfilled the objective perfectly. At the same time, a programme of field-research was instituted that ran for the next twelve years, with members collecting galls and rearing insects all over the country; the records being collated through a central MapMate recording program. The results were impressive. Simon’s original desk research showed there were 14 possible inhabitants – a causer, a single inquiline and 12 parasitoids, of which one was an ichneumon and the rest chalcids. The field work concluded eventually that there were actually 18 possible inhabitants. Among these, three chalcid species were discovered that were completely new to the British bedeguar although present in other British galls. It is interesting to note that the bedeguar causer does not have two generations, but reproduces in the usual manner, unlike oak-gall causers.