Do Gallwasps Compete?

Galls sometimes exist at enormous densities on a given leaf or shoot. Does limitation in the availability of egg-laying sites, or of resources in a particular host plant part, lead to direct interspecific and intraspecific competition in gallwasps? An important distinction needs to be made here between interactions among gallwasps in different galls, and among cynipids within the same multilocular gall. There is evidence for competition between individual galls, but within individual galls the interaction among the cynipid larvae is more likely be one of facilitation.

Several studies suggest that where many individual cynipid galls exploit resources channelled through the same host organ there can be intra- and interspecific competition. The most informative study is on the leaf galls of the asexual generation of Cynips divisa. The galls develop along the lateral veins on the underside of oak leaves in summer and autumn, and this study examined the impact on three traits (gall size, female fecundity and survivorship) of gall density on individual leaves and of gall order along an individual vein. Survivorship decreased significantly with gall density, and survivorship, fecundity and gall size decreased with distance of a gall from the leaf midrib on multiply galled veins. These patterns are consistent with intraspecific competition for limiting resources.

Many gallwasps induce multilocular galls (containing many larval chambers), and the abundance of this trait in many other gall-inducing insects suggests that individuals developing in the same structure need not compete. Oak gallwasp galls containing a larger number of larvae are themselves larger and heavier, suggesting that the effect on plant investment in gall tissue of multiple larvae is additive rather than competitive. There may, however, be potential for competition between the offspring of different mothers (foundresses) developing within the same multilocular gall. It has usually been assumed that the larvae within a single multilocular gall are the offspring of a single female, but population genetic evidence suggests that this is not always the case. A proportion of the multilocular galls of Biorhiza pallida and 4 Andricus species (A. coriarius, A. lucidus, A. pantelli and A. seckendorffi) all contain the offspring of more than one female (multiple founding) (Atkinson et al 2002, 2003). In all cases, one female was inferred to be the mother of most of the wasps emerging from a single gall.

This pattern suggests two alternative explanatory scenarios. First, females may lay eggs in the same bud in equal numbers, but the eggs of one female come to dominate gall induction, and their development smothers those of subsequent foundresses (as for lethal inquilines). This might well happen if one batch of eggs was laid long enough before the second to initiate gall induction. An alternative non-competitive hypothesis is that the females contributing smaller numbers of emergents actually laid smaller numbers of eggs. Distinction between these two requires an ability to assess how many eggs are laid in the same oviposition site by each foundress.

A second issue arising from the demonstration of multiple founding is what causes more than one female to use the same site. All of the species surveyed only ever gall a small proportion of host buds. Are eggs only laid in this small proportion, or are eggs laid in a much larger distribution of buds, but fail to induce galls? There is good evidence that oak cynipids have specific oviposition preferences. If cynipid requirements for oviposition sites do depend crucially on the state of individual targets, and host organs vary in suitability in an individual host, then (as for the plant vigour hypothesis) female traits allowing recognition of high quality sites should have been favoured by selection. In this case, multiple founding could result from competition for limited suitable oviposition sites. If this is true, experimental manipulation of the relative density of suitable sites and ovipositing females should result in predictable changes in levels of multiple founding. Again, lack of knowledge of the precise requirements of cynipids makes the design of such experiments extremely difficult!