Wasps with Gall!

Here's an attractive little critter from last week:

Gall Wasp (Cynipidae), Gander Mt. FP, Lake Co, IL  5/3/2014

Gall Wasp (Cynipidae), Gander Mt. FP, Lake Co, IL  5/3/2014

Emphasis on little. This is a Gall Wasp (family Cynipidae), one of 750 species. They earn their name by laying their eggs inside plant tissues, often including the oak trees found where I spotted this one. I don't know which species of plants this one parasitizes, since I have no way of knowing which one it is. (Bugguide shows more galls than adults, at least identified to species.)

Once the plants are parasitized, they form a structure called a gall. Some are simple balls while others can be impressively spiky; apparently many species of wasps can be identified by the form of the gall. (1) These galls are also used as homes by a number of inquiline species, similar to the Ant Cricket from a few weeks ago. The wasp larvae are sometimes parasitized by other hymenopterids as well. On top of all that, in some cases the galls are tended by ants, since they tend to produce honeydew. (2) Put all of this together, and you end up with a rather complicated little ecosystem growing up around these galls.

In the case of the ants, their presence both reduces parasitism rates on the Cynipid larvae and alters which parasites are likely to succeed. (2) Since gall-making comes at a cost to the plant host, (1) a reduction of parasitism on the wasps can't be seen as a good thing from the plant's point of view. This suggests that there's a negative relationship between the ants and the plants, not something I'd have guessed. On the other hand, it's possible that the ants reduce predation directly on the plant, and if so, the overall nature of their relationship could hinge on how important each of those factors is.

Inouye & Agrawal's work (2) focused on Argentine Ants (Linepithema humile), an invasive species from (you guessed it) Argentina. They note that interactions between the wasps, wasp parasites, and native ants show different dynamics, with some interactions dependent upon the density of galls that aren't so dependent with Argentine Ants.

So here we have an introduced ant in a mutualistic relationship with a native wasp, allowing, perhaps, for increased parasitism on a native plant, by reducing the success of other (mostly?) native parasitoid wasps. What effect that has at a slightly higher trophic level would be fascinating to find out.

(1) Stone, G. N., Schönrogge, K., & Atkinson, R. J. (2002). The population biology of oak gall wasps (Hymenoptera: Cynipidae). Annual Review Of Entomology, 47633-668.

(2) Inouye, B. D., & Agrawal, A. A. (2004). Ant mutualists alter the composition and attack rate of the parasitoid community for the gall wasp Disholcaspis eldoradensis (Cynipidae). Ecological Entomology, 29(6), 692-696.