Where are the Galls?

by Carl Strang

In site monitoring we are recording the ongoing story of a place, and change is the currency in which we trade. Some changes are easier to notice than others. When something newly appears on the scene, there is a good chance we will pick it out. A tree is down that was standing the previous day. A bird new to the site starts singing. The mother raccoon’s tracks now are accompanied by her babies’.

Less easy to notice are absences, things that have failed to appear as usual, especially if they are small. Such was the case for me this year at Mayslake Forest Preserve with respect to the goldenrod galls. Last week as I made my way through the prairies and meadows, I realized that there were very few galls, and I started to pay attention. I saw no spindle galls of the moth caterpillar (Gnorimoschema gallaesolidaginis), but usually there are few of those anyway. In the three previous winters there were dozens to hundreds of bunch galls (produced by the midge Rhopalomyia solidaginis ) and especially of ball galls (stimulated by the gall fly Eurosta solidaginis) along the route I was taking.

A goldenrod bunch gall

A goldenrod bunch gall

This year I counted 2 bunch galls and no more than 15 ball galls. If only one species were involved, I would suspect that a specialized disease or parasitoid had caught up with them and caused a population collapse. There were two, however, and furthermore they belong to different families of insects.

A goldenrod ball gall

A goldenrod ball gall

The ball galls mostly were in little bunches, suggesting that each group was the product of a single female. The places they were clustered seemed unremarkable, though possibly the plants were larger in those spots (smaller and thinner in most places). It seems unlikely that parasites of both species would have knocked them down in the same year. It’s tempting to blame the drought for all of this, as it was the oddest aspect of this past year, but it’s also true that the winter was relatively mild, and other factors I haven’t perceived may have come into play. For now this must remain an open question.

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