100 More Species #22: Aphid killer
Aphid killer, Tribe Syrphini
Alexis noticed this tiny caterpillar surrounded by aphids and ants on the underside of a nasturtium leaf. We deployed our excellent caterpillar guide, looking especially at the slug caterpillars, but came up empty. I looked for nasturtium in the list of caterpillar host plants, but only found the cabbage white, which this was not.
I decided to try bugguide.net, searching for caterpillar + aphids, figuring this relationship probably had been documented before. I found a post making the same wrong assumption we had: that this was a caterpillar. The assumption had been corrected: no dear fool, this is a fly larva. More precisely, it is the larva of a hover fly. Some hover flies start their lives as aphid killers, sluglike maggots that creep along plants gobbling up aphids as they go. There's yet another reason not to kill insects that resemble bees and wasps if you're not sure what they are. The bee-mimic hover fly you kill today won't produce any aphid killers tomorrow.
Astute readers will have realized by now that I have, yet again, cheated. This larva may very well be the same species as one of the adult hover flies I've already counted this year. Alas, there's no way to know, without collecting the aphid killer and rearing it to adulthood--these animals are not as well studied as many others, alas. But I thought the story and the creature were interesting enough that they belonged here, in spite of possibly repeating the same species.
