Fly with a fondness for legs - aphid midges, Aphidoletes aphidomyza

Last week we met voracious flower fly larvae as they ravaged populations of brown ambrosia aphids on my silphium cup plant. This week things have gone from bad to worse for the pesky aphids as populations of sneaky predatory aphid midges arrived in force and rained carnage on the aphids. In past episodes, we visited many wild predators like firefly larvae, tiger beetles, and spiders that employed powerful jaws or fangs to slice, dice, and consume hapless prey. Today we meet a predaceous midge, Aphidoletes aphidomyza, with a fondness for the legs of aphids. Hold on, a fondness for legs, what’s that about? Here's the story. Larvae of predaceous midges are many times smaller than those of the large flower flies we met last week. In fact, they are much tinier than the aphids on which they feast. These diminutive predators sneak up on the victims and with persistence and luck, they attach themselves to the joints on the aphid’s leg. After they are plugged in, they inject paralytic venom, likely produced in their salivary glands, into the blood stream of the aphid. In a matter of minutes, the paralytic action of the venom takes hold immobilizing the aphid. As I watched one of these attacks, I saw a small midge larvae intermittently strike and bite  the left foreleg of a small aphid. As the movements of the aphid subsided, the midge slowly slithered beneath the aphid and attached itself to the right hind leg of the moribund aphid. Half an hour later the midge moved to the left hind leg. With its victim motionless, the predator finally settled on rump of aphid for its meal. After subduing their victims, midge larvae may move to meatier parts of the aphid’s body such as the thorax where they may be joined by other larvae.  Some accounts report that these tiny assassins will kill more aphids than they consume. Not sure how to explain that. The aftermath of the midge’s carnage is blackened hulks of shrunken exsanguinated aphids. 

Aphids dance away from a predaceous midge larva searching for a victim. This one snared the left foreleg of a small aphid and is injecting paralytic venom. Next it moves to the right hind leg of the moribund aphid. Then on to the left hind leg. With the aphid motionless, the larvae settles on ‘rump of aphid’ for dinner. Nearby, two midge larvae feed on the head of an aphid. When these tiny monsters are done, all that remains of the aphid colony are shriveled black carcasses.

These brown ambrosia aphids are not entirely defenseless. Watch as one drop kicks a predatory midge right off a leaf. Here’s the instant replay at one tenth normal speed.

Bug of the Week thanks Dr. Shrewsbury for spotting predaceous midges in the aphid outbreak on the cup plant. The informative articles “Entomo-venomics: The evolution, biology and biochemistry of insect venoms” by Andrew Walker, Samuel D. Robinson, David K. Yeates, Jiayi Jin, Kate Baumann, James Dobson, Bryan G. Fry and Glenn F. King and “Aphid Predatory Midge” by Jim Walgenbach were used as references for this episode.