The Aphid Midge - Aphidoletes aphidimyza

The aphid midge is a tiny mosquito-like fly whose young (larvae) prey on aphids. Widely used in commercial glasshouses, these insects will eat 60 different aphid species, and are a very useful predator. Some growers even keep their midge populations going by providing "banker plants" like grasses or nettles where aphids like to congregate.

The midges fly around searching for aphids, and when they find them they lay their eggs very close to them so that when the young emerge they have plenty of food. (Aphids are a bit of a sitting target as they anchor themselves to a leaf with the long sucking pipes that they use to feed on the plant sap). The midge larvae are little orange maggots too small to see with a naked eye, but they have large powerful jaws and quickly start devouring aphids leaving blackened, collapsed bodies attached to the leaf. When they have gorged themselves on their prey and gone through two or three "moults" they reach about 1-3mm long. They can be seen with a lens or microscope - fat red or brown maggots in among the aphid clusters

Research in USA (outdoors) suggests that you need quite high densities of midges to combat a significant infestation of aphids.  The estimate is one larva to every 15 prey. Nevertheless this predator has one or two advantages over other methods of aphid control, and in particular...

• Midges are nomadic, so they will fly around seeking out their prey. They will probably be able to find the first aphids of the season before you do.
• Because they travel far they are suitable for treating trees, even outside. They are widely used in orchards in the USA.
• The larvae will eat all the different aphids typically found on plants, and are not adapted to any particular species
• They can thrive on relatively low amounts of prey, so are a good preventative measure, which is the most successful type of biological control.
• This is a killer rather than a parasite - this is important for ornamental plants because the leaves aren't spoiled. 

The larvae pupate on moist soil and then the adult insect emerges and flies around looking for a good nursery site. Adults eat honeydew (the residue from aphid attacks) and thrive best at about 20-26 degrees with high humidity. When the short days of autumn arrive the insects slow down and may overwinter as pupae.

Aphidoletes aphidimyza as pupae mixed in vermiculite. Unit: 1000 in 250ml bottle.