Yesterday I strayed past a cluster of crabapples with cupped leaves dripping with honeydew – the work of aphids. No worries, closer inspection revealed that the good guys had arrived. In this case the good guys were larvae of flower flies, maggots in entomological jargon, or the immature stage of a fly. At first glance these predators conjure images of Jabba the Hut – legless, undulating masses of insect flesh. Like Jabba, many species of syrphid larvae have a particular fondness for flesh and in this case the flesh was the horde of aphids on the crabapple leaves.
Flower fly larvae navigate the leaf surface in a series of undulating motions. Although they lack a true head, the anterior end of the maggot is loaded with surveillance equipment. It searches to and fro until it contacts an aphid on the leaf surface. It then snares its hapless prey with some rather nasty mouthparts and sucks the blood from the victim. This often involves rearing back and lifting the aphid into the air. When the prey is sufficiently drained, it casts the carcass aside and wriggles off to find the next tasty morsel. There are reports of flower fly larvae consuming as many as many as 1200 aphids during the course of their development. Little wonder that aphid problems can disappear before your eyes once the syrphids are in the house. Not all syrphid larvae are predators of soft-bodied insects like aphids or thrips. Some eat fungi or pollen and others such as the rat-tailed maggot live in polluted waters where they filter food from the water.
Adult flower fly on flowering perennials. They mimic bees and wasps.
What of the adults? These fantastic flyers are known by a variety of names: flower flies, hover flies, syrphid flies, and drone flies. If you grow perennial plants such as daisies or chrysanthemums or walk through flowering meadows, you will see these bee or wasp-like insects resting on flower heads or searching plants for colonies of aphids. Take a moment and count the wings. If you see two wings, not four, then these are likely to be the adults of the aphid-eating maggots described above. They have the ability to hover and fly forward and back much like a helicopter.
It's easy to see how someone might mistake the adult flower fly for a bee or wasp.
Their striking resemblance to stinging insects such as bees and wasps probably affords protection from birds and other predators that have learned to steer clear of stinging insects. The adult flies are not predators at all. They feed on nectar and pollen. When the female hover fly encounters an aphid colony she lays a small white egg near the soon-to-be victims. The eggs hatch and the syrphid larvae enjoy a feast of aphid tartar. So, to gain the benefit of these marvelous maggots and enjoy the antics of hover flies, provide a constant array of flowering plants in your garden and landscape.