Hover flies pollinate flowers and eat aphids
Many of our native pollinating insects are not bees. Included in this group are the hover flies, also known as syrphid flies, flower flies, or drone flies. These are true flies—in the order Diptera—and they are easily recognized by their ability to hold a seemingly motionless position in the air.
Some of the hover flies have distinctive stripes on their abdomens—black and yellow bars that mimic the markings of stinging bees. These markings are a biological adaptation that protects the flies from certain predators even though they are completely unable to sting. If you are unsure if you are looking at a bee or a fly, remember that a fly has one pair of wings while a bee has two pairs.
Many adult hover flies survive on nectar and pollen, but the larvae eat a much more varied diet that may include other insects or decaying plants and animals. It is hard to generalize, however, because the family Syrphidae consists of about 6000 species—all with slightly different habits. They are found on all continents except Antarctica and are harmless to humans.
Some species of hover fly are highly prized as biological control agents because the larval forms of those flies snack on aphids. Hedgerows and cover crops can be planted which attract hover flies to cropped areas. Flowers that produce pollen and nectar provide the energy the adult flies need to produce large numbers of eggs—all of which turn into aphid-munching larvae.
An excellent publication on hover flies as biological control agents can be downloaded at http://anrcatalog.ucdavis.edu/pdf/8285.