Last week we visited hordes of female aphids, sipping sap on the luxuriant springtime growth of garden plants. As these ladies sip sap, they produce copious amounts of the sweet sticky liquid called honeydew. Honeydew is what remains of plant sap after aphids remove the goodies. Were it not for a posse of natural born killers who have a particular fondness for aphids, plants would likely disappear each spring under the sheer weight of aphids! The odors of honeydew and smells emanating from the plants provide airborne signals to many kinds of aphid hunters, including flower flies – a.k.a. hover flies or syrphid flies. These curious flies are well known to most gardeners. Often brightly colored and sometimes hairy, many hover flies resemble bees or wasps. This mimicry affords them protection from birds and nosy humans.
Adult flower flies are important pollinators of many kinds of flowering trees, shrubs, and herbaceous plants. Flower flies have the unique ability to hover like a helicopter and fly forward and backwards as they search plants for aphids. As aphids feed, excreted honeydew creates an aroma that acts like a dinner bell ringing “come and get it”. The more aphids and honeydew on a plant, the more likely it will be discovered by flower flies. Once the infestation is detected, the females fly lays a small white egg near the colony of aphids. The egg hatches into a gelatinous, wriggling maggot whose sole purpose is to hunt and eat soft-bodied prey. With no true eyes, this mass murderer discovers its victims by swinging its head to and fro, searching for prey with sensory structures located on the front end of its fleshy head. When it bumps into an aphid, it quickly snares its victim and sucks the fluids from its body.
Flower fly maggots have prodigious appetites. In the laboratory, I have watched these predators consume more than 25 aphids in a day. Reports of aphid carnage in the literature puts the casualty figures in excess of 200 aphids during the course of development for each maggot. In agricultural systems, flower flies are believed to provide 75% to 100% control of aphids. In my experience with aphids on roses, flower flies, with a little help from lady beetles, can entirely wipe out populations of aphids by the end of May. So, before you reach for the aphid spray, carefully look to see if the maggot brigade is at work. It is wickedly entertaining to watch these grotesque larvae hunt and capture aphids on the leaves and stems of roses. The aerial acrobatics of adult hover flies are equally engrossing as they hunt for aphids and pollinate plants.
In a matter of minutes, the flower fly maggot sucks the life from an aphid.
The interesting article “Oviposition behaviour and host colony size discrimination in Episyrphus balteatus (Diptera: Syrphidae)” by J.P. Sutherland, M.S. Sullivana, and G.M. Poppy was a reference for this Bug of the Week.
For more information on the biology of flower flies please visit the following web sites: