While visiting the market last week, I was intrigued by a bed of roses gracing the parking lot. On each bush, leaves glistened with shiny liquid and flower buds were speckled with the white cast skins of bugs. These were the classic signs of an aphid infestation. Springtime is aphid time and in this season of rebirth, female aphids give birth to spectacular numbers of babies all of which are females. 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. The poor roses would probably collapse beneath the sheer weight of suckers were it not for a posse of natural born killers who have a particular fondness for aphids. The odors of honeydew and smells emanating from the plant 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 hornets. This mimicry affords protection from birds and nosey humans. Adult flower flies are important native 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 back 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 female 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 victims by searching to and fro with sensory structures 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 put the casualty figures in excess of 200 aphids during the course of development for each maggot. In some agricultural systems, flower flies are believed to provide 75% to 100% control of aphids. In my experience with aphids on roses, flower fly maggots, 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 grotesque larvae hunt and capture aphids on the leaves and stems of roses. The aerial acrobatics of adult hover flies are equally amusing as they hunt aphids and pollinate plants.
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.