Bands of yellow brown on flower flies mimic those found on stinging insects. This may deter would be predators from attempting an attack on the harmless flower fly.
The Brooklyn Botanical Garden is one of the true treasures of New York City, boasting some of the finest interpretive plantings and displays of plants to be found anywhere in the world. Last week while visiting the garden on a brilliant autumn afternoon, I was delighted to behold an incredible montage of late summer pollinators including monarch butterflies, carpenter bees, and skippers busily working autumnal bloomers like tartarian asters and foxgloves. Among the most beneficial of these seekers of nectar and pollen were the wonderful flower flies, also known as hover flies or syrphid flies. These curious flies are well known to most gardeners. Often brightly colored and sometimes hairy, many flower flies resemble stinging aerial acrobats such as bees, wasps, and hornets. This mimicry affords protection from hungry predators like birds and also deters close inspection by 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 for food.
On bright summer days in autumn, flower flies will be seen on a wide variety of late bloomers.
In addition to providing the critical ecosystem service of pollination, flower flies are important members of Mother Nature’s hit squad. You see, the larvae of many species of flower flies are maniacal predators of garden pests such as aphids and spider mites. As aphids feed, they excrete sugary liquid called honeydew. Honeydew creates an aroma that serves as a dinner bell ringing “come and get it” for aphidophagous flies. The more aphids and honeydew on a plant, the more likely aphids will be discovered by flower flies. Once the infestation is detected, the female fly lays small whitish eggs near the colony of aphids. The eggs hatch into gelatinous, wriggling maggots whose sole purpose is to hunt and eat soft-bodied prey. With no true eyes, these mass murderers discover victims by searching to and fro with sensory structures on the front end of their fleshy head. When a larva bumps into an aphid, it quickly snares its prey with a mouth hook and sucks the fluids from its body.
After hatching from eggs laid by the female flower fly, flower fly maggots go to work devouring soft bodied pests like aphids. The pulsing organ running along the midline of the maggot’s back is its heart.
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 can entirely wipe out populations of rose aphids in a matter of weeks. But to have this important service of aphid destruction in your garden or landscape there must be a source of nectar for adult flower flies nearby. So, as you plan your garden for 2017, do like the folks at the Brooklyn Botanical Garden have done and be sure to add beautiful autumn flowers to the mix. In this way you can enjoy the beauty of blossoms and pollinators like flower flies, while reaping the benefits of the maggot brigade as they remove pests from your plants.