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Have you ever seen a small flying insect with black-and-yellow bands that looks much like a bee – but unlike a bee, it hovers like a helicopter over flowers and plants? It can fly straight up or down, backwards or turn in almost any direction too. If you have seen this agile insect, you have seen a hoverfly.
Also called a flower fly, this insect often mimics a bee’s coloration to keep predators away. A hoverfly does not sting, though, and you could hold one in your hand without fear.
Scientists have identified more than 6,000 species of hoverflies (more than 200 live in Massachusetts), so there is not space here to describe each one. Hoverflies have many shared qualities that I can discuss.
Adult hoverflies consume nectar and pollen. Female hoverflies need protein from pollen to produce eggs. In the process of dining, hoverflies pollinate flowers (especially white or light-colored flowers). Next to bees, flies are the most important pollinators. And hoverflies are the most impressive of the fly pollinators. As these flies visit flowers, pollen sticks to their body. The flies transport these pollen grains to other flowers, where the pollen grains fall off and fertilize that flower, allowing it to fruit. Hoverflies can carry pollen over greater distances than bees and pollinate many plants that grow in your garden or yard. Look for these flies on carrots, apples, peppers, onions and strawberries.
Many species of hoverflies produce carnivorous, predatory larvae. These larvae feed on aphids and other soft-bodied, sap-feeding garden pests. Some larvae can eat 400 aphids per week – making hoverflies a gardener’s friend. According to Cornell’s College of Agriculture and Life Sciences, hoverfly larvae can reduce aphid populations by 70 percent to 100 percent.
Some female hoverflies look for aphid colonies near which to lay their eggs. Each egg is small and white. The female glues the egg to the plant. In a few days, a tiny maggotlike, legless larva hatches from the egg. Each translucent larva is blind and relies on touch to find aphids. The larva wiggles its head back and forth, feeling for prey; once it brushes against an aphid, it pierces the skin with its mouth hooks and sucks out the fluids, leaving behind the carcass. One large larva can devour an aphid in fewer than two minutes, consuming as many as 50 per day. The more aphids the larva eats, the larger it grows. As it outgrows its exoskeleton, it sheds it, progressing through several stages. After it reaches the last stage, it turns into a teardrop-shaped cocoon (pupa) on the leaf or in the soil. Pupae need dampness or high humidity to develop into adults, so if its leaf is dry, the larvae will move away from its host plant to pupate elsewhere. It searches for moist leaf litter or loose, damp soil. The pupal stage can last for weeks or months. Eventually the pupa metamorphoses into an adult fly.
Hoverflies that eat aphids go through a complete life cycle, from egg to adult, in about two to four weeks. They have five to seven complete generations per year.
You can tell hoverflies from bees because flies have large eyes and only two wings. Bees and wasps have small eyes and four wings. It is also easy to tell most hoverfly males from females: males have eyes that touch each other, and females have a gap between their eyes. Although we mostly see adult hoverflies, this stage does not last long. Most adults live only a few days – just long enough to mate and lay eggs.
What can you do if you want to encourage hoverflies in your garden? That’s easy! Studies have shown that hoverflies stay in areas longer when there is plentiful nectar and pollen, so you can plant flowers that attract hoverflies. Scientists at the University of New Hampshire have learned which flowers attract the most hoverflies. Sweet alyssum (Lobularia maritima L.) is the clear winner because not only does it attract the most hoverflies, but it blooms the longest – from late spring until the first frost. (It is not native and prefers cooler temperatures, though, so during the hottest summer months there might be a lull in flower production.) Other plants that attract hoverflies include cilantro, dill, fennel, yarrow, butterfly weed, monarda (wild bergamot), cosmos, marigolds, lavender and goldenrod.
The most abundant and common hoverfly in North America is the margined calligrapher (Toxomerus marginatus), an aphid eater. In Massachusetts, you might spot this fly anywhere from mid-March to early October. Other common hoverflies are the transverse-banded flower fly (Eristalis transversa), also an aphid eater, and the narcissus bulb fly (Merodon equestris).
Not all hoverfly larvae are carnivorous aphid-eating machines: Some hoverfly larvae consume decaying organic matter (look for maggots in your compost) and still others scavenge from bee or wasp nests. A larva of the narcissus bulb fly, as you might have guessed, eats your flower bulbs.
If you spot a hoverfly darting among your flowers, appreciate what it is up to: pollinating your blooms and ridding your garden of insect pests.
Reader photo
Connie G. spotted this eight-spotted forester moth in her Colorado backyard on June 20.
Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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