We have been seeing syrphid flies (family: Syrphidae) in great abundance this summer over at the Garden Ecology lab’s research garden, so much so, that our field research team has begun to call it the year of the syrphids! These bee-mimicking, skittish pollinators have particularly loved the native and cultivar yarrow we have planted in our plots. Although their abundance has recently dipped–likely because Yarrow (Achillea millefolium) is done with its first round of bloom–we still see them buzzing around.
Syrphid flies, also known as flower flies or hover flies are a common visitor of gardens. You may see them buzzing around bright flowers or fighting mid air. They are important pollinators and feed off of nectar and pollen in their adult stage. Additionally, in their larval form, they are great at reducing aphid populations, but are extremely susceptible to pesticides.
The life cycle1 of syrphids start with the adults laying eggs in leaves of infested plants. After about three days, they hatch into their voracious, blind, larval stage.
The larvae feast on small pests like aphids, leafhoppers, scales, and thrips. The larvae do this by moving along plants, lifting their heads to try and seize and pierce their prey with their triple-pointed dart inside their mouth2. After slurping their prey dry, they will discard the exoskeleton.
Larvae will develop through a few instars and after 1 to 3 weeks will go into a pupal stage on the host plant or on the soil. After two weeks, an adult emerges.
In the pacific northwest, our common syrphid is Scaeva pyrastri. It is unique in that rather than overwintering as a larvae, S. Pyrastri overwinters as an adult. Three to seven generations occur in a year, with possibility for the higher counts depending on the region and species. Another species, originally native to Europe, the drone fly (Eristalis tenax3) is named after male honey bees because it is mimics them so well. Other mimics in Syrphidae lay their eggs in the nests of bumblebees or social wasps, where the larvae eat dead bees and detritus.
Their quick movements and bee-like appearance can make syrphid flies hard to identify.
To identify a flying insect as a syrphid, look for a single pair of wings. Flies (Order: diptera) do not have a second pair of wings like bees. Instead they have a vestige of hind wings called halteres that look like little nubs beneath their wings. These act like gyroscopes to help the fly balance during unique in-flight maneuvers. Also look for large, forward facing compound eyes typical of any dipterans. In our lab, we’ve see a wide range of size and different colors. Syrphids can be anywhere from a tenth of an inch to half an inch long, and have black or brown bodies with white or yellow spots and stripes. Fun fact: most hover fly mouths are extendable ‘sponges’ that mop up nectar and pollen.
Flower flies are extremely important to pest control and pollination, 40% of syrphid species larvae feast on the previously mentioned prey, and each larvae can eat up to 400 aphids during development!
Unfortunately, the larvae of syrphids are similar to many other species so are hard to identify. However, they are usually on pest infested plants and may be seen near adult syrphids. Look for their typical ‘stretching’ behavior while they are on the hunt. If you have a pest problem, avoid using pesticides or insecticides! These kill the syrphids that can help with pests. Instead, promoting syrphids or other pest eaters like ladybugs and lacewings by providing a variety of insectary plants can help you in the long run.
As previously mentioned, yarrow (Achillea millefolium) has been our most successful syrphid-attracting plant in our lab this year. Syrphid flower preference varies based on the subfamily, according to studies. The subfamily Eristalinae is attracted to white flowers, Pipizinae prefer white and yellow, and Syrphinae is more general. Link to an article going more in depth on syrphid flower preference here4.
Not coincidentally, native yarrow is primarily white, while our cultivars are yellow and pink. Observationally, syrphids visit yellow yarrow at a similar rate as they do the native, while our pink cultivar saw next to no syrphid visitors. We recommend planting yarrow as well as a variety of native flowering plants to support these pollinators. Leave leaf litter and debris around flowering plants, too. These provide protected overwintering sites which syrphids rely on7.
Currently, no syrphid species are on the U.S. Endangered Species Act lists, but like many insects, this underappreciated pollinator is understudied and biodiversity of this group is not well tracked. In Britain, however, some hoverflies have been placed on their Biodiversity Action Plan.6
Whether syrphids are endangered or not, we can help biodiversity by promoting native pollinators and planting native plants in our yards and gardens.