San Luis Obispo may soon see a surge of unexpected guests making a home in the airspace in front of downtown businesses as summer approaches.
Swarms of hoverflies, named aptly for the way they “hover” in flight, have become a familiar sight in downtown doorways and storefronts in recent years. This year, however, an unseasonably warm winter and lack of a freeze period may lead to a population boom, presenting challenges for business owners in the area, according to Ryan Perry assistant professor of entomology at Cal Poly
Due to the mild winter, insects became active much earlier in the season, according to Perry. Warm, sunny weather also accelerates the hoverfly’s life cycle, causing extra generations of the insect to appear each year.
The plethora of foliage and trees that line the streets draw hoverflies to downtown San Luis Obispo. These plants usually harbor aphids, small insects that feed on plant sap. The unusual weather this winter and spring caused aphids to boom in population, leading to more food for hoverflies and prime conditions for their populations to grow, Perry said.
Hoverflies are parasitoids, meaning they will lay their eggs near clusters of aphids, sometimes finding them by picking up chemical signals. Once the juvenile hoverflies hatch, they pierce into the aphid, sucking out sustenance then moving on to the next, according to Perry.
“They’ve basically just got this buffet of insects to go find, lay their eggs near and have the cycle continue,” Perry said.
In a typical year, hoverflies will keep the aphid population down through their consumption, but the warmer winter has caused both insect levels to soar.
“In a year like this, where it’s just super warm, it’s an arms race, and the aphids are winning this time,” Perry said. “You know, there’s more of them. Yes, there are more hover flies, however, they are unable to achieve complete control.”
In 2019, the city faced an overpopulation of houseflies, linked to an increased number of pigeons and whiteflies in downtown, as previously reported by The Tribune. Methods like putting pigeons on birth control and trimming the ficus trees infested by whitefly — whose secretion attracted the houseflies, were used to control housefly population.Â
Business owners respond to influx of hoverflies
Although houseflies and hoverflies are different species, the irritation they present to downtown business owners is similar.
Cherisse Sweeney, owner of Basalt Interiors, has worked at her Higuera Street location downtown for 15 years. She said the number of flies downtown has gotten “progressively worse” over the years.
Sweeney said there are times where she needs to sit in front of the store and shoo away the flies from the door to prevent them from swarming inside. Other times, she will close the door to her showroom to keep the flies out, which Sweeney says makes her showroom feel less welcoming and open.
“It’s not pleasant to have to walk through a funnel of flies to get into the show room,” Sweeney said.
Still, Sweeney said she would not like to see pesticides used on the ficus trees and plants downtown to diminish the aphid population.
“I don’t know what the right abatement solution would be that’s the least toxic,” Sweeney said. “I think to start with just if we can’t get rid of them easily, at least making the sidewalks feel like they’re clean. It just feels dirtier when the sidewalks are dirty, the streets are dirty, the atriums are dirty then there’s hover flies flying around.”
If the streets were to be cleaned with the consistency they once were, Sweeney said, the hoverflies would be a “little more tolerable.”
Management strategies for aphids need to start early and be “more strategic” due to the mild winter and summer-like conditions this spring, according to Perry. Once populations start increasing to unmanageable levels, a pest management strategy might be considered, he said.
“If the numbers get too high, you would then go in and spray the insecticides to just knock down that population to something that’s manageable,” Perry said.
Business owners also use mesh door curtains, strategically-placed fans and even Irish Spring soap to keep hoverflies away from their stores.
Steven Wick, one of the owners at Zen Shak, has resorted to using an electric fly swatter to prevent the flies from coming into the store. During hot months, he notices the flies get “pretty bad” out front, and eventually, they try to make their way into the cooler store.
“Sometimes I have to go out there and use [the electric fly swatter],” Wick said. “Just because if they’re out there, they’re gonna eventually come inside. But it gets rid of them real quick, [I’m] doing my part.”
While some business owners are bothered by the flies downtown, others see them as a trade-off to having a tree-lined downtown. Angela Hemler, the manager of Zen Shak, says she was initially “appalled” by the swarms of flies when she moved to San Luis Obispo. For Hemler, having an ecologically rich downtown, especially the ficus trees, which she calls a “staple,” is worth instances of overactive insects.
“Over time, I actually noticed that as you walk through them, they don’t hit you, they don’t bother you,” Hemler said. “It actually didn’t really seem to make an impact on me.”
Although their presence might be an annoyance, hoverflies are harmless and beneficial to the environment, according to Perry.
“It’s just people, they, you know, they have this natural gut reaction to see a fly,” Perry said.
“They’re gonna see flies, they assume filth and gross, these ones that are, you know, basically just hovering around like this, no, they’re, they’re looking for a mate, and then they want to go lay their eggs somewhere else.”
Perry said that the increase in hoverflies is part of a natural cycle, influenced by changes in the climate and the local ecosystem.
“It’s really just understanding this circle of life, if you will, of excess plants, excess heat equals excess herbivorous pests, or what we call pests. Because of that, there’s more of the predators and the parasitoids,” Perry said.
This story originally appeared in the May printed edition of Mustang News. Check out more stories from the issue here.