What’s with all the flies in SLO? Here’s what the city’s doing to get rid of them

If you’ve shopped or dined in downtown San Luis Obispo recently, you may have noticed annoying swarms of flies buzzing around outdoor gathering spots.

There are so many, in fact, that the city has received complaints from downtown business owners and visitors who have encountered the nuisance flies, and it’s launching a pest management strategy to get the bugs under control.

The flies have been particularly prevalent in recent weeks at patio at spots like Doc Burnstein’s Ice Cream Lab, Urbane Cafe, F. McLintocks and Blast 825 Taproom, among other downtown hubs.

To make matters worse, their numbers are expected to grow now that summer is nearly here, city officials say.

So why are they here, where are they coming from and what can be done?

One pest linked to others

The housefly spike is connected to increased numbers of pigeons and whiteflies in the downtown area, which have an interconnected ecological relationship, said Freddy Otte, the city’s biologist leading the pest management strategy.

The pigeons and whiteflies create prime breeding and feeding habitat, respectively, for dark-colored houseflies.

“We have a multitude of efforts moving forward in the downtown core right now,” Otte said. “Everything is connected.”

A pigeon roosts in the creek tunnel that exits near Luna Red in San Luis Obispo. The city says the birds’ droppings are a breeding ground for houseflies, which have been infesting the downtown. David Middlecamp dmiddlecamp@thetribunenews.com

One big problem is a community of about 500 pigeons living in the tunnel that channels San Luis Obispo Creek under the city, exiting near Luna Red and The Network, Otte said.

“The waste and filth from the pigeons are what’s providing the breeding habitat for the houseflies,” Otte said.

Additionally, houseflies are attracted to the honeydew secreted by whiteflies that suck juices from the roughly 400 ficus trees in the downtown area.

The tiny whiteflies typically aren’t noticeable to people passing by as they tend to hover high up in the trees, Otte said.

Fly populations also develop more rapidly in warmer conditions during and after spring blooms, meaning the problem could grow this summer, Otte said.

“The black houseflies don’t bite,” Otte said. “They just buzz around. But it’s a nuisance for people walking in the downtown core.”

Whitefly crop.jpg
The adult form of a whitefly, the type found in downtown San Luis Obispo on ficus trees. Courtesy City of SLO

Measures to reduce pigeons

The city is using a multifaceted pest control approach, starting with pilot programs, to gauge the effectiveness of the measures, Otte said.

The mitigation plan includes placing pigeon feed laced with birth control on the roofs of private properties in downtown SLO. The bait interferes with the ability of the pigeon’s eggs to hatch, making them sterile.

“The pigeons will not be harmed and we’ve informed several groups, including PETA and Fish and Wildlife, that we’re doing this,” Otte said. “The birds will be fine, but they just won’t reproduce.”

Otte said the pigeons produce more than 12 tons of waste annually, factoring in that 100 pigeons can produce more than 4,000 pounds of waste.

The pigeon droppings also have contaminated the creek, which recently tested positive for excessive fecal coliform levels, Otte said.

Pigeons have a life span of two to three years, thus it likely will take a few years to see the downtown pigeon numbers dwindle.

Otte said that environmental permits aren’t required to use the pigeon birth control.

Measures to reduce whiteflies

Over the past few months, the city also has trimmed several of the ficus trees, which has helped to reduce the habitat for the whitefly populations, city officials said.

“Intermittently, most of the pruning was completed after the holidays and as recent as (last May),” said SLO arborist Ron Combs. “We have completed this work for the most part except for a couple stragglers outside of the heart of the core.”

The city will also deploy non-stinging wasps and beetles to combat the whitefly problem in the trees, Otte said.

Later this month, a microscopic, parasitic native wasp will be used to attack a species called the ficus whitefly, Otte said.

ficus wf Encarsia emergence hole.jpg
A nymph that was parasitized by a local native, microscopic, stingless wasp. The hole visible in the photo is where the wasp emerged to kill the pest whitefly. Courtesy City of SLO

Local native, microscopic, stingless wasps parasitize the whitefly nymphs (infants) and kill the pests.

In July, the city is planning to release a nephaspis beetle to attack nesting whiteflies.

Otte said he has consulted with Cal Poly agriculture professor David Headrick, who specializes in pest management, to help address the issues.

Headrick, who previously worked on whitefly control in the city, is helping by coordinating the transport of the beetle that has been used to effectively attack whiteflies in the San Diego area.

“Hopefully, by three months and by next summer, we’ll see improvements to make further decisions about what to do with all these pests in the downtown,” Otte said.

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Nick Wilson covers the city of San Luis Obispo and has been a reporter at The Tribune in San Luis Obispo since 2004. He also writes regularly about K-12 education, Cal Poly, Morro Bay and Los Osos. He is a graduate of UC Santa Barbara and UC Berkeley and is originally from Ojai.