by Courtney Llewellyn

Flies may have been given their name because of what they do, but a more appropriate moniker may be pests. And even though they’re called house flies, their impacts on animal facilities are noteworthy: They are a nuisance to facility employees and those living in farm areas, they transmit a number of pathogens (including E. coli and salmonella) and they leave behind “fly specks” (spots of regurgitation or defecation).

There are a number of reasons you might want to monitor your house fly populations, according to Alec Gerry, a veterinary entomologist at UC-Riverside, who presented on the topic during this year’s virtual World Ag Expo. “As part of an IPM program, you want to maintain the fly population below the economic ‘injury level,’” he said. “Identify an action threshold – a population level we don’t want pests to get past – to avoid nuisance complaints and to decrease incidence of disease.”

In addition to making sure farms don’t reach these injury levels, there may be regulations requiring fly monitoring at federal or state levels. Monitoring is part of the egg safety final rule from the FDA.

Gerry said over time, you can easily collect data to see how fly activity changes over seasons or even years. That will help you see a historical “normal” amount of fly activity.

There are many ways to conduct fly monitoring, but Gerry said the most important one is catching the flies. The first patented conical trap for this purpose was created in 1872, and other options and designs were invented over time. “The problem with an instantaneous count is that fly activity varies by time of day,” he noted. “In the morning hours, there is higher fly activity than in the afternoon or evening. It also varies with environmental conditions, like light intensity and temperature.” Counts will be higher on warm and hot days.

You can do station counts (at a particular spot in your facility) or use a moving fly ribbon (a sticky ribbon carried in front of an employee walking through your facility) for slightly more accurate counts.

“We generally recommend fly monitoring devices that work over multiple days. The fly ribbon, which was invented in 1908, is very easy to use,” Gerry said. The baited jug trap (patented in 1979) also works, and you can make your own by taking a gallon milk jug, cutting holes in the sides and placing fly bait inside. There are a variety of other sticky traps too.

Another recommended fly monitoring device is a spot card – often just a white index card. Since resting flies leave fecal and regurgitation spots, this allows for a measure of overall fly activity, as several fly species will leave spots.

“How many monitoring devices are needed? It depends on the sampling precision you’d like to have and the cost and time,” Gerry said. “It’s a balancing act between the two considerations.”

For poultry houses, Gerry recommended fly ribbons, jug traps and spot cards. You should record activity for two to seven days. For outdoor facilities (beef, dairy and equine), you’ll have to use some different methods. They must be able to withstand dust, wind and rain. Sticky traps, spot cards and baited traps should be spread across your facility in protected locations, and you should record activity for seven days.

(If you don’t want to take the time to count all the tiny dots on a spot card, Gerry said you can use veterinaryentomology.org/flyspotter-house-fly-monitoring, which offers a scanning and counting feature.)

“Once you’ve been doing monitoring for some period of time, it gives you a historical monitoring record to determine the fly activity level associated with negative impacts,” he said. “Choose the monitoring device that’s appropriate for your facility type. Spot cards are probably the simplest to use, but one day we hope to have an automated fly monitoring system. We’re not there yet, but some labs are working on this.”