Dr. Sonja Swiger, Texas A&M, said houseflies are more than just a nuisance. They contribute to the issue of antibiotic resistance.

“Houseflies are known pathogen carriers of bacteria resistant to antibiotics,” said Swiger. “This is a concern when we’re taking care of animals and also trying to get rid of insects.”

The muscid (house) fly is heavily driven by its food source and what it wants to consume. “Food sources are both sugars and proteins,” said Swiger. “Adults need both of those, especially female stages. Most flies need a protein source to produce eggs.”

Flies are attracted to livestock for several reasons, including their preference for manure and eye and nasal secretions. “That’s when there are issues,” said Swiger. “Flies are going to manure then going to those secretions, which leads to transmission of pathogens.”

One of the main ways to limit flies is to reduce access to their food, which in turn reduces the number of eggs they can produce. “Most flies lay 10 to 100 eggs at a time,” said Swiger. “They’re in clusters, then other flies join in and lay their eggs in the same spot.” Larvae of different fly species thrive in groups thanks to shared resources and the ability to break down manure and other food sources.

When flies have completed development – which takes several days to several weeks – they pupate. “At this point they want to find drier substrate, so they may come to the top of the food source or crawl out of the resource they were using during the larval stage,” said Swiger. “That allows them to dry their exoskeleton and emerge when they become adults.”

Fly populations can get out of control quickly. After maturity, there’s 10% survivability – 10 flies have lived. Out of those 10 flies, roughly five are female. Each of the five females potentially lays 100 eggs – again with 10% survivability.

“Now there are 50 surviving females that can each lay 100 eggs,” said Swiger. “Within a month, they have almost tripled or more, so it doesn’t take long to have a fly problem. It only took one female, and now there are hundreds to continue.”

Manure management and IPM

Adult housefly. Photo courtesy of Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org

Pesticides are the main means of controlling flies, but pesticide resistance has become an issue – especially with houseflies. They are the hardest to kill because they’re resistant to all the chemicals on the market.

Swiger explained a study in which fluralaner (Bravecto) was used to manage a fly population. Although fluralaner is not on the market for houseflies and is not used in any application for them, when it was used for research on wild-caught flies, there was resistance in all female and most male houseflies.

“That’s really concerning,” she said. “It’s crossover resistance because of the use of other chemicals similar to fluralaner. Flies didn’t die even though they had never been exposed to the product.”

There are two types of pesticide resistance: behavioral and physiological. Behavioral resistance is due to flies not coming into contact with whatever is put out to kill them. “They’re no longer interested in bait or going to the attractant traps,” said Swiger. “It doesn’t mean the product isn’t killing them – they just aren’t exposed to it.”

Physiological resistance involves a change in the way the chemicals impact the insect. Flies develop a mutation or they aren’t breaking down the product as they did in the past. This can lead to cross-resistance, which was shown in the fluralaner study.

With flies, generations evolve rapidly, and new generations are present in days or weeks. “We select for resistance when we overuse products,” said Swiger. “Start with a cow that has flies that don’t have pesticide resistance; treat the cow and all the flies die because there’s no resistance. Unfortunately, products are often used more than once, and if the same products are used too close together or are not rotated, flies are overexposed to chemicals and don’t all die.”

If a pesticide-resistant fly is in the mix of flies on an animal, that fly won’t die when animals are treated. The single resistant fly lays 100 eggs or more, and the cycle continues, regrowing the fly population. Over time, the resistant population of flies becomes dominant. One fly passed the traits that prevented it from dying, and there’s now an abundance of pesticide-resistant flies.

Because fly control products have just enough similar chemistry, crossover resistance occurs, and despite using a new product with different chemistry, the new product is less effective. “There may be flies that die, or they may not die,” said Swiger. “We’re never sure if it’s crossover resistance but we are seeing evidence of it.”

A good IPM program can help combat insecticide resistance. “It can be more labor-intensive but that’s part of the point of being integrated,” said Swiger. “We want to be mindful of how we use pesticides when we have to use them.”

IPM emphasizes cultural practices such as good sanitation and habitat changes. One of the keys is to eliminate breeding sites where larvae are found – manually remove resources and take away places where flies lay eggs and larvae grow. Swiger suggested searching for fly habitat, and said while habitat may differ among farms and species, there will be similarities when moist, decomposing substrate such as manure is present.

Physical and mechanical control promote good sanitation, such as fans that prevent flies from landing, as well as screens and traps. Biological controls include predators and parasites that feed on fly larvae. If parasitoid wasps are used, be sure they are the appropriate species that will pursue the fly species present on the farm.

Chemical control should be used only when needed to combat an out-of-control fly problem when other practices have not managed the problem.

Guidelines for good pesticide use include following the labeled instructions for application amount and storage. If animals are being treated directly with a product, it’s important to have accurate animal weights to ensure the correct amount of product is applied. Using the incorrect dosage leads to resistance.

“Think about when you’re applying a product and how much impact there will be, whether it’s humans, pets or the animals themselves,” said Swiger. “Use good annual rotation – switch from a pyrethroid to an organophosphate or a macrocyclic lactone. This isn’t just changing the product, it’s changing the active ingredients. Think about areas where you’re using different pesticides and the impact on flies.”

by Sally Colby