Insecticides in cattle feed to combat flies ‘significantly lower’ dung beetle populations, which control flies naturally

Anyone who has walked through a barn or cattle pasture in the summer knows that flies are a nuisance and even a health hazard. Face flies can spread diseases like pink eye to cattle, and horn flies—biting flies that live on cows and take up to 20 blood meals per day—in large enough numbers can impact animals’ health and growth. But insecticides frequently used to combat these pests may actually be reinforcing the problem by killing dung beetles, which naturally control flies, and potentially harming other beneficial insects.

Researchers with the Cornell Integrated Pest Management program have been working in collaboration with farms across New York state to understand how feed-through pesticides—insecticides added to cattle feed to kill flies—impact dung beetle populations. The researchers are also sharing alternative strategies to control pest species, such as using walk-through fly traps, providing shelter, and recruiting poultry to eat fly larvae.

“These flies can cause major problems for herds. If you’re raising steers, you want them to gain weight quickly, and the annoyance, injury and disease that flies can cause in large numbers can affect the animals,” said Ken Wise, livestock coordinator for Cornell Integrated Pest Management (IPM).

“However, the broad-spectrum use of any insecticide is not an integrated approach to controlling flies. I know it’s a pain to do, but if you can estimate the number of flies on your cows and treat the animals only when they need it, you’re going to have a lot less insecticide in the environment.”

Feed-through insecticides harm dung beetles and don’t control face flies

Both flies and dung beetles lay their eggs in manure pats. Larvae eat the manure and then hatch as fully-grown insects. Dung beetles control flies by competing for the same manure for food and shelter. Other species of beneficial beetles that inhabit manure include predators such as rove beetles, hister beetles and water scavenger beetles, which also eat fly larvae. And beetles’ benefits go beyond fly control: When they create tunnels in manure pats and in the soil beneath them, they help break down waste more quickly and recycle nutrients back into the soil, helping to increase soil health and fertility.

In the current research, Wise, Bryony Sands, assistant professor at the University of Vermont, and Hannah Tolz, extension support specialist with Cornell IPM, are exploring how two feed-through insecticides impact fly and dung beetle populations. One is a broad-spectrum insecticide and the other is an insect growth regulator (IGR), designed to kill fly maggots before they can hatch from manure pats. Cattle eat feed treated with insecticides; after passing through the animal, the products kill insects that eat or dwell in manure.

While pesticide use is sometimes necessary to protect crops and livestock, overuse of these substances has repeatedly been shown to cause negative unintended consequences in the environment. For example, separate Cornell research has found that wild foraging bees exposed to certain pesticides suffer “reductions in brain function, foraging and nest locating ability, growth, and reproduction.”

Initial findings suggest that farms that use feed-through insecticides have “significantly lower” dung beetle populations and beetle species diversity. In addition, the research showed that horn fly numbers rarely exceeded thresholds at which treatment is needed to prevent economic loss. Face fly populations were lowered by insecticides but almost-universally exceeded problematic levels, even at farms using insecticides, suggesting the treatment was not addressing the problem, Wise said.

IPM methods to help control flies

Kate Marsiglio is one of the 19 farmer-collaborators working with Wise, IPM and Cornell Cooperative Extension on the research. For 20 years, she has been operating Stony Creek Farmstead in Walton, NY and raising beef, lamb, chicken, eggs and pork.

Roughly half of the farms involved in the research use feed-through insecticides, and half don’t. Marsiglio has never used insecticides to control flies on her cattle, relying instead on rotating animals frequently across her 300-acre farm, and sending chickens in after the cattle have moved.

“The chickens come through and scratch out the cow pies, spread them all out, and then eat the fly larvae. I also love watching wild birds follow our cows: You see birds on their backs, eating insects and creating this great beneficial relationship,” she said. “We’re trying to add as little unnecessary chemicals into the environment as possible. Because if you feed an entire herd of cattle insecticides, it ends up in their poop and then it passes into our soil and our water.”

When using pesticides, Cornell IPM recommends that farmers:

• Only treat cattle when fly numbers exceed action thresholds. For horn flies, that’s 200 per animal. For face flies, 10 per animal.
• Use targeted insecticides, such as back rubbers, face rubbers, dusters and direct spray, rather than feed-through products.

Instead of using across-the-board insecticide treatment, Cornell IPM recommends growers:

• Provide shelter when fly pressure is high. Horn and face flies are reluctant to go into a darkened enclosure in the summer.
• Use poultry to control pasture flies in manure pats.
• Use mechanical tools like fly traps or walk-through “Bruce Traps”—these are darkened chutes which cattle walk through. Because horn flies dislike enclosed spaces, they fly off, move toward light and are trapped by double-screened walls. These traps “can control 50–70% of horn flies over time,” according to Cornell IPM.

Further research is planned for this summer to understand how differing ingredients in feed-through insecticides impact beetle numbers at farms in New York and Vermont.

“Insecticides in the environment are residual—they stay there for a long time and can potentially cause off-site effects to pollinators, plants, soil and water,” Wise said. “We encourage an integrated approach that focuses on prevention and avoids overuse of insecticides.”