Horn flies (Haematobia irritans), pictured above, and stable flies (Stomoxys calcitrans) are serious pests of the North American cattle industry, causing annual economic losses exceeding $1 billion. In the open-access Journal of Integrated Pest Management, a new in-depth review of existing research highlights the promising repellent and insecticidal qualities of more than 20 different botanical compounds, such as neem oil, pyrethrum, and even nepetalactone, the essential oil of catnip. (Public domain image: Wikimedia)

By Meredith Swett Walker

We generally don’t associate cows with pleasant aromas, but in the future you may catch a whiff of mint, lemongrass, or chrysanthemum when visiting a barn or feedlot. It turns out that extracts and oils from these and other plants show promise as repellents and insecticides for use on cows.

Meredith Swett Walker

In a guide published today in the open-access Journal of Integrated Pest Management, Allan Showler, Ph.D., of the U.S. Department of Agriculture’s Agricultural Research Service reviews research on more than 20 different botanical compounds with potential to repel or kill two common fly species that pester cows. Evidence shows that flies are becoming resistant to the synthetic pesticides currently used to control them, and so alternative methods are needed.

Showler’s review concerns control of two species of fly, the horn fly (Haematobia irritans) and the stable fly (Stomoxys calcitrans). Both flies feed on blood as adults and lay their eggs in manure or decaying vegetation, where larvae feed and pupate. Horn flies primarily bite cattle, but stable flies have a wider range of hosts including cattle, horses, humans, and dogs. Fly infestations are stressful to cows and can cause weight loss and reduced milk production. These flies not only make cows miserable, but they also cut into ranchers’ and farmers’ bottom line. Estimated economic loss in North America due to horn flies is about $1 billion per year, including costs for control measures. Stable flies cost U.S. cattle and dairy producers $100 million to $150 million or more annually.

Infestations of horn flies (Haematobia irritans) and stable flies (Stomoxys calcitrans) are stressful to cows and can cause weight loss and reduced milk production, and they cost ranchers and farmers in North America an estimated $1 billion per year. (Public domain image: Pixabay)

Some of the botanical compounds that Showler reviews may be familiar. Neem oil, which is extracted from the fruits and seeds of the evergreen tree Azadirachta indica is widely used in organic agriculture to control pests on plants. Neem extracts disrupt the endocrine system in insects, reducing reproduction or causing death. Pyrethrum, which is derived from chrysanthemums, is the most widely used botanical insecticide and affects the insect nervous system. Several pesticides collectively known as pyrethroids (such as permethrin and deltamethrin, for example) are synthetic versions of the pyrethrin compounds found in pyrethrum.

Other botanical sources may seem like odd choices, but could be surprisingly effective. Treating cows with essential oil of catnip (Nepeta cataria) conjures images of inebriated barn cats pestering their bovine roommates. But nepetalactone, the compound in catnip that gets cat high, is also a powerful insect repellent. Preliminary research indicates that catnip oil effectively deters horn flies from biting. When nepetalactone is hydrogenated, one of the resulting compounds is a highly effective stable fly repellent—twice as effective as DEET.

The stable fly (Stomoxys calcitrans), pictured above, and the horn fly (Haematobia irritans) feed on blood as adults and lay their eggs in manure or decaying vegetation, where larvae feed and pupate. Horn flies primarily bite cattle, but stable flies have a wider range of hosts including cattle, horses, humans, and dogs. (Public domain image: Wikimedia)

Plants are not the only source of potential fly control chemicals. Fungi, especially those in the genus Neotyphodium, may prove useful too. These fungi live inside grasses, including grass species that cattle feed on. The fungi have a symbiotic relationship with the grass and produce chemicals called alkaloids that protect the grass plant against insect herbivores. Apparently that protection transfers up the food chain. Cattle eating grasses containing fungal endophytes had 50 percent fewer horn flies on them according to a 2013 study by researchers at the Universidad de La Frontera in Chile. Further research showed that the manure of these cattle proved a lethal environment for any horn fly larvae that hatched there.

Showler says as resistance to synthetic pesticides becomes a bigger problem, “botanically based materials can avert rapid development of resistance, because many of those materials have multiple bioactive compounds with multiple modes of action.” But he says much of the research he reviewed was very preliminary: “Many of the studies … merely show effects for the first time.” Showler adds that skeptics tell him botanicals have ‘already been researched and there is nothing more to learn,’ but “reality shows that the opposite is true,” he says.

Plants clearly have much left to teach human chemists about fending off insects. Further study of these botanical (and fungal) compounds may allow us to keep our cows happier, healthier, and pleasantly scented.

Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs Pica Hudsonia and The Citizen Biologist or follow her on Twitter at @mswettwalker.

[Editor’s note: This post has been updated to clarify the distinction between pyrethrum, pyrethrin, and pyrethroids.]