Horn flies (Haematobia irritans), shown infesting cattle above, are a costly pest to the cattle industry. A recent study on the potential use of a fungus that attacks horn fly larvae found that the fungus acts too slowly to serve as an effective means of biological control for the insect. However, research continues in the effort to identify a faster-acting strain of the fungus. (Photo credit: Craig Sheppard, University of Georgia, Bugwood.org)

By Ed Ricciuti

Soon after silkworm eggs were smuggled out of China in the 12th Century, northern Italy and France emerged as centers of silk production, an industry almost brought to a standstill in the early 1800s by a mysterious disease that killed the caterpillars on which it depended. Bacteriologist Augustino Bassi determined the disease was caused by a fungus, leading him to describe how some diseases are caused by parasites, an insight anticipating the later affirmation of the germ theory of disease by Louis Pasteur and Robert Koch.

Ed Ricciuti

The lethal qualities that made Bassi’s fungus the bane of silk producers have proven a boon for agriculture and public health. Named Beauveria bassiana after its discoverer, it is a potent biological control agent against a host of harmful insects, including grasshoppers, locusts, bed bugs, and house flies. When the fungus contacts its host, spores germinate, enter the body, and grow internally, killing the insect within a few days. New spores arise from a white mold that covers the dead insect like the wrappings of a mummy, contributing to the infection’s name of white muscardine disease.

Given the virulence of this fungus, pest control managers have considered whether it might also be effective against one of the most damaging cattle pests, the horn fly (Haematobia irritans), but research recently published in the open-access Journal of Insect Science suggests that, while it “holds promise,” lots of work remains to be done. The fungus samples tested in the research had stopping power but did not work fast enough to kill the flies before they reproduced, starting three or four days after attaining adult stage, which lasts less than two weeks.

Morphing from larvae that hatch out of eggs deposited under the edges of manure pads right after cattle defecate, blood-sucking adult horn flies swarm in cattle pastures and cost the U.S. cattle industry as much as $1.6 billion yearly. Although one of these tiny flies sips less than 2 milligrams per meal, it feeds several times daily, so hundreds of flies can drain a substantial amount of blood from a cow. The main reason why the health of infested cattle declines, however, is that they are agitated the day through, as they try to shake and swish off their winged tormentors.

Insecticides have been the main weapon against horn flies, but employing them against mobile swarms of flies on large numbers of beef cattle outdoors tests the ingenuity of pest managers. Dairy cows are easier to treat because farmers milk them indoors twice daily. A variety of techniques and devices to facilitate treatment  have been used, ranging from run-of-the-mill spraying to impregnated ear tags and even a device resembling a paint gun, which  shoots blobs of insecticide at cattle. No matter the method, horn flies have a “phenomenal” resistance to insecticides, says lead scientist on the new study, Phillip Kaufman, Ph.D., associate professor of veterinary entomology at the University of Florida. Hopes are that a fungus could provide an alternative treatment.

“The use of fungal pathogens is one underutilized option that could aid in management of this difficult pest,” says Kaufman.

B. bassiana has attracted interest as a horn fly control because, highly contagious and aggressive, it has been reported in hundreds of insect species. Militating against using B. bassiana against horn flies is the fact that, so far, it takes at least a week for the spores to emerge and kill the flies, giving them more than enough time to lay eggs. Moreover, the fungus has multiple strains, many of which are specific to their insect host and, as recent research suggests, may actually qualify as distinct species.

Isolating the strain matching the flies to be treated is among the problems facing scientists. Kaufman and his colleagues tested one strain taken from the lone horn fly carrying the fungus out of 7,000 they collected on Florida ranches. They passed the fungus through seven generations of flies, perpetuating it with samples in each case from the fly that died first or, as Kaufman says, “was first one to the finish line.”

Demonstrating the tricky nature of tailoring a biological control to a pest species, the fungus took hold but was not sufficiently fast acting. The researchers could not kill enough flies before the insects died naturally. Even so, a bright spot was that almost all the flies showed evidence of the white muscardine, including the development of spores.

Perhaps, says Kaufman, the problem lies with using a sample from “only one fly, taken from one farm, on one date.” Or else, factors in nature, such as bacteria in manure, could impact the efficacy of the fungus.

“There might be better strains out there,” he says. Scientists now need to find strains that not only polish off flies but do it quick time.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.