Skip to content

A Year-Round Approach to Black Fly Control Yields Big Results for Twin Falls, Idaho

When Kirk Tubbs was hired on as the manager of the newly formed Twin Falls County Pest Abatement District four years ago, he wasn’t quite sure what to expect. With a background in wildlife biology, Kirk had experience working with subjects much larger than insects (bears and wolves, for example), so he had a good understanding of disease vectors and habitat surveillance. The world of Culicidae and Simuliidae was still largely a mystery, but the scientific method for finding answers was the same, and he knew he could make a difference.

The abatement district itself was initially established as an emergency response to concerns of West Nile virus in the Twin Falls region. Two years later, the taxpayers voted to establish a permanent district. However, some questioned whether the positions needed to be full time, as insect problems only occurred in the summer months.

Before the district was created, black fly control was conducted on a limited basis by local livestock producers, the irrigation companies that maintained the canals in Twin Falls, and the county weed department. Up to that point, control efforts had consisted of simply treating a 10-mile stretch of canal once the nuisance became too great — and until the funds ran out.

Assessing the situation

Kirk’s first task was to gain a better understanding of the black fly life cycle in Twin Falls. And, with more than 2,000 miles of canals supplying irrigation to the agricultural businesses throughout the county, that was no small task. “The survey ropes we placed in the canals came back with so many black fly larvae and pupae coating them, we couldn’t get an accurate count and had to simply report the percentage of rope that was covered.” This, of course, raised the question of where all these immature black flies were coming from. “We can find larvae everywhere, yet a study done by the University of Idaho in the 1970s found them only in limited areas,” Kirk explains. “Several years ago, the irrigation companies were using anti-moss chemicals that killed everything in the canals. Since then, they’ve gone to safer moss control that kills only moss. In addition, changes in farming practices and how the canals are maintained have improved water quality, which has improved the habitats for black flies.”

The traps also revealed that Twin Falls was dealing with two types of black flies: Simulium vittatum, which attack cattle, causing nuisance and spreading disease; and Simulium bivittatum, which will bite humans.

A chilling discovery

Knowing what types of black flies were present didn’t answer the question of where they were coming from or how they emerged so fiercely when the irrigation canals filled each spring. Through his research and observation, Kirk learned that both species of black fly survive the winter in the larval stage, rather than as eggs. “We saw that it represented a weak link in their biology, and we wanted to take advantage of it.” Kirk realized that the areas where black fly larvae could survive were significantly smaller in the winter than during the summer months, since the canals dried up during the winter months and river flows were low due to reservoir conservation.

He also learned that, having spent extra time feeding in the larval stage, emerging females can reproduce autogenously, skipping the feeding and mating patterns common for black flies in the summer months. And this cycle of autogenous reproduction can repeat itself quickly as temperatures rise, providing exponential growth in adult populations at any time. This was confirmed by placing sticky traps adjacent to small streams to catch adults emerging on warm days. Even during the winter, on any sunny day where the temperature reached 50 degrees, adults were found.

When Kirk proposed winter treatments in 2009, he was met with some skepticism. Why would he want to use resources when no one was currently complaining about the black flies? But, he was given the latitude to pursue the approach and began by treating a 10-mile stretch of river with VectoBac® 12AS larvicide where high numbers of overwintering larvae had been identified.

The first point in the 2009 line represents a typical spring emergence. Applying control measures temporarily reduced the populations that year, but it was the winter treatments that made such a big difference in the black fly population at the beginning of 2010. “Treating the source was the key to staying ahead of the exploding population that spring,” says Kirk. “With the low flow rates in the late fall and winter, we can treat with a smaller amount of product than we’d use in the spring and summer, even though the larvae are less active and eat less than they would when it’s warmer. But, the ‘downstream’ effects are huge. By controlling the population where the larvae spend the winter, we can greatly reduce their ability to populate other waterways and establish breeding grounds there.”

Based on the results of the 2009–2010 test, Kirk expanded the winter treatments county-wide, and the results have been equally impressive throughout the district. In the spring of 2011, it was rare to find a single black fly in the district’s traps. The slight upswing in October of 2011 was the result of excessive runoff through the canal system, which made effective treatment of the waterways impossible. The high runoff continued into 2012, limiting the overwintering treatments. But, the district’s sustained efforts have successfully kept black fly populations down. “The important thing is to treat the sources based on geographic boundaries, as opposed to political ones. Adult black flies can travel 10–20 miles,” Kirk adds.

Building on success

Today, in its fourth season of operation, the Twin Falls County Pest Abatement District is firmly established as a valuable county resource. The livestock producers have reported greatly reduced black fly populations and improved weight gain in their livestock as the animals can once again graze near the waterways. The residents of Twin Falls are enjoying the outdoors with much less irritation, as well.

Kirk is also looking to share his district’s success with neighboring communities. “We’re currently working to expand the effectiveness of the winter treatments, but none of the adjacent counties have dedicated abatement districts established. Hopefully that’ll change in the future. We’re limited by scope and budget, but we’re always willing to offer our services as we can.”