Concerns of Impacts to Weed Biocontrol from Mosquito Abatement

Thank you to Mikenna Smith, Teton County Weed and Pest Entomologist, for her contributions to this blog from her recent research presentation.

Biocontrol and Mosquito Control: Are They at Odds?

As an entomologist working in both biological control (biocontrol) of weeds and mosquito abatement, I often find myself in a unique position to explore how these two practices interact.

When researching whether mosquito control affects weed biocontrol agents, I found surprisingly little direct research. However, there’s a lot of literature on how mosquito pesticides affect non-target insects in general. That’s what I’ve used to guide this discussion.

Pesticide Impacts with Lab & Field Studies

Research on pesticide impacts often falls into two categories: lab studies and field studies. Both are important but they often conclude two different things. Lab studies are controlled and usually suggest mosquito pesticides are harmful to many insects. In contrast, field studies often find little to no measurable impact. This difference is largely due to realism—or lack thereof.

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Lab studies often don’t reflect how mosquito control works in the real world. Many test scenarios use unrealistically high pesticide doses or expose insects in ways that don’t happen in the field—like dipping leaves in pesticides and feeding them to caterpillars. In reality, mosquito control programs are careful, cost-conscious, and only apply pesticides in ways that are effective and targeted.

Aquatic Habitat: Larvicide Impacts on Non-Target Species

For instance, microbial larvicides like Bti and Lysinibacillus sphaericus are applied directly to water where mosquito larvae live. These products are highly selective for mosquitoes and black flies, and field studies show they don’t significantly impact other aquatic insects. Spinosad, another larvicide, is less selective but used in highly organic, polluted water where other larvicides fail.

Insect growth regulators like methoprene are also used in water and are highly effective at very low doses. While they can affect small insects that share mosquito habitats, field studies show minimal impact on most aquatic life. Methoprene is the most commonly used IGR for mosquito control.

Terrestrial Habitat: ULV Impacts on Non-Target Species

When it comes to adult mosquito control, ultra-low volume (ULV) sprays are typically applied over large areas using a truck or airplane to target flying mosquitoes. Droplet size, timing, and distance from the spray path are all factors that reduce non-target exposure. When conducting ULV treatments, the equipment is calibrated so the diameter of the droplets is between 10-30 microns, which is the “goldilocks size” for knocking down and killing flying mosquitoes in the flight column. Too small and those droplets will shoot straight up into the air, too large and they will fall to the ground. But just right and they will suspend in the air in that mosquito flight column.

When applying ULV adulticides we also have to control our application rate by controlling the vehicle speed. The slower you go the more pesticide you put out and vice versa. Pesticide label rates are often given as low, mid, or maximum rates. It is very uncommon for mosquito districts to use the maximum label rate, that usually only happens when they have a highly resistant strain of mosquito or during a virus outbreak. Applications are most commonly done after sunset for several reasons, including the need for a temperature inversion to target the mosquito flight column during peak activity, and to protect non-target species like honey bees, which are inactive at that time.

In Summary

Overall, field studies suggest that mosquito control, when done properly, has limited impacts on non-target insects and reflects real-world conditions more accurately than lab studies. But there’s still more to learn—especially about long-term or sublethal effects, and how biocontrol agents might respond differently than native species.

If you're working on a weed biocontrol program and are concerned about mosquito spraying, consider partnering with a local mosquito district. They’re often eager to collaborate on studies like this. Let’s not study these issues in isolation—let’s build partnerships and find real-world answers together.

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