ULV & Adulticides

What are Adulticides?

Pesticides that target adult mosquitoes are called adulticides. They work by killing mosquitoes when they come into contact with small droplets of the spray material.  These droplets are sized to specifically target mosquitoes while minimizing impacts to other organisms.   Delta Mosquito & Vector Control District uses adulticides to help reduce the spread of mosquito-borne diseases, such as West Nile virus.

The decision to treat an area is based on:

• Detection of virus in mosquito samples
• Reports of human disease cases
• Abundance of mosquitoes in the area
• Availability of staff and equipment

Ground ULV treatments are just one part of an integrated mosquito management program, which also includes larval control like removing or treating standing water sources.

How is an Area Selected for ULV? 

1. Trapping – Over 400 traps are placed throughout the District and monitored weekly.
2. Counting & Testing – Traps are checked for mosquito numbers and tested for viruses.
3. Inspections – Areas with high mosquito counts are inspected for water sources.
4. Larval Control – Standing water is treated or removed when possible.
5. ULV Treatment – If virus activity or mosquito numbers remain high, adulticides are applied using ground ULV.

How does ULV work? 

What do we use?

The District uses two main types of adulticides:

• Pyrethroid-based products: Synthetic versions of natural insecticides found in chrysanthemum flowers (Tanacetum cinerariifolium). These are commonly found in household sprays and pet shampoos.
• Malathion: An organophosphate insecticide approved for mosquito control, used safely in public health programs for decades.

Both products are applied at very low dosages, carefully calibrated to be effective against mosquitoes while posing minimal risk to people, pets, and the environment. They also break down quickly in sunlight and water, making them both effective and short-lived in the environment.

How Small is the Dose?

To help visualize: during ULV, about 2 tablespoons of adulticide are dispersed over an area the size of a football field (1.3 acres). That’s far less than what is used in common household applications.

Because of the small amount used, residents do not need to take any special precautions during ULV events.   Residents may wish to move indoors to avoid any discomfort during the event. 

ULV applications are made at night when mosquitoes are most active, and when people and pollinators like bees are less likely to be exposed.

Why We Use ULV?

• To slow the spread of mosquito-borne diseases like West Nile virus, especially when virus activity is detected in an area.
• To quickly reduce the adult mosquito population, especially those potentially infected and capable of spreading disease.
• To protect public health when risk levels rise, especially in populated neighborhoods.
• To complement other mosquito control methods, such as larval control, habitat reduction, and public education.

ULV is a fast-acting tool used in response to surveillance data, and it helps reduce immediate risk during peak mosquito activity.

It is not, however, a long-term tool to control mosquitoes in an area.  The populations of mosquitoes will quickly rebound unless the larval sources are identified and controlled.  Removing potential mosquito sources from your own residence, maintaining pools and other water features, and reporting observed mosquito sources to the district, are all ways residents can help eliminate the mosquito problem.

What can you do to help?

• While ULV treatments help reduce adult mosquito populations, they do not eliminate the problem entirely. Here’s how you can help support mosquito control in your community:

  • Eliminate standing water where mosquitoes lay eggs—check buckets, flowerpot trays, tires, birdbaths, and gutters.

  • Maintain swimming pools, spas, and fountains—even if they’re not in use.

  • Report neglected pools or mosquito concerns to the District.

  • Understand that ULV doesn’t affect larvae or eggs—removing water sources is essential.

  • Protect yourself by using EPA-registered repellents and wearing long sleeves and pants during peak mosquito hours (dawn and dusk).

  Together, ULV treatments and community action provide the most effective mosquito control.

Understanding ULV Applications

Over the area of a football field. 1.32 acres.