Lessons in Larviciding


By, Cami Adams, SWCMCD Entomologist

“What are you spraying?” is one of the most common questions posed to us at the district. Most people are familiar with seeing the adulticide truck at night treating for mosquitoes. So, when one of our mosquito trucks is encountered in the middle of the day spraying standing water, there tends to be some confusion. Contrary to initial belief, these trucks are not spraying for adult mosquitoes but rather their aquatic, immature stages. Mosquito larvae thrive in water that can stagnate in a multitude of places, such as tires and buckets, rain-filled woodland pools, and roadside swales. While the district employs different larval control techniques depending on the breeding source, the quickest way to treat non-flowing ditches and roadside swales is through a truck-mounted hose that disperses liquid product.


The two active ingredients used for this type of treatment are either Bacillus thuringiensis israelensis (Bti) or Bacillus sphaericus (Bs) which are developed from naturally-occurring soil bacteria that do not affect non-target organisms like bees, fish, plants, animals, people, or other aquatic invertebrates such as dragonfly nymphs. Additional to the targeted environmental benefit, the products have been noted to reduce larval mosquito populations within 24-48 hours. Fast efficacy leads to quicker control and less adult mosquitoes biting our residents. How do these active ingredients work? The bacteria produce crystals that are toxic to mosquito larvae, fungus gnats, black flies, and some midges. Within mosquito larvae, ingestion of these crystals disturbs the larval gut, killing the larvae sometimes within just a few hours.

Larviciding missions are usually triggered by a big rain event, such as the one South Walton experienced recently with Hurricane Sally. Standing water can produce adults in as little as a week. Ample amounts of water mean more mosquitoes breeding, and it is important for the district to treat these retention areas as quickly and efficiently as possible to minimize the number of mosquitoes emerging. If the ground is saturated and the water will not soak up as quickly, a longer lasting formulation is used to provide longer control.

There are no special precautions to take when Bti or Bs is applied. You do not have to leave the area where treatment is occurring, and children and pets are safe to be outside during application. Because the district commercially uses these products, we wear the required personal protective equipment that is meant to protect the applicator during mixing of the product and application. This is only to prevent skin or eye irritation from direct contact and does not mean the product is toxic. In fact, Bti is approved for pest control in organic farming processes. Using these active ingredients to control mosquito larvae helps SWCMCD uphold part of our mission statement to “utilize a variety of methods in such a way as to minimize potential effects on people, wildlife, and the environment while maximizing the value to the district’s taxpayers.” So, the next time you see us out and about spraying standing water, give us a wave and know we are here to serve you.

For more information, please visit www.southwaltonmosquitocontrol.org.

Mosquito Control