Reduction

The majority of the TNRD Nuisance Mosquito Control campaign is focused on reducing mosquitoes while they are in their larval stages, for two primary reasons. Firstly, larval control is much more efficient than adulticiding – it is possible to treat larval mosquitoes in very high concentrations in larval development sites, while adult mosquitoes tend to disperse soon after emerging. Secondly, larvicides such as Vectobac 200G, Aquabac 200G and Altosid Pellets are species-specific, affecting only aquatic members of the Order Diptera, which includes mosquitoes, black flies and midges.

Adulticides such as malathion and pyrethrin are wide-spectrum insecticides, with the ability to kill beneficial insects as well as pests, and can also be toxic to vertebrates including fish, birds and mammals.

At the request of the Ministry of Environment, areas containing fish or areas that are permanently contiguous with fish habitat are ineligible for larval nuisance control treatments, since the larvicide could potentially affect fish productivity by affecting a food source (mosquito larvae).

Vectobac (PCP 18158, a.i. 2.80%) & Aquabac (PCP 26863, active ingredient (a.i.) 2.86%) and are trade names of the two biological mosquito larvicides used in the TNRD Nuisance Mosquito Control Program. The active ingredient of these products is the bacterium Bacillus thuringiensis var israelensis (Bti) which is formulated as a granule product with crushed corncob as a carrier. The product is effective at controlling mosquitoes and other members of the order Diptera in the larval stage, and is applied by hand, backpack blower, or by helicopter to standing water containing significant populations of mosquito larvae. Bti produces an endotoxin that contains five different proteins that can be digested in the alkaline gut of mosquito larvae. Once digested the proteins become toxic to the larvae and work to destroy the larvae’s gut.

The benefits of the Bti larvicides are many. Bti larvicides are considered to be very specific to mosquito larvae and non-toxic to fish, amphibians, reptiles, mammals and most other insects. They are easy to apply and have no residual effect, along with being the most economical of the larval controls. Death of the larvae usually occurs within 24 hours.

This product is applied at rates ranging from 3 kg/ha to 20 kg/ha depending on the organic content of the water being treated. The TNRD contractor typically applies around 10,000 kg of Bti larvicide each year (larvicide amounts vary depending on the amount of flooding and thus the size of larval development habitats).

Altosid Pellets (PCP 21809) is the trade name of a mosquito larvicide that contains the active ingredient methoprene (4.25% a.i.). Methoprene is a mosquito growth regulator that mimics mosquito juvenile growth hormone. In mosquitoes, juvenile growth hormone production ceases when a mosquito pupates. This allows the mosquito to develop into an adult while in its pupal stage. When methoprene is present in the water, the pupae is not able to develop into an adult and the mosquito dies in its pupal stage. Altosid is formulated in slow-release pellets that release a constant concentration of methoprene into the water for up to 30 days. This larvicide can be applied to dry ground before a flood and will only activate when submerged in water. If the site dries up, the pellets will stop dissolving and will begin to dissolve again if re-submerged. This is the larvicide of choice in habitats such as ditches that become wet intermittently, and in fields where farmers practice flood-irrigation (or over-irrigation) and many generations of mosquito larvae can be produced.

The TNRD Mosquito Adulticiding Program is a REQUEST ONLY, GROUND-BASED program. Residents of the TNRD can request adulticide spray by contacting the mosquito control contractor at (250) 372-5700 and byhaving neighbours sign the TNRD Adulticiding Petition. When conditions for adulticiding have been met, Adulticides are applied via truck-mounted Ultra Low Volume (ULV) foggers. Both malathion and resmethrin can be used in these sprayers. Historically, aerial and ground application of these pesticides constituted the primary means of controlling mosquito populations, and large amounts were used annually. However, more recently, due to environmental concerns associated with the toxicity of adulticides, application of these pesticides has occurred on a much smaller scale, and only in response to specific requests. The adulticide program is only available to residents in Electoral Areas ‘J’. ‘L’, ‘O’ and ‘P’, as residents in Electoral Area ‘A’ elected to ban the spraying of adult insecticides for mosquito control.

Due to excellent results from the larviciding practices, no adulticides have been used in the TNRD since 2003.

Malathion is an organophosphate insecticide that works by inhibiting cholinesterase and thereby causes paralysis of the insect. This insecticide is the most commonly used chemical control for adult mosquitoes in North America. It is applied by ground ULV spray and thus no mixing or dilution is needed. This product has a low mammalian toxicity, moderate avian toxicity and variable toxicity to aquatic organisms.

The benefits of this adulticide are its low cost (compared to other adulticides), and ease of use (no mixing required). The disadvantages include strong odor, a propensity to cause paint damage (if applied incorrectly) and, in some areas of North America, resistance among mosquito populations has developed.

Pyrethrin is a natural chemical that is produced in the flowers of chrysanthemums. Pyrethrin pesticides affect sodium channel function in the neurological system of the insect. Natural pyrethrins are formulated with additives such as piperonyl butoxide and N-octyl bicycloheptene dicarboximide to enhance the effectiveness and stability of the pyrethrins.

There are many benefits of the pyrethrins. First, they have rapid knockdown properties. They are unstable when exposed to light and break down quickly. Unlike malathion, pyrethrins are non-corrosive and have a less offensive smell. They are considered to have a very low mammalian toxicity.

There are disadvantages to the use of pyrethrins as well. Pyrethrins are very expensive insecticides. Also, they pose a slightly higher safety risk as they need to be diluted prior to use (thus there is increased handling and increased chance of spill). The biggest disadvantage to pyrethrin use is that it is highly toxic to fish.

The Thompson-Nicola Regional District offers a request-only adulticiding program for residents of Electoral Areas ‘J’, ‘L’, ‘O’ and ‘P’. There are number of criteria that must be met prior to adulticiding. First, an attempt must have been made to control the mosquito when they were in their larval development stage with larvicides. This means that a landowner cannot refuse larval control, and then later request adulticide spray. Next, the population of adults must be exceptionally high. As a rule, a bite-test must be performed and an individual must be getting an average of 3-BITES-PER-MINUTE on ONE EXPOSED FOREARM. Basically, a volunteer must stand outside with one sleeve rolled up and count the number of bites received in one minute. When the bite count numbers are sufficient, a landowner can request an evaluation of his/her property to determine if all Ministry of Environment regulation can be met. Crews must be able to leave a 110 meter buffer zone around any fish-bearing stream (ie North and South Thompson Rivers and their tributaries), drinking-water well, bee-hive, or neighbour that has refused spray. It is recommended that interested landowners take the TNRD petition to their neighbours and ask if they would like to be included in the program, since adulticiding is more effective if a greater area is sprayed. NO ONE WILL EVER RECEIVE ADULTICIDE unless they have met all of the above conditions and have signed the TNRD Petition. If adulticiding is approved, spraying is conducted between dusk and dawn, to minimize impacts to beneficial insects such as bees and to lessen exposure to humans and domestic animals. Instructions are given to landowners prior to a spray campaign to keep windows and doors shut, turn air-conditioners and fans off, and keep pets in the house or under cover, and to wash all fruit, vegetables and children’s toys following the spray.

BWP Consulting Inc. is a biological consulting company based in Kamloops, B.C. This company has conducted all mosquito control related activities for the TNRD since 1999.

You can contact Crew Supervisor Cheryl Phippen with questions related to Mosquito Control in the TNRD or West Nile Virus. To contact Cheryl, leave a message on the 24-hour TNRD Mosquito Advisory Line at 250-372-5700, or email BWP Consulting.