The mosquito is nobody’s friend. And when certain mosquito species carry diseases such as malaria or West Nile virus, public-health authorities need to take action. Surely, anything that helps to kill mosquitoes must be good?
But there are other considerations. Can we reduce the mosquito population without damaging other parts of the living system on which we depend for long-term survival? At what stage of development are mosquito populations easiest (and therefore cheapest) to control? How much pesticide are we willing to live with as we control mosquitoes? Which pesticides are the safest for us to use? What sorts of non-pesticide measures are available to us? How much money are we willing to spend?
Applying insecticide seems clean and easy; besides, “the government” can do it. They should have at least one qualified and experienced entomologist in charge who can supervise the job properly. The mosquito entomologist knows how to make the mosquitoes’ lives miserable, where and how to apply insecticide, and when is the most effective time. So why not just give him/her the resources to go ahead, apply the insecticide, and kill the mosquitoes?
For many years, I was the head of the Pesticide Research Laboratory at the University of Manitoba, involved in projects to do with pesticide chemistry in the environment. So I know there’s more to the story.
Killing adult flying mosquitoes is intrinsically inefficient. Adult mosquitoes hide during the day under protective vegetation to stay moist and cool, and they aren’t easily accessible to applicators who must stay on city streets and lanes and apply adulticiding insecticides at night. By contrast, mosquito larviciding is intrinsically efficient, focused, and much more cost-effective. Reducing available breeding sites is the first step.
In experimental testing of mosquito adulticides, mosquitoes in small cages are placed in a row on short poles in an open test area and, with the wind blowing gently toward the cages, a truck-mounted sprayer is driven by with the mosquito insecticide being atomized behind it. The wind wafts the insecticide to the cages, and the insecticide enters the cages and kills the mosquitoes. The technique kills mosquitoes effectively. But in the real world, adult mosquitoes, whether nuisance species or disease-carrying ones, aren’t conveniently caged, nor are they all downwind of the applicator trucks.
Real application of adulticides involves the application of insecticides to residential areas from roadways, both upwind and downwind of mosquito-infested yards. If the wind doesn’t carry the insecticide to the mosquito, the mosquito doesn’t receive the lethal dose. If the mosquito is in the back yard and the truck drives by the front, it is highly likely that the mosquito and insecticide won’t meet; a similar result can be anticipated if the mosquito is hiding in vegetation.
During actual mosquito infestations, there are many more times the “tolerable” population of mosquitoes present. Under favourable conditions, the kill will be 50-per-cent to 80-per-cent successful and the reduction of mosquitoes may last 12 to 36 hours. However, there will enough mosquitoes left after an adulticiding event to cause the person with the back-yard barbecue considerable annoyance. If we want to kill disease-carrying species, the results aren’t acceptable.
The treatment of sports facilities, picnic sites, cemeteries and formal gardens is localized and can be carried out more thoroughly; control of the mosquitoes in such locations can be expected to be more complete. It may even last for two to three days. And the areas being treated are small enough that whole populations of non-target organisms — those that we like and which may eat mosquitoes, and those that make up the complicated web of life and do us no harm — will likely be able to repopulate the limited areas treated.
The day-to-day variability of the mosquito population is affected by temperature and humidity. It’s often impossible to tell if an application of insecticide is responsible for a reduced mosquito population.
Yet the public has great faith in the efficacy of insecticides, particularly for the control of mosquitoes. While this faith is well-placed in the case of larviciding — when the mosquito larvae are localized in pools of standing water — it is much less well-deserved when applied to the control of adult mosquitoes.
The use of more insecticide may marginally assist in killing more adult mosquitoes. However, it brings with it the risk of killing beneficial insects and other susceptible forms of life. In particular, higher levels may adversely affect the lives of vulnerable humans (especially children and the elderly) who are exposed to the insecticide.
The reliance on larviciding appears expensive, particularly compared to the no-action alternative. However, modern biological larvicides based on Bacillus thuringiensis israelensis (Bti) are capable of killing larvae without harming virtually all of the non-target organisms. Comprehensive larviciding requires planning and selective widespread application to all breeding areas. And it is intrinsically effective. By comparison, the application of adulticides is much more expensive, is of dubious benefit, and must involve the entire area rather than just the breeding sites.
The decision to apply mosquito adulticides is political. Citizens who are terrified of mosquito-borne diseases demand relief. Does adulticiding of residential areas actually provide good value for money? Probably not. Does the city or regional entomologist have any alternative other than to follow the short-term dictates of city or county council? Only with forethought and planning.
As authorities confront the challenge of controlling mosquitoes bearing the West Nile virus, they should remember that effective and timely larviciding, and reducing mosquito habitat, could make costly and intrusive adulticiding unnecessary. Adulticiding seems attractive, but benefits at best last for a day or so.
It isn’t easy being the government entomologist, but such expertise is essential. We as citizens must have realistic expectations about his or her capacity to make our mosquitoes disappear.
Barrie Webster, vice-president of Websar Laboratories Inc., is a former head of the Pesticide Research Laboratory at the University of Manitoba.