The earth’s vegetation is part of a web of life in which there are intimate and essential relations between plants and animals. Sometimes we have no choice but to disturb these relationships, but we should do so thoughtfully, with full awareness that what we do may have consequences remote in time and place.
— Rachel Carson, Silent Spring
There are widespread reports that Merial’s topical flea and tick treatment Frontline has become ineffective in a disturbingly short period of time. The active ingredient in Frontline is fipronil, an insecticide in the phenyl pyrazole family. Fipronil blocks g-aminobutyric acid (GABA) receptors and interferes chloride passing through GABA-gated channels in non-mammalian species. At low concentrations, it disrupts nervous system function and at higher doses causes death.
Fipronil has been used in flea and tick preparations around the world since the mid-1990’s. As Heather noted in comments to the previous post, widespread problems with counterfeit Frontline have been documented. (So much so that I strongly recommend you go to this link to an EPA page that provides information on how to identify counterfeit Frontline.) While I’m sure that the sale of counterfeit product is part of the issue with Frontline, I’m not convinced that it’s the only problem — or even the most troubling one.
Pesticide resistance occurs when a species adapts over a series of generations to have a decreased susceptibility to a specific chemical. Since no pesticide is completely effective in the natural environment, there will always be some organisms that survive exposure. Organisms can receive sublethal doses for a variety of reasons, but because only the most resistant organisms survive to pass their genes on to offspring, resistance can increase exponentially in a population. As Rachel Carson predicted in Silent Spring, problems related to pesticide resistance are increasing all over the world.
My friend Heather quite understandably professed skepticism that exposure to fipronil products applied to our pets would be sufficient to create an environment where widespread resistance could occur. If this was the only route of exposure, I suspect she would be right. But pet products only represent a very small percentage of fipronil use.
Fipronil is active against a wide array of pests other than fleas and ticks, and it is used in a number of different applications. Products containing fipronil were first marketed in 1993 and they are sold under a variety of brand names including Frontline, Regent, Termidor, Combat and Maxforce and they are used:
- As flea and tick sprays for indoor and outdoor use.
- In turfgrass management programs at parks and golfcourses.
- As a termite preventative in some plywood products.
- To control parasites on livestock in South and Central America.
- As a replacement for the pesticide Dursban which was de-listed for on lawns and public areas.
- In baits and other ant control products.
- As a termite treatment.
- As a cockroach treatment.
- As a seed coating
- As a broad spectrum insecticide treatment on potatoes, corn, cotton, sweet potatoes, bulb onions, cotton and vegetables.
As you can see, there are a lot of places where fleas and ticks can be exposed to fipronil. And in many cases, they’re inadvertently exposed to ineffectively low doses of the compound.
Not only is fipronil used in a wide range of environments, it also has an unfortunate tendency to stick around the environment for a long time. Pesticides like fipronil that break down slowly and remain on soil or vegetation can contribute to selection for resistant organisms for a long time after they are applied. Fipronil degrades slowly on vegetation and relatively slowly in soil. It is listed as being highly persistent on land with a terrestrial field test half life of 75 days.
Fipronil is phototransformed to a variety of breakdown products. One of these breakdown products, fipronil-desulfinyl, is more toxic and more persistent than fipronil. The persistence of this breakdown product and its high neuroactivity, suggest that it may be a significant contributor to the effectiveness of fipronil. Unfortunately, it’s probably also a significant contributor to the development of fipronil resistance.
It’s possible that less effective (diluted) couterfeit fipronil containing products (especially those that are land applied) may be part of the problem as well. As we see in antibiotic resistance, exposing target organisms to ineffective doses of an pesticide will wipe out some, but not all, of the organisms. The surviving organisms may then become more resistant to the product and spread increased resistance to their offspring.
The low-dose effect may be a problem even when products are used at full potency because pests are exposed to lower doses of the product at the boundaries of the application area. Dilution, diffusion and breakdown effects also create situations where lower doses of persistent products can remain in and adjacent to sprayed areas for weeks or months after application.
I’m not the only one who believes that the indiscriminate use of acaricides (tick-killing agents) all over the world is leading to the selection of acaricide resistant tick strains. And fleas and ticks aren’t the only species that fipronil has had a significant effect on. Regent (a fipronil containing pesticide listed for agricultural use) is effective against a variety of pests, but there are increasing concerns about its environmental and human health effects. It was demonstrated to be responsible for a precipitous drop in bee populations in France. This occurred because fipronil causes bees to become disoriented and unable to return to their hives.
Fipronil provides us with a rather sobering example of the law of unintended consequences — the hubristic belief that humans can control the environment around us. It’s a reminder that management will always eventually fail — and Mother Nature will always bat last.
These sprays, dusts, and aerosols are now applied almost universally to farms, gardens, forests, and homes — nonselective chemicals that have the power to kill every insect, the “good” and the “bad,” to still the song of birds and the leaping of fish in the streams, to coat the leaves with a deadly film, and to linger on in soil — all this though the intended target may be only a few weeds or insects. Can anyone believe it is possible to lay down such a barrage of poisons on the surface of the earth without making it unfit for all life? They should not be called “insecticides,” but “biocides.”
— Rachel Carson, Silent Spring