The Return Of Bromethalin

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May 1, 2000

Bromethalin is an interesting rodenticide. It was available to our industry under the trade name Vengeance™ for about 12 years until just a few years ago when its most recent sponsor, AgrEvo Environmental Health, decided to remove it from the marketplace. But bromethalin is now "back" marketed by two manufacturers: Bell Laboratories with Fastrac™ and JT Eaton with Top Gun.™ Both companies offer the rodenticide in pellet and block formulations at concentrations of 0.01 percent (100 ppm).

But what might be some reasons for you as a pest management professional to use bromethalin as part your rodent control? How does bro-methalin differ from conventional anticoagulant rodenticides? What is its mode of action? And, does bromethalin pose any unique hazards to non-target animals? Let’s examine these questions a bit further.

Mode of Action. As a rodenticide, bromethalin is classified as a non-anticoagulant compound. Obviously this is because its mode of action has nothing to do with the circulatory system and the interference of the blood’s coagulation process. However, you would nearly need to be a pharmacologist to be able to understand bromethalin’s physiological mechanisms.

Technically, bromethalin uncouples the oxidative phosphorylation process at the cellular level, thus interrupting the vital production of ATP necessary to maintain essential metabolic function. The end result, basically, is that rodents become progressively weaker over a couple of days and loose the ability to feed, move and ultimately, breathe. Poisoned rodents are frequently found with a characteristic pose of the back legs stretched out behind them. It’s not likely many of us will be trying to explain all of this to the everyday client who asks, "How does that stuff kill my mice?" On the job, it’s easiest to just state, "bromethalin interferes with the process of energy production within living cells," or that it "attacks the central nervous system." Those needing more information can surf the Web or contact manufacturers for technical sheets.

Single-Feed/Stop-Feed. Much is made about bromethalin’s unique action to provide single-feed/stop-feed action. The current labels state that "rats and mice may consume a lethal dose in a single feeding with first dead rodents appearing one or two days after bait consumption." Obviously, such characteristics are attractive in a rodenticide. The time it takes bromethalin bait to kill a rodent is dose dependent. On average (i.e., sometimes more, sometimes less) it requires about 2.5 grams of bait to kill a mouse, and 7 to 8 grams for rats. Considering mice consume 2 to 4 grams of food daily, and rats 15 to 35 grams, these amounts are well within realistic expectations, especially if the bait is of good quality and highly palatable to the rodents. Death occurs from 12 hours to 4 days after a rodent has ingested a lethal dose.

I have baited infested poultry houses with bromethalin and began observing dead rodents 15 hours later. Research has shown that rodents cease feeding after consuming a toxic dose of bromethalin bait — even though they may not die for another day or two. Of course, not all rodents feed at only one spot in a particular evening, especially if an account has plenty of other acceptable foods available to rodents (again, illustrating the importance of combining sanitation efforts with rodent control efforts). But by placing the bait into the rodent’s high activity zones (as a result of thorough inspections), we can maximize the chances of stop feeding and quick-kill based on a single-night’s feeding.

Two Important Advantages. As I see it, the return of bromethalin to our industry is a plus. I say this for two reasons. First, because bromethalin offers a quick-kill potential it can provide a valuable tool for some food, pharmaceutical and warehouse accounts. For example, anticoagulant baits work on a delayed effect basis. A rodent feeding on an anticoagulant bait outside a food warehouse may continue to forage for another 3 to 7 days before succumbing to the effects of the bait. I call this a "dead rodent walking" (and eating). During this time, the poisoned rodent may still forage into the warehouse or plant and contaminate and/or infiltrate product. Such a scenario could also occur with bromethalin baits, but the "dead rodent walking" time is likely to be significantly reduced.

Second, and perhaps even most important, is the issue of future potential anticoagulant resistance. The baiting of rodents with second-generation compounds for the past 20 years has been unrelenting by professionals and homeowners alike. Thus far, resistance to second-generation anticoagulants in the U.S. has not been formally recorded. But resistance to the bromadialone compound does exist in several countries outside the United States and there is no reason to believe that rodents in the United States won’t eventually follow. It may even be possible that pockets of second-generation resistance are currently present in the United States but remain undetected or unrecorded. Still, it is important to note that resistance to all second-generation anticoagu ants is not a foregone conclusion.

To date, for example, no formal resistance has been noted in any country for brodifacoum (e.g, Talon and Final), despite more than two decades of continued use of this anticoagulant applied successfully to thousands of significant rodent infestations.

Should resistance to anticoagulants develop within the next 5 to 10 years, it is a matter of serious concern. We must consider how expensive it is for a manufacturing company to develop, research and launch a novel rodenticide in today’s regulatory environmentally conscious climate. It is becoming more and more difficult for the marketplace to be able to support such up-front expenditures.

Possible Concerns? In the past, questions have been raised regarding the palatability of bromethalin baits. However, most of these concerns are a result of experience with the earlier formulations, and particularly with mice. Bell Laboratories maximizes Fastrac’s palatability by minimizing byproducts and impurities combined with highly palatable quality baits. JT Eaton also claims better formulations for their Top Gun than the bromethalin rodenticides baits of past. Because both Fastrac and Top Gun became available only very recently, the true test — the pest management professional’s use of these products over the next several years — will be the deciding factor of bromethalin’s acceptance by rodents (and thus by pest professionals).

Another concern with the bromethalin rodenticides is that unlike anticoagulant rodenticides, bromethalin doesn’t have a specific antidote. Although this is true, it would require relatively large amounts of finished baits to be ingested by non-target animals to be lethal. And due to the stop-feed qualities of bromethalin, the placement quantities are small. Considering these two factors together, the chances of lethal ingestion should be extremely small.

If non-lethal doses of bromethalin are consumed, the toxic effects are reversible and may be countered by administration of finely powdered charcoal, a diuretic (to remove tissue fluids) and general supportive therapy, which is a standard treatment mode for many chemical poisonings. It also bears mentioning here that secondary poisoning (e.g., a dog consuming the carcasses of rodents poisoned by bromethalin) does not readily occur with bromethalin.

In summary then, having a non-anticoagulant as part of our industry’s rodent control arsenal gives us extra tools — tools all pest management professionals need. And, as most of us know, rodent control can be challenging and seems to be becoming even more so each year. We need as many tools as possible and we should continue to welcome new products into the marketplace.

The author is president of RMC Pest Management Consulting and can be reached at rcorrigan@pctonline.com or 765/939-2829.