I recently had an extensive e-mail conversation with UMO cooperative extension agents re: the safest poison options for dealing with mice/rats in/around garden and livestock areas. I'll post her pertinent responses.
My initial question was regarding a mouse infestation in my high tunnel. The mice had decimated one of my beds of greens, and was starting on the second one. Here's part of her response:
Below is a response from the State Toxicologist with the Maine Board of Pesticides Control, Pam Bryer. With respect to your first question, I would avoid eating the remaining greens if you are certain they’ve been contaminated by mouse droppings.
As prep for a talk I did recently I went to my local big box store to see what types of rodenticides they were selling. You are right in that many of them are the neurotoxic bromethalin but there are also two others you might be interested in: cholecalciferol and zinc phosphide. And, by the way, if you were to hire a commercial pesticide applicator they would be able to use a different set of pesticides as well (namely the anti-coagulants).
Bromethalin: persistent in soil, doesn’t volatilize, doesn’t wash out, highly likely to transfer up the food chain, not carcinogenic, moderately toxic to soil invertebrates, and highly toxic to mammals. If it were used in the scenario you mentioned it would persist in the soil for some time but it would be difficult for it to be translocated into the plant tissues. Essentially, because it binds tightly to organic matter and is a large molecule it would be difficult for the plant to take it up in significant quantities. Also, you are looking at a volume dilution effect. Rodents are more susceptible to these poisons than humans so a lower dose is used to kill them than would kill us — plus we are much larger and would need many more times the dose than it would take to kill some mice. That said, because of its persistent nature and bioaccumulation potential and toxicity it would be best to avoid that if you can in a garden space.
Cholecalciferol: not persistent in soil, doesn’t volatilize, doesn’t wash out, low likelihood of transferring up the food chain, not carcinogenic, moderately toxic to soil invertebrates and mammals, and low toxicity to birds and fish. Cholecalciferol is another name for vitamin D. Rodents are more susceptible to vitamin D overload than humans, and other mammals, are which makes this a less risky pesticide to use around humans. Because it breaks down in the soil quickly, 97% will be degraded within ~25 days, this becomes a better choice for a garden area. Again, like above, the dilution of the chemical within the soil, and then the plant, and then folks who eat that plant, the actual amount of active ingredient would be unlikely to cause effects. This rodenticide acts by disrupting calcium balance in the body; excess calcium is released from the bones and leads to multiorgan disfunction.
Zinc phosphide: not persistent in soil, doesn’t volatilize, doesn’t wash out, not carcinogenic, low toxicity to soil invertebrates, moderate toxicity to fish, high toxicity to mammals. This compound to changed by the low pH of the rodent stomach into phosphine gas. Phosphine is very volatile and moves quickly into the lungs were it acts to prevent normal respiration and death ensues. The amount of phosphine that is generated is unlikely to harm humans and it ends up dissipating quickly. Phosphine (or a precursor) is used in agriculture as a soil fumigant and agricultural storage fumigant.
Based on this information, I have bought some Cholecalciferol. D-Con is now changing all of their formulation to Cholecalciferol. My research, after reading the links provided by the UMO extension agent was that Cholecalciferol is the ONLY mouse/rat poison that is approved for use around organic farms. I would never buy mouse/rat bait that is not provided in chunk or bar form. The very last thing I want a rodent to do is pick up bait pellets and transport them to a nest. So, no matter what chemical one buys, options are automatically limited if you want it in chunk, block or bars that fit into bait stations.
Further reading tells me that traps may work for a short time if there are only a few mice/rats. However, they quickly become trap wary, and won't touch a trap, or... in the case of the traps set in my HT, IME, they learn how to snap them, so they can take the bait freely. If any feed sources are available: bird feeders, compost piles, gardens, just a few spilled grains from your livestock, hay or straw bales, the rodents will move in and proliferate. The best way to deal with them is to remove ALL feed sources. That may not be possible if you have livestock, or a garden. Next up is a combination of traps and poison. Sadly, if you want to keep rodents from setting up housekeeping with resultant population explosions, IMO poisons must be employed.
