How to kill rats, inside and outside dengarden

The University of California at Davis Integrated Pest Management Program, our San Mateo County pest control agency, and the Centers for Disease Control all have advice on how to close off openings that rats might use to enter a house. The idea is that you can’t hope to eliminate rats in the great outdoors—humans have never been able to do that—but you can minimize some outdoor temptations like dog food and barbecue grease, and you can separate the human space indoors from the rat space outdoors. Securing the House to Keep Outdoor Rats Outdoors

If you close off all openings to the basement or crawlspace, around the pipes, and the bottom of the house, and leave none larger than a half dollar, you can theoretically keep the Norway rats out; and if you can do the same for the attic and roof area, and leave no holes bigger than a quarter or so, you can keep the roof rats out.


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Dry dog food must seem ideal to rats; portable, storable, full of protein. In fact, it makes a good bait for rat traps. If the dog food is outside, or where rats can reach it, they will get it, and stash it in their nests, including in parked cars. Access to a large supply of cat and dog food can create a horrific infestation, as in this veterinary hospital where rats were able to get into the building and eat the pet food. Keep your dog food locked up, not in dishes on the porch or an open garage. Barbecues Attract Rats

One way rats find out what’s good to eat is that they avoid gorging themselves on a food or bait they aren’t familiar with, a behavior called “bait shyness.” They sample a new food or food-like substance and see if it makes them or other rats sick before they eat any large amount. Rats do this cautious nibbling not only because they are clever and conservative, but because they are physically unable to vomit.

But since 2011 there has been ample evidence that this delayed effect of a buildup of poison in the rat’s body, such as from second-generation anticoagulants, causes rats to poison other animals who eat them. The rats may not die for days after they start eating the poison. In the meantime, any cat or dog who eats the poisoned rat will be poisoned themselves, as will any bobcat, lion, fox, coyote, hawk, owl, or eagle. The California Department of Fish and Wildlife, explaining this problem in detail, confirmed 400 deaths in these wildlife species and many others from SGAR rat poisons between 1994 and 2014 and they say the effects are likely much more widespread since poisoned wildlife may hide and die out of sight. CDFW even found “tertiary” poisonings, two mountain lions who died from eating coyotes who had eaten poisoned rats.

The State of California banned four SGARs for household use in 2014, and EPA prohibited their marketing to households in 2015. EPA also prohibited the marketing of pellet-sized baits for home use (because the pellets might be eaten by children and pets). Lisa Owens Vianni of Friends of the Earth notes, however, that the 2014-2015 bans did not go nearly far enough to protect wildlife from anticoagulant baits. The four banned pesticides are still available for commercial or professional use—for example, in those bait stations you see in parking lots and malls and private property—and can still be bought online in quantity. Other long-lived anticoagulants like diphacinone ("Tomcat Liquid Concentrate") are still available for home use, and are toxic to wildlife. Anticoagulant rat poisons have been found in the carcasses of fishers (rare forest carnivores), spotted owls, and barred owls, and blamed for the deaths of suburban bobcats and mountain lions, even after the 2014-2015 law.

(1) It used to be sold from feedstores as "Wafarin" and still may be as far as I know. It works because it makes the rat bleed to death internally. However, it’s not lost on some old timers (like my husband) the irony of that — because "coumadin" a blood thinner often prescribed for serious heart problems — is the manufacturer name for generic "wafarain." At 68 he’s convinced they are poisoning him.

(3) Strange but true — in the building my son, his wife, and two year old live in (in Hong Kong) rats are a huge problem. For about 4 months the building manager kept poisoning the rats on the common porches, everyone was upset because the dead rats decaying bodies had produced a horrible smell (or so they thought). Turned out the elderly lady above their apartment had expired and no one knew it as her corpse rotted for all those months. After the discovery, the manager posted a notice that no one is allowed to poison the rats and must only use traps — to ensure that situation does not repeat itself.