This is the second part of a three part series.
Part 1 - Introduction
Part 3 - Ultimate Bloodshed!
Norway rats are very smart, very social animals that are successful because of two important principles: 1) they use their highly evolved senses to minimize their risk of being killed, and 2) they have learned how to take advantage of human beings and the places we ignore. Let's get something out of the way really quick: you will not be "eradicating" Norway Rats from an outdoor location. Soon after you stop harassing them, they will return.
Another bit of housekeeping: one of the worst things you can do with your rat problem is to leave open blocks or pellets of poison out in the open for them. First, the rats will most likely take it and stockpile it. They may never eat it. Second, you risk killing other animals that might eat the poison. Third, and this is true for all poison methods, secondary toxicity to rat predators (when animals eat a poisoned rat) is a real thing. One of my favorite previously available rat poisons was Ramik Green. Then one day at the farm supply store, the clerk said, "buy however much you want - the EPA says it kills hawks that eat the rats, so they're banning it starting next month." I had no idea!
As I mentioned, rats function across time and space in a risk-sensitive manner. They prefer not to be in the open, slinking along fence lines, under downed logs, and under decks and sidewalks. Anything to stay away from predators. Mature, dominant rats rely on sign (rat poop) and the existing trails of juvenile rats to get from place to place. The trails lead to food sources and back to hiding places. This behavior pattern has to be disrupted if any trapping, poisoning, or shooting results are expected. How to disrupt their patterns - realistically?
1) Remove as much of their favorite food supply as often as you possible can.
2) Clean up secondary food sources, like dog poop or trash bags in your own yard.
3) Investigate your hard surfaces. You'll eventually start seeing all the shit piles left behind by rats, each rat thinking, "If I crap here, everyone will know it's safe AND IT'S MINE."
4) Exclude rats from their favorite hideouts - highly recommend 1/4" galvanized fencing under existing sub-grade holes. In theory, chicken wire has small enough holes to exclude mature rats. However, they will bend the metal until it breaks under stress. Sigh.
This set of actions will confound Norway Rats. They'll briefly become more wary or "spooky," but ever the adapters, they will settle into new routes and routines after several days. Only in rare circumstances does the removal of a convenient outdoor food source predict that the rats will abandon their colony. In almost 20 years of chasing rats, I've never seen it happen, though animal rights "non-lethal control" websites "guarantee" the method. In reality, once safe burrows have been dug, the rats will simply go farther, less often, for food. However, it might decrease the colony's size from increasing at its normal (exponential) rate. And this is where the concept of rat "control" - not total rat elimination - comes in.
Thresholds for rat populations around humans do exist. Indoors, that threshold is roughly "zero." "Zero" rats are acceptable within our homes. This is for cultural, sanitary, and economic reasons. Something "greater than zero" is the acceptable in our gardens. Again - food source control may not get you there, especially when your garden is a food source. Part III of this series will deal in more detail with the concepts of thresholds for rat damage and density.
To summarize basic rat biology for your control plans:
1) Rats are sensitive to changes in their environment. Including trap placement.
2) Rats use a system of smoothed trails, hideouts, and hidden areas to move between their permanent burrow and their most dependable food source.
3) Rats will hoard, not eat, any loose bait left out for them in their feeding areas. It smells tasty, but it doesn't smell "quite right," and they know that.
4) Rat exclusion (electric fence, 1/4" galvanized mesh, cement fill, others) is an effective deterrent and management tool for rats' travel patterns. However, exclusion, in itself, is unlikely to have an impact on local rat densities.
5) Rat management that does not start with open food source control is guaranteed to fail. Secure the trash and compost. Pick up the dog and cat poop. Or stop complaining about rats.
Thanks for tuning in - Part III to drop shortly!