Controlling rats in Evanston

By Lawrence Heaney, Curator of Mammals, Field Museum of Natural History, Chicago

Rats in Evanston and other Chicago communities cause large amounts of damage and may transmit disease. Controlling, or better yet, eliminating them, has no down-side, since they have no positive value. But how to do this effectively is not a simple matter. Here’s what the research shows.

Pest rats in Evanston are “brown rats”, or Norway rats (Rattus norvegicus, their scientific name, was given to them by a Swede). These are not native to North America: they originated in the Middle East and Central Asia, and began living with people probably by 8,000 years ago. This has given them a lot of time to develop means of living with us. In most parts of the world, this species is almost entirely dependent on humans for food and shelter, and do not live in natural habitats - and this is true in Illinois.

Brown rats are capable of exceptionally high rates of reproduction: adult females can produce four to eight litters per year, with an average of seven but up to 15 young per litter, often producing 50 young in a year. Females often begin to produce babies when they are four months old. The number of young in a litter is mostly determined by the amount of food available: the more available food, the more babies.

Brown rats start off pretty naive, but learn very quickly. The first time a young rat encounters a trap baited with peanut butter, it will eat the bait and be caught by the trap. Other rats see and smell the dead rat and peanut butter, and will never go near peanut butter. If they see a rat (usually a young one) eat poison bait and soon die, they will never eat the poison bait.

Given the very high rate of reproduction, quite a few babies can die and serve as a warning to others without causing much decline in the rat population, and new babies quickly take their places. If lots of babies are killed, this leaves more food for the (experienced, cautious) adult females, who are then able to produce more and larger litters.

Feral cats are sometimes promoted as a means of controlling rats. Several studies have shown that cats will catch young rats, but virtually never catch adults. Rats are large and aggressive, and cats learn not to mess with the adults. The attached photo shows a cat and an adult rat facing off in an alley in Baltimore, in a study by a team at Johns Hopkins University, which showed that feral cats have almost no impact on reducing rat populations. When people report that cats are killing rats, it is the young ones being killed, not the adults. In the Chicago area, there are native predators that kill and eat rats. Coyotes are especially effective; studies in downtown Chicago show that most of their prey are rats. Owls can be very effective also.

All of this leads to one conclusion. Reducing rat populations is quite possible, but only by one means: reducing the amount of food. With less food available, females are able to produce fewer young, and if food is eliminated, the adult rats either move on or simply cease to reproduce. Currently, many alleys have trash bins that have holes easily used by rats, and bags of food trash often accumulate for several days before pickup. As long as we provide a “rat cafeteria”, rats are likely to reproduce more rapidly than they can be removed.