Mice & Rats

Rats and mice are one of the biggest nuisance pests to man. They damage and destroy property, eat and contaminate food, and can be a vector of diseases posing concerns to human health. The following information will provide some important facts about rats and mice and hopefully help you to understand them and why the control of their populations can be of such importance.

Biology

There are two basic species of rats: Roof and Norway rats; and two basic species of mice: house and meadow. They can differ greatly from each other in many ways, especially in how they are controlled.

Roof Rats are smaller, leaner rats with a long tail that they use to balance when climbing. Their body length is usually between 6-8 inches with a tail measuring between 7-10 inches. Their average weight can be from 5-9 oz but as much as 12 oz. Their fur is soft and brown and some black intermixed, and can even be gray on top. Underneath they can be gray, white, and occasionally black. Their noses are pointed; and they have large eyes and ears (their ears are large enough to be folded over their eyes). Their droppings are ½ inch and pointed on the ends.

Norway Rats are the largest of the rats and are also known as sewer, brown or wharf rats. They are usually between 7-10 inches long and can weigh around 7-18 oz, but can grow to 20 or more oz under prime conditions. Their fur is coarse and they have a rounded or blunt nose, small eyes and ears (their ears cannot be folded over their eyes). Their droppings are long, about ¾ inch and shaped like a capsule with both ends being blunt.

House Mice are the most common of all rodents. They are much smaller than rats and grow between 2.5-3.5 inches long. They usually weigh between ½ -1 oz and are dusty gray on top and light gray or off white underneath. They have a pointed nose with small eyes and large ears, and their droppings are small and lack ridges. This is important because their droppings are very similar to that of the American cockroach and this is vital to identification.

Meadow Voles are mice that live in surrounding landscape. House mice can and will also live in the landscape but prefer to find their way into a structure where their superior climbing ability makes it easy for them to move around. Meadow Voles are stocky with small prominent black eyes and their ears are very small. Their tails are about as twice as long as their feet. The reason behind their small tail is that they do not use it for balance when climbing since this species prefers to burrow into the ground. They have a dense shaggy fur which can be anywhere from gray to brown on top with the underneath being gray or even a bluff color.

Breeding

Both Roof and Norway rats reach sexual maturity in 2-5 months. Roof rats have an average litter of 6-8 young and can have between 4-6 litters per year. They live for only a short time in the wild due to a variety of circumstances; their life cycle usually is around 9-12 months.

Norway rats have litters between 7-8 young per litter and they can have between 3-6 litters per year. They usually live no longer than one year in the wild.

The house mouse is one of the highest breeding rodents as far as numbers are concerned. They reach sexual maturity in 35 days! Their pregnancies only last around 19 days and they can have up to 6 young per litter and have as many litters as 8 in a year.

Meadow voles can have about the same number of litters as a house mouse, usually 3-6, with as many young in a litter as 11. They also breed all year long but spikes in population can occur in the spring. Females reach sexual maturity in 40 days and their pregnancies last around 21 days on average. Meadow mice are the only one of the species to be somewhat active during daylight hours; while activity may be confined to the early day light hours or dusk, they are still rarely seen.

Diseases and Damage

Each year rodents cause millions of dollars in damage either by eating, gnawing, or contamination in a variety of ways. Rodents in California cause damage to landscapes by feeding off of plant roots, gnawing the bark off of small plants and bushes and large trees, even to the point of killing the tree.

They can enter homes and cause damage to wiring, and can contaminate a home with urine and fecal droppings. These droppings can spread disease is some cases and even the fleas they carry can vector diseases. Rodents are for the most part nocturnal and are rarely seen; most times the only evidence of their presence is their droppings or hearing them move through out a home or structure.

The biggest issue caused by rodents is not entirely disease but is more often the damage they cause. Replacement of electrical wiring, heating and air conditioning ducts, and many times plant material, are the result of a rodent infestation.

Control of Rats and Mice

Control of both rats and mice can overlap depending on where the infestation has occurred. The most common means of control are trapping and baiting. There are several forms of baiting that can be done, depending on what type of rodent needs to be controlled and where. We will cover all three aspects of rodent control most commonly encountered in California.

Trapping is the most common means of control that people are aware of, but only a small part of controlling rodents. The traps are physically set and if rodents are caught, they are removed from the structure. Glue boards are also used, and very effective against mice. However, they are rarely used in rat control because rats are large and fearful, and they learn very quickly to stay away from any kind of trap if they are not caught the first time they encounter them.

This type of control is rarely, if ever used, on the exterior. This is for several reasons: first, traps can only catch one rodent per trap. Treating an outside area that may have a large rodent population would be costly and drawn out. Second, the traps do not hold up well in moist conditions. Even plastic traps can pose a threat; they can harm non-target animals and children.

Baiting can be carried out in several ways. Bait stations are most often used to control both rats and mice. Bait is placed in a hard plastic or metal container to protect pets and any other non-target animals, people, or children from having access to the bait. Bait stations are also staked to the ground and labeled in accordance with state laws and regulations. Bait stations also protect the bait from exposure to the elements, preventing the bait from getting moldy; this can cause the rodents to develop an aversion to the bait. The rodents enter the station and feed on the bait, which is coated in wax.

The wax covering provides the rodents a means of gnawing which they must do to maintain their teeth. If not, their teeth would grow to a point where the rodent could not close their mouth and they would starve. Inside the wax covering is the bait, which must remain fresh since rats are especially particular to what they eat. This partiality to food can be problematic, especially when bird feeders are around; rats actually have a food preference and may abstain from feeding on bait if a more appealing food source is available.

Broadcast Baiting is a form of rodent control that most companies do not perform due to lack of knowledge, and in some cases lack of proper licensing. Broadcast baiting is done with a pellet form of bait which is spread within thick, dense brush to control rodent populations outside the range of feeding of a bait station. This method is the most control and cost effective when addressing large slope and densely covered brush areas. These pellets are small and contain less than ½ percent of active ingredient. If applied properly, there is little concern to non-target animals.

When developing any type of rodent control program, all options must be looked at. The most important key in any rodent control program is identification. Once you are aware of what type of rodents you are looking to control and where the infestation is located, then your options can be examined more thoroughly. Having the proper rodent control program can mean the difference between getting results and getting frustrated.