Several California mosquito and vector control districts offer their residents general information on rodent control. This information on roof rats emphasizes conditions in the City of Davis and what preventative practices citizens can do to prevent or reduce rodent problems.
The roof rat or black rat (Rattus rattus) is responsible for having spread the plague in Europe during the Middle Ages. Here in Davis it poses both a health and safety hazard. Besides plague, roof rats may spread other diseases to humans like murine typhus, leptosporosis, salmonellosis, trichinosis and rat bite fever. They can also spread diseases to domestic animals and are suspected of transmitting ectoparasites from one place to another. Besides consuming and contaminating stored food and feed they will gnaw on wiring (posing a fire hazard), wood, and tear up insulation for nesting material. Rats will feed on fruits and vegetable in many gardens as well as damaging young trees by feeding on their bark. Unprotected compost piles as well as pet food are common food sources for them in our neighborhoods.
The roof rat’s cousin, the Norway (or sewer) rat (Rattus norvegicus) is a burrower. It is larger than the roof rat and prefers ground level dwellings.
Roof rats are nocturnal like their cousins the Norway rats, but the aerial nature of the roof rat is what distinguishes them and has implications for their control. Roof rats prefer to forage above ground; they are agile climbers and travel through trees, vines, rafters, wires and rooftops. They use trees and utility lines to reach food and enter buildings. They can also be found foraging in dense ground cover. Secluded areas above ground are ideal for nesting such as attics, overhead garage storage, in the vine cover of fences or buildings and in wood piles. They prefer non-deciduous trees or trees with hollow cavities and crowns of palm trees, especially when old fronds are not removed.
Roof rats have high reproductive potential, with 3 to 5 litters/year and 5 to 8 pups/litter.
Prevention through methods of exclusion and habitat modification can provide long term control by preventing infestation. Exclusion from homes or buildings is difficult since they are excellent climbers and can get through very small openings. Rodent proofing a home or building requires sealing all possible entry points especially where pipes or utility lines enter a building.
- Seal openings ½ inch wide or greater to the outside of a structure with concrete mortar, steel or copper wool or metal flashing
- Cover attic and foundation vents with ¼ inch wire mesh or heavy wire screen
- Use rat guards made from sheet metal 18-24 inches wide to prevent the rats from climbing
Habitat modification makes an area less suitable and less attractive to roof rats. They are very sensitive to changes in their environment -- even slight habitat changes may cause these rodents to move or redirect their activity patterns. Improving general sanitation is a big step towards eliminating food sources:
- Secure garbage in rodent proof containers
- Store materials properly
- Harvest fruits and vegetables in a timely manner and pick up fallen fruit promptly
- Remove attractants such as pet food, bird feeders and standing water.
- Compost in closed compost containers
Eliminate or reduce protective cover that these rodents use:
- Prune shrubs so that the ground below is visible
- Mow, trim or remove ground cover plants that are over 1 foot in height
- Stack firewood or lumber at least one foot away from walls and fences and at least 18 inches off the ground
Roof rats travel along vines, tree branches, fences and utility wires. Eliminating these aerial pathways can dramatically reduce roof rat travel.
- Eliminate or severely prune back vines growing on buildings or fences
- Remove tree limbs that overhang roofs
- Prune trees so that the branches do not touch fences, overhead wires or branches of adjacent trees
- Prune skirts of trees so that branches do not hang down to the ground
These preventive practices will help in reducing roof rat infestations.
