Vertebrate Pest Conference Proceedings collection


Date of this Version

March 1986


A program to control roof rats (Rattus rattus) has been conducted by the Orange County Vector Control District since 1975. Orange County is located in southern California just south of Los Angeles and is composed of 782 square miles of coastal foothills and alluvial plain with a population of 2 million. Urbanization takes up about one-half the total area and it is mainly a semi-desert situation with usually less than 15 inches of rain a year. However, because of the balmy temperature and imported water, vegetation grows rampant. Vegetation is used by the rats as harborage (Algerian ivy, bougainvillea, dracaena, etc.) and a food source (oranges, avocados, and various ornamentals with small fruits).

The Orange County Vector Control District is a special district with its own appointed governing board and 32 employees. There are 16 vector control technicians assigned to 16 geographic zones to answer complaints, make inspections, and control rats, flies, mosquitoes, and chironomid midges. They answered 7,281 complaints (service requests) on rats in 1985. Technicians visit approximately 41,000 properties for inspection and treatment per year. They are examined and certified in rat and other vector control by the State Health Department.

The roof rat (Rattus rattus) is a difficult animal to control. It is wary of traps, shows bait shyness, and some of its food (avocado) has high amounts of vitamin K, an anticoagulant antidote. It is secretive, an excellent climber, and can get through very small openings. It is typically an outdoor rat but we have seen an increasing incidence of animals going into the home. Over a 10-year period, there has been a 3 to 20% increase. They take advantage of every resource in their environment for harborage and food. The district has very few problems with Norway rats.

The program is basically a complaint/response program. We have tried neighborhood surveys and inspections and found them to be not cost-effective. Our process is to receive a service request, record the location data, and have the technician call with 48 hours to set up an appointment to visit the service requester's property. The technician visits the property and inspects for rat signs, harborage, food sources, and entrances into the structure. He does not go into the home because any work there is the responsibility of the homeowner or a private pest control company. The technician writes out and gives to the property owner a set of recommendations and an educational pamphlet. If chemical control is needed, then a release is signed by the property owner and bait blocks are placed in an appropriate place. It is the judgment of the technician that determines if bait is placed, under what conditions it is placed, and where it is placed. The technician also visually checks the adjacent properties for rat signs and harborage. If there is suspicion of rat activity, then the technician visits and inspects those properties. We will not place bait if the resident doesn't want it. Since our workload in the summer doesn't allow the technician to recheck a property within 3 months, we try to revisit in the slower winter months to check for bait activity, new rat signs, and clean-up by resident.