The term Vertebrate refers to animals with backbones such as mammals, birds, reptiles, fish and amphibians. Although a variety of vertebrates can sometimes become pests including birds, bats, skunks, opossums, raccoons and even deer, the species which will be briefly discussed here are rodents since they are the only ones for which pesticides are routinely used as a management method. However, any sanitation/exclusion used to prevent rodent infestations will almost certainly help with other vertebrate species as well. Also, since the pesticides used to control rodents are very unlikely to enter surface water through urban runoff, rodent management options will not be extensively covered in this Program.

For additional information on rodent identification and control options, contact ACWM using the resource information provided in Appendix H. The rodent species most likely to be to require management are the non-native commensal rodents and native burrowing rodents. A brief discussion of each is provided in the following.

SAFETY ALERT! – Do not handle rodents without rubber gloves. All dead rodents should be buried or placed in plastic bags in the trash.

Natural Predators

All of Los Angeles County’s rodents have natural predators including hawks, owls, coyotes, weasels and snakes. Protection and conservation of these species will help control rodent populations to an extent; however, they will never be able to overcome high rodent populations caused by poor sanitation or other anthropogenic modifications to the habitat. Feral cats should not be introduced or encouraged in an attempt to control rodents because, although they may kill a few rats, they will also destroy many native birds, reptiles and other small mammals. In most cases, feral cats must receive supplemental food from people in order to survive which, if not managed carefully, may result in other pest problems or the cats themselves becoming too numerous.

Commensal Rodents

The commensal rodents are non-native invaders and include roof rats, Norway rats and house mice. The word “commensal” refers to the fact that these species live in close association with humans and are rarely able to survive in truly natural areas. They are frequent invaders into homes, buildings, warehouses, etc. Roof rats are way more common in Los Angeles County than the larger and more aggressive Norway rats which are usually more closely associated with dense human population, trash, food processing etc., than places like landscaped areas and parks. However, Norway rats can be lured to almost any area by poor sanitation and harborage where their extensive tunnels and large, open burrows distinguish them from roof rats. Any infestation suspected to be Norway rats should be reported to the Department of Public Health.

High populations of roof rats can girdle tree limbs causing the limbs to die back. Beyond this, the amount of actual landscape damage caused by commensal rodents is probably minimal; however, steps can and should be taken to ensure an area is not attractive to them for the following reasons:

  • All of the commensal rodents have the potential to spread diseases to humans and pets*
  • The sight of a rat at a lunch area would be an unpleasant (not to mention potentially unhealthy) experience for employees and visitors
  • If commensal rodents are present in any number, they will exert constant pressure to enter buildings
  • Chewing damage to a variety of substrates including vehicle and irrigation wires
  • A County facility should never be the source of rodents invading the surrounding community

Unlike many of the pests covered in this document, commensal rodents are usually only able to live in an area due to anthropogenic (human origin) food sources. Table 1 below outlines conditions that contribute to an infestation of commensal rodents and which should be avoided or eliminated.

Table 1. Conditions that Contribute to Commensal Rodent Infestations

Accessibility of food and food-related trash If any of the following conditions are allowed to persist, there WILL eventually be a problem with rats, mice or both;

  • sloppy employee or visitor lunch areas
  • overflowing trash or trash cans without tight fitting lids
  • food leftovers at picnic areas
  • improperly stored, leftover or spilled pet food or feral cat feeding locations

If trash dumpsters are routinely filled to overflowing, it’s time to get a bigger one, an additional one or schedule extra pickups. Locating dumpsters 25 feet or more away from the exterior of a building helps reduce the overall quality of habitat for commensal rodents.

Bird Feeders/Bird Feeding The leftover and spilled seeds from bird feeders can contribute to severe commensal rodent problems. If a bird feeder is absolutely essential to the character of the facility for some reason, it should be tightly covered or brought in each night and the spilled seeds cleaned up.
Water Leaking sprinklers, faucets and over irrigation provide an almost constant supply of water for unwanted rodents.
Harborage Commensal rodents don’t like to be out in the open and need a place live and hide. Eliminate things like dense vegetation, brush piles, piles of scrap wood and building materials, and pallets.

Glue Boards

The most common methods of non-chemical control of commensal rodents are glue boards and traps. Glue boards are completely non-selective and will trap almost anything that attempts to walk across them. If a non-target animal is caught in a glue board, it is virtually impossible to release it unharmed. For this reason, they should only be used in locations where non-target animals, including birds, do not have access to them. Glue board covers or “tunnels” are commercially available which may provide a margin of safety for non-target animals in some situations.

Photo of bird trapped on glue board

*Dr. Joe Ramirez of the Department of Public Health, Environmental Health Division, reports that 68 cases of murine typhus were confirmed in L.A. County in 2013. It is his opinion the disease is greatly underreported.