Proceedings of the Vertebrate Pest Conference

The General Chairman of this first “Vertebrate Pest Control Conference” introduces the philosophy of vertebrate pest control and the need for sharing information on this topic, including publication of a Proceedings that will serve as a handbook on method for controlling non-game vertebrates in North America that at times become pests. The term “vertebrate pest” is defined, and the author emphasizes the value of artificial regulation of the density of many species of vertebrate animals as being common sense, and also as good conservation of natural resources.

The author provides a history of the development of vertebrate animals control as undertaken by the Bureau of Biological Survey, and its successor, the U.S. Fish and Wildlife Service. Early efforts to protect agricultural crops are cited, as are discoveries about the role that vertebrate animals play in public health concerns such as Rocky Mountain spotted fever, bubonic plague, and rabies. Efforts by the Forest Service within the U.S. Department of Agriculture are described, as authority was given to this agency to control rodents and predators on federally owned and controlled lands in the western U.S. California’s development of Departments of Agriculture at the state and county level are noted, as programs to control animal damage were developed and expanded, eventually also including control methods for crop-destroying pest birds. The author concludes by noting that conservation and pest control efforts can be complimentary, and people of good will working together can find reasonable solutions for pest problems. Sharing information, as can occur with this Conference, can point the way toward needed research into better solutions for vertebrate pest problems.

Known methods for controlling both venomous and nonvenomous snakes are discussed. Among the techniques described are exclusion of snakes from structures, use of exclusion fencing, habitat change to reduce or remove food and cover favored by snakes, and searching for and killing snakes (often when rattlesnakes or copperheads first emerge from dens in the spring). Various types of snake traps are discussed. Toxicants employed to kill snakes include nicotine sulphate in water, strychnine in eggs, and various insecticide sprays. Fumigation of burrows and dens is discussed, along with cautions to prevent nontarget hazards as well as hazards to the applicator. Several chemicals, including the insecticide DDT and others, may be effective snake repellents or can be lethal to snakes under some conditions. Miscellaneous methods include use of domestic animals (geese, ducks, chickens) that may kill and eat snakes. Bounty systems have been widely tried, but they are ineffective and subject to abuse.

The author provides a thorough review of methods for controlling coyotes, bobcats, and foxes, noting that “control” is not intended to exterminate these species, but rather to remove individual problem animals or to reduce economic losses to a practical minimum by local population reduction. Coyote control measures discussed include use of traps including description of various types of trap sets, snares, coyote-getters, denning, still-hunting, calling and shooting, aerial hunting, and poisoning using either strychnine drop-baits or impregnated bait stations using Compound 1080. Precautions to limit nontarget poisoning are provided. The use of exclusion fences is briefly mentioned. Bobcat control methods discussed include trapping and hunting with dogs. Control methods for red fox described are trapping, snaring, denning, coursing with hounds, organized drives, calling and shooting, and aerial hunting. Advantages and disadvantages of all methods are discussed.

Past and present practices to control wolves in British Columbia, Canada are discussed. Control is carried out chiefly for the protection of livestock. Reduction in use of strychnine occurred after the 1940s, and Compound 1080 (sodium fluoroacetate) became increasingly effective and useful after the mid-1950s. A bounty system, which had been in place previously and was in direct competition with government-organized predator control programs as well as subject to fraud, was discontinued in 1955. Aerial application of 1080 baits over larger areas under appropriate conditions in winter, or to spot treat locations where predation has occurred at other times of year. Precautions are taken to safeguard desirable species as much as possible, including collection and disposal of baits in late winter or early spring. Regulations governing toxicant use are described, including landowner agreements if used on private lands. Present predator control methods and techniques have proven to be very effective, especially with wolves, for the protection of livestock.

Methods of controlling meadow mice of various species are reviewed: Microtus californicus in the West, M. pennsylvanicus in the East, M. ochrogaster in the Midwest, and Pitymys spp. in the East and South. Control methods employed have included habitat modification, use of individual guards or repellents around tree trunks in orchards, and burrow fumigation against Pitymys spp. and M. ochrogaster. Trapping is generally practical only on a small scale, using wooden-based snap traps baited with pieces of apple and/or rolled oats, or using pit traps. Rodenticides generally have provided the best control of meadow mice. While strychnine is still used by some orchardists, zinc phosphide had been shown to be more effective and presents less nontarget hazard. The practice of trail baiting was developed and is discussed, either by hand or by use of mechanical bait dispensers such as the trail-builder. Ground sprays were developed by the early 1950s, and endrin was found to be the most effective toxicant against pine mice (Pitymys spp.), however at times it has given poor results in some Midwestern states. This method is not recommended by the Bureau, due to potential nontarget hazards as well as environmental concerns about continued used of chlorinated hydrocarbons. Currently, broadcast zinc phosphide rodenticide applications in orchards provide the most economical control solution and are considerably less hazardous to non-target species. A list of relevant publications is provided.

Methods for the control of problem opossums, bats, raccoons, and skunks are discussed. Basic biology of these animal species is provided. For the opossum, control methods include trapping, physical capture at night, or use of a strychnine formulation injected into eggs. The presence of bats in homes or other structures is a concern due to their potential infection with rabies. Bat biology and behavior is summarized, and bat-proofing of structures is recommended as a technique. Bats repellents for use in attics or other closed spaces are discussed. In some situations, professional fumigation of a structure to kill bats is needed, but it has several disadvantages and does not provide a permanent solution. Raccoon biology is summarized. When numerous, they can cause economic damage to gardens and crops and are especially destructive to corn fields. Raccoon trapping using a variety of traps and types of trap sets is described in detail. Exclusion of raccoons is possible by adding a single wire strand electrified by a fence charger to an existing fence. Where raccoons cause extensive damage to crops, they can be controlled using strychnine baits; formulas and instruction for strychnine-egg baits and strychnine-corn baits are provided. Skunk biology and behavior is summarized. Preventing their entry into structures is preferred over skunk destruction, as they have beneficial aspects by eating mice and insects. Techniques for excluding skunks are described in detail, including exclusionary methods to keep them out of poultry houses. Insecticides properly applied to lawns will reduce skunk damage to turf by eliminating insect and grub food sources. Methods to trap skunks are described, as are techniques and a formula for making strychnine-egg baits to kill skunks.

Control tools and methods for moles and for woodrats (Neotoma spp.) are discussed in detail, with emphasis on California and other western states. Categories of control methods included are exclusion, repellents, traps and trapping, burrow fumigants, and toxicants. In regard to mole toxicants, experience indicates that sodium fluoroacetate and thallium sulfate are considered the most effective, although these materials are restricted because of their possible nontarget hazard. For woodrats, sodium fluoroacetate has given excellent control, while zinc phosphide is probably the most commonly used rodenticide. Anticoagulants when used properly can also be effective. References on the biology and control of these species are listed.

Pocket gopher biology, behavior, damage, and control are discussed extensively in this paper, with emphasis on Thomomys spp. found in California and elsewhere in the western states. Habitat modification by use of herbicides to reduce forbs, which are preferred by gophers, has been demonstrated to be effective on Forest Service rangelands. Limitations of burrow flooding or burrow fumigation are discussed. Exclusion techniques are described as most useful in home and garden situations. Gopher control is most effectively done by poisoning and/or trapping, both of which are discussed in detail, as are types of traps commonly used. Effective poisons are Compound 1080 and strychnine, if correctly formulated and applied. The recently developed mechanical gopher bait applicator implement, the burrow-builder, is discussed. Pertinent references are provided.

Description of control techniques and methods for grounds squirrels in California are provided in detail, as well as some history of their development. Poisoning, trapping, and burrow fumigation are the only known procedures that give satisfactory control, with poisoning usually the best method. Of the toxicants used, sodium fluoroacetate is the most effective. Burrow fumigation has proved successful with both carbon bisulphide and methyl bromide, and the former is described in detail. Techniques are described for bait formulation, bait acceptance testing, and bait application that will allow use of these rodenticides in a manner that does not seriously endanger nontarget wildlife or domestic animals.

Control methods used against meadow mice (Microtus), kangaroo rats, prairie dogs, and cotton rats in California and other western states are described, with emphasis on proper formulation and application of rodenticide baits. For meadow mice, habitat management by removal of vegetative cover is discussed, as are exclusion barriers that can protect individual plants. Reduction of meadow mice populations requires use of rodenticides, which are formulated using apple cubes, oats, cracked corn, or wheat; zinc phosphide and strychnine recipes are given for formulating these baits. For kangaroo rat baits, rolled oats, rolled barley, or milo maize are preferred bait materials, generally treated with zinc phosphide or strychnine. For prairie dog control, burrow fumigants or trapping are sometimes employed, but more commonly larger control operations use rodenticides with Compound 1080, thallium sulfate, zinc phosphide, or strychnine formulated on steam-rolled oats or whole oats. Cotton rat rodenticide baits are made using materials such as oats, corn, wheat, maize, carrots, sweet potatoes, or white potatoes. Toxicants employed are strychnine, zinc phosphide, or thallium sulfate. Recipes are given for use of oats or sweet potatoes.

Muskrats, originally native to only two small areas of California, are now widespread in their distribution, where they can cause damage to some crops, but are of particular concern because of their burrowing in structures of irrigation canals and ditchbanks, waterways, and levees. Methods of canal and waterway construction can reduce or prevent damage, as can use of materials that exclude muskrat burrowing. Some materials have been shown to be repellent to muskrat burrowing when used to treat the soil near waterways. Reduction or elimination of weedy plant growth can be a deterrent in situations where no other food sources are nearby. Muskrat populations can be reduced or controlled by use of traps, burrow fumigants, shooting, or use of toxic bait; details of recommendations for use of these methods are included.

Nutria, which have increasingly been brought into California to be ranched for fur within the past decade, have know to have escaped, with the first known wild population seen in 1948. Initial attempts at control by use of carrot or sweet potato baits using zinc phosphide or strychnine were unsatisfactory. A trapping program begun in 1951 by the Bureau of Sport Fisheries and Wildlife (USFWS) is now considered the most effective control method. Use of No. 3 steel traps is described, as are the merits of shooting either during nighttime on feeding grounds, or in daytime.

Damage to orchard crops and field crops in California by several species of native birds is described, covering biology, behavior, and typical damage caused by linnets (house finches), horned larks, crowned sparrows, and woodpeckers. The legal status of this birds is unique in California, as these migratory birds may be controlled under the authority of the California Department of Agriculture and their associated County Agricultural Commissioners. Bird exclusion, frightening, or other deterrent techniques should be tried first, and only when these are ineffective or impractical should lethal methods be used. Specific bait formulas and toxicants for bird control are under the authority of the state and are not provided here. Control of house finches in agricultural crops typically involved use of prebait followed by toxic bait exposed in orchards in bait troughs; trough design, placement, and use are discussed. For horned larks, lethal control is best accomplished by closely observing feeding behavior, and then exposing prebait followed by toxic bait in furrows, trails, or other similar depressions in the soil within crop fields. Frightening has also worked well, using carbide exploders, the “stake and flag” method, or “continuous string flagging,” especially when these scare tactics are installed in advance of or at the first sign of horned lark damage. Crowned sparrows prefer dense vegetation, so habitat management to remove such cover (including weedy borders along fields or fence rows) near crops or orchards can entirely prevent damage. In smaller plantings, wire frames to exclude crowned sparrows or dense flagging can prevent damage. Where lethal control is needed, prebait and then toxic bait is exposed in shallow v-shaped troughs 2 to 3 feet above ground level, alternately throughout the period of crop damage. Woodpecker damage can be a serious problem in walnut and almond orchards, or they can damage wooden structures. Selective shooting can be effective, as can application of chemical repellent materials on areas that are being damaged or adjacent ledges. Occasionally, woodpeckers have been successfully removed by use of Victor rat traps, or use of toxic baits in holes where woodpeckers store acorns.

The limited distribution of yellow-billed magpies in California is described, with emphasis on Merced County, California. The state’s Fish and Game Code was temporarily amended in 1959 and again in 1961 to remove this species’ protected status temporarily, allowing lethal control. Use of strychnine baiting using walnut or almond meats is described, preceded by prebaiting, typically during January through May. Control also involved removal of nests by knocking the nests out of trees and then removing and destroying nesting materials.

Damage by starlings and blackbirds is becoming more serious and widespread. Various control measures are being used, but a satisfactory answer to many bird problems is still to be found. Frightening devices (firearms, rope firecrackers, gas exploders, distress calls, electric perches, airplanes, etc.) are useful in many situations but lose their effectiveness as birds become familiar with the frightening principle. Effective reduction in bird numbers is difficult to achieve because of the inaccessibility of the birds, the costs, and the dangers involved. Efforts at using cage traps, light traps, and elevator traps are described, as is use of poisoned baits and roost control techniques. Until more is known about the habits and movements of blackbirds and starlings, or until new or improved techniques are evolved, it is hard to see how present difficulties are to be quickly overcome; in the meantime, perseverance with the measures that can be used conveniently and safely seems to be the most logical course. It must be emphasized that a combination of two or more methods increases the efficiency of each. Strong publicly supported and planned programs would certainly help to reduce the problem. A list of references is provided.

In this informal dinner speech associated with the First Vertebrate Pest Control Conference, the Dean of Agriculture at the University of California - Berkeley discusses in broad terms how man has changed the environment, especially in California, that has led to new and diverse conflicts with vertebrate pests. He provides examples of new and emerging vertebrate pest conflicts that involve agriculture, public health, and other concerns. He describes the University of California’s efforts in this area, which lag behind research and teaching in some of the other pest disciplines. These include a new curriculum in wildlife management being developed at UC Davis, an added faculty position specializing in teaching and research and wildlife, as well as expanded facilities on the Davis campus.

A detailed description of problems of Norway rat infestation of sanitary sewer systems is provided, as well as a review of control methods and materials. Design and maintenance of urban sewer systems can reduce rat problems, but ongoing effective rat control programs typically must include use of rodenticides. Commonly used active ingredients are discussed, as is the formulation and application of baits appropriate for use in sewer systems. The need for prebaiting when using acute toxicants is noted, as is the desirability of including mold inhibitors into bait formulations when using paraffin baits or anticoagulant baits, which require repeated feeding to be lethal. A method for locating rodents burrows that are connected to main sewer lines or laterals is described, using smoke bombs and portable blowers to force smoke through sewer systems and reveal locations where rats have points of access. Components of an effective strategy to control Norway rat infestations in sewers are discussed, and literature citations are provided.

Developments in control of commensal rodents – Norway rats, roof rats, and house mice – are summarized with emphasis of changes in available tools and materials during the 20th century. Emphasis is on rodenticide active ingredients recently or currently in use. Research efforts under way at the USDI Denver Wildlife Research Center on better materials for controlling commensal rodents are mentioned.

An extensive review of the importance of ectoparasite control on mammals and its relation to public health. Fleas, ticks, mites, and lice are discussed in detail, as well as their roles as vectors of disease of public health importance in the U.S. Ectoparasite control methods are described. The role of flea control and/or rodent control in the prevention or control of enzootic plague is discussed. Specific recommendations on use of insecticides within bait boxes are provided, as is the practice of burrow dusting for flea control. Control of lice that infect humans is discussed, as well as the potential role of lice in epidemic typhus in some areas of the world. Mites that affect humans, including chiggers and the tropical rat mite, are mentioned and control recommendations are provided. The role of ticks in serving as vectors of various rickettsia, bacteria, viruses, and protozoa is discussed in some detail, especially in relationship to human diseases that tick bites may cause. Tick biology, identification, and control are described in detail and some case studies are provided. A list of literature citations regarding ectoparasites is included.

Methods of controlling domestic pigeons, European starlings, and house sparrows are discussed in detail, with emphasis on urban bird problems. Trapping, exclusion, and roost dispersal by means of frightening devices (pyrotechnic tools such as cracker shells and fuse ropes, in particular) and roost sprays are described in detail. For capture of pigeons, both cage traps and cannon nets are described, with illustrations provided. Case studies are provided of successful dispersal of starling roosts by use of sprays formulated with aqueous ammonia and a wetting agent. Brief mention of use of avicides is included.

While no epidemiological data exist for disease risk particular to pest control operators, several potential diseases are discussed: rabies, leptospirosis, plague, murine typhus, and ornithosis. Contact with rabid animals most likely may occur from skunks, foxes, and bats. While leptospirosis is typically found in rats, it may also occur in opossums, foxes, raccoons, and skunks. While rat-borne plague caused outbreaks in the U.S. at the beginning of the 20th century, more recent incidents have resulted from human contact with wild rodent species and their ectoparasites; epizootics have been noted in squirrels, prairie dogs, rabbits, and pack rats and the disease is found in the 15 western states. Murine typhus, not to be confused with classical louse-borne typhus which is transmissible from person to person, is typically contracted by the bite of the rat flea, Xenopsylla cheopis. Recent cases have been confined to 11 Southeastern states. Ornithosis (or psittacosis) is often mild or undetected and common reservoirs in the U.S. include parakeets, turkeys, and pigeons. Thus, pest control operators involved in pigeon control may be at risk by inhaling droplets or dry particles as aerosols of dried droppings and bird exudates. Precautions to avoid exposure to these diseases are discussed, and literature references are provided.

The history of development, physical properties, mode of action, methods of use, and typical applications vertebrate pesticides are reviewed and discussed. Active ingredients reviewed are strychnine, sodium monofluoroacetate (Compound 1080), zinc phosphide, cyanide, red squill, ANTU (alpha-naphthylthiourea), five anticoagulants, arsenic trioxide, endrin. Of these, ANTU, anticoagulants, and red squill have been used in tracking powders for rats and mice. Fumigants used for control field rodents in their burrows have included calcium cyanide, carbon disulfide, and pyrotechnic (gas) cartridges that produce carbon monoxide. A list of references is included.

The history and development of pesticide regulation regarding active ingredients and formulated products used against vertebrate animals is described and discussed, from early efforts up to and through the 1947 passage of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), as well as more recent developments and amendments. Discussion includes the development of pesticide labels and their importance.

Concluding remarks for the First Vertebrate Pest Control Conference are provided by the author as Technical Director of the sponsoring organization, the National Pest Control Association. Also discussed are the recent developments in the understanding of two fungal diseases of humans associated with birds and with bats, histoplasmosis and cryptococcosis. Case histories of these two public health risks are provided, as is discussion of how pest control operators can protect themselves against disease exposure.