BIOLOGICAL CONTROL OF CONIFER SEED DAMAGE BY THE DEER MOUSE (Peromyscus maniculatus)

This paper describes the development of a biological technique that successfully controls conifer seed damage by the deer mouse. Eleven experiments have been conducted on three study areas at the University of British Columbia Research Forest, Maple Ridge, B.C. Populations of deer mice have been monitored in all experiments with data from 56,000 trap nights. The technique involves a mixture of conifer seed (Douglas fir) with sunflower seed and oats which is uniformly distributed on logged areas in the late winter-early spring. Survival of conifer seed with these alternate foods was excellent, compared with control Douglas fir by itself, for an 8-week period during this time of year. Populations of seed-eating birds and chipmunks are not present on clearcut areas from late October to early April. In addition, deer mice are at their lowest density during the spring. Thus, the use of alternate foods with Douglas fir seed in a direct-seeding operation at the appropriate time of year will result in successful regeneration of cutover forest lands. INTRODUCTION For almost half a century foresters in North America have used direct seeding as a method of regenerating cutover forest lands. Destruction of the seed supply by small mammals and birds has had adverse effects on the success of these reforestation projects. Black (1969), Radwan (1970), and Radvanyi (1973) have provided comprehensive reviews on this subject. The deer mouse (Peromyscus maniculatus) is considered to be the most important seed predator in the Pacific Northwest. Chipmunks, microtines, and several species of seed-eating birds have also contributed to the problem by their consumption of conifer seeds (Black, 1969; Radvanyi, 1973; Pank, 1974). There have been two basic approaches to reducing conifer seed predation by small mammals and birds. Firstly, there are conventional control methods which have mainly involved chemicals and secondly, there is biological control. Conventional control methods have predominated and the following discussion about them is based on reviews by Radwan (1969, 1970). Control, used in the context of this paper, means to control damage. Since the early 1900's, forest managers in the Pacific Northwest have tried numerous methods for protecting coniferous seeds from rodents and birds. These include mechanical devices, poison baits, and toxicants and repellents applied directly to seeds. Mechanical devices such as screens have been too expensive and not practical for large scale field projects such as reforestation. Poison baits are now considered ineffective because control of rodents is only possible for very short periods of time. Elimination of the initial rodent population is incomplete and reinvasion from surrounding areas is often rapid (Gashwiler, 1969; Radwan, 1969; Hooven, 1975). Poisons, either as baits or as toxicants on seeds, are hazardous to nontarget species and the fate of these chemicals in the environment is unknown (see review by Evans, 1974). In addition, the direct application of toxicants and repellents to seeds has often resulted in reduced seed germination (Radwan, 1969, 1970). According to Evans (1974), a substantial change in priorities has taken place, from exclusive reliance on chemicals to research directed towards nonchemical methods of control. There is a trend to phase out persistent pesticides (toxicants and repellents) and replace them with less persistent and less hazardous chemicals. In the meantime, perhaps methods of biological control can be developed which will provide long-term solutions to this reforestation problem. A broad concept of biological control with respect to insects has been discussed by van den Bosch (1971). This concept involves an attempt to reduce the average density of a pest population by the action of parasites, predators, or diseases. However, biological control can also be more generally defined as a type of control in which the damage caused by the pest is reduced or eliminated by a biological agent or process. This approach has been discussed by Howard (1967) where he includes habitat modification resulting in reduced food and cover, or intentional alteration of the pest's environment resulting in lowered numbers of the pest species. Assuming the pest population does not necessarily have to be reduced, biological control may also refer to the use of preferred (buffer) or alternate foods. This process has shown some success in luring waterfowl away from grain and vegetable crops (Howard, 1967). Flocks of dark-eyed juncos have been easily diverted from fir seed by supplying artificial food at just one place on a cutover (Hagar, 1960). Prunings piled around the base of orchard trees as a buffer food have often minimized damage from meadow voles (Fitzwater, 1962). Browsing of conifer seedlings by deer may be reduced by the availability of alternate and more preferred species of browse (Campbell, 1974). A biological technique for controlling conifer seed predation in the deer mouse is discussed in this paper with respect to the functional and numerical responses of deer mice to varying densities of Douglas fir seeds. If the upper limits for the number of mice and number of seeds eaten per mouse can be determined, then the use of alternate foods with Douglas fir seeds could result in reduced predation on the conifer seeds. Presumably, the mice develop what might be termed an "olfactory search image" to exploit a food source. Howard and Cole (1967), Howard et_al. (1968), and Howard and Marsh (1970) have provided evidence for the use of olfaction by deer mice in detecting food. Therefore, by introducing alternate foods of varying palatabilities, densities, and distributions, the predators may switch to a new food source with a resulting increased survival of conifer seeds.