A Program for Control of Damage by the Field Vole,Microtus agrestis (L.), in Seed Orchards of Forest Trees1

For providing high-quality seed for forest nurseries, a large-scale seed cultivation program, including laying down 3,790 ha of seed orchards of pine, spruce and other forest trees, was established in Finland in the early 1960's. In particular, pine grafts are extremely vulnerable to damage by rodents, especially the field vole, Microtus agrestis (L.). An integrated rodent control program for all the existing seed orchards was established in 1969. From that time, rodent populations were surveyed every year according to a common scheme based on the Small Quadrat Method (SQM). Short-term forecasts and specific control instructions were issued shortly after the survey. Earlier, the principal method of controlling voles was surface treatment with endrin. At present, the bulk of grafts is protected by mechanical guards round the stems. Investments in guard materials, as well as labour costs of preparing and installing the guards, constituted more than 90% of all control costs from 1969 to 1976, and amounted to about 5 million Fmk. Total costs of rodent surveillance and supervision of the program were about 6% of control costs. Since the establishment of this integrated control program, the absolute numbers of grafts destroyed has considerably decreased, even though the area of seed orchards had more than doubled; thus the large investments in vole control have already been recovered.