THE NEED OF SURFACE SPRAYS FOR THE CONTROL OF MICROTINE RODENTS MOGENS LUND, Research Biologist, Danish Pest Infestation Laboratory, Lyngby, Denmark ABSTRACT: Four Hicrotine species, the field vole (Hicrotus agrestis), the continental vole (Hicrotus arval is), the \vater vole (Arvicola terrestris) and the bank vole (Clethrionomys glareolus) are the most harmful rodents in forests, fields, orchards and gardens in Northern and Central Europe. Except for the latter they are all herbivorous, their food consisti.ng to a very low degree of seeds and grain. As a consequence dry poison baits are not well accepted most of the year. The only economic and effective control method until now has been surface spraying with the chlorinated hydrocarbons endrin and toxaphene. As these chemicals are now black-I isted in many European countries, and their use severely restricted in other countries, no effective means for controlling these rodents exist for the time being. New surface sprays without the persistence of endrin, but with a more long-lasting effect than parathion, are severely needed if the extensive damage to trees and crops shall be reduced. Leaving out of consideration the brown rat and the house mouse, both of which can be coped with if reasonable economical and organizational efforts are taken, only five rodent species can be considered major pests in Europe. They all belong to the subfamily Hicrotinae. The largest species being the muskrat (Ondatra zibethlca) which constitutes a special problem discussed by Dr. Becker, Germany . First, I shall give you a short description of the other four important species, their food habits and the typical damage for which they are responsible. The water vole or black water rat (Arvicola terrestrls) is the size of a young brown rat with a somewhat shorter tail and ears hidden in the black-brownish fur. It is found all over Scandinavia, Denmark, Germany, Central Europe, Holland, and Belgium. Westwards, in England, France, Spain, and Portugal it is replaced by other subspecies, not always of the same importance. Being a true vegetarian, it prefers grasses and herbs in the sunmer, and the storage organs of various plants, as well as roots of trees and shrubs in winter­ time, when it is mainly subterranean. All habitats with grass-cover are suitable for the water vole, which - in spite of its name - is not bound to water and streams as is its harmless close relative in Britain. The water vole is feared by fruitgrowers, foresters , private garden-owners, farmers and telephone companies. Except for the last group mentioned the most important damage occurs in the winter when roots of all sorts of trees and ornamental shrubs are severely gnawed . Fruit trees are preferred, and certain kinds as Cox orange apples are selectively chosen. Bulbs of various plants, potatoes, carrots, and other plant storage organs are collected in autumn in the extensive burrow system. Beets, clover, luzerne, and other field crops are damaged in the surrrner season. Telephone cables are of course endangered the whole year. The field vole (Hicrotus agrestis) is a much smaller animal, but in some countries, such as Norway, Sweden and Finland, it is considered a much rrore important forest and orchard pest. As a real herbivorous rodent , it is strictly bound to areas with a permanent grass-cover. When the caloric value of this food is at a minimum in late winter the voles often turn over to feed on the bark of trees . As it cannot climb, the damage is confined to the lower 15-20 cm of the stem, but most often the effect is fatal to the trees. Old abandoned fields turned into forests by large-scale plantation programs all over Scandinavia, and to some degree in Central Europe, are ' ideal habitats for the field vole and are particularly susceptible when peak densities occur almost every four years. The continental vole (Hicrotus arval is) is very similar to the field vole and can only be distinguished by experts. It is an open land species not dependant on trees, but just as much bound to grass cover and herbs. In West Germany, Central Europe, France, Belgium, and Holland, it is more abundant than~· agrestis and causes heavy losses to clover and luzerne fields , grass-seed fields, beets and other field crops. In most of its areas, it reveals a rhythm in population densities with a peak every three years. 22. " ' ' .. .. : . I; ) '.'." ! DLIFE The bank vole (Clethrionomys glareolus) with a reddi%O~Qi8rQfn ' the i batl<a'nd ' t'be : 1size of a field vole, differs in some Important respects from ~h~ " other' 1 -vo1es ' mentioned. It prefers a habitat of mixed deciduous trees with high bottom vegetation of scrubs and is not dependant on grass as food or cover. It is more granivorous, the diet consisting for a large part of seeds, leaves of trees , animal food, and bark. As It is an excellent cl Imber the damage to the trees Is generally not concentrated to the lower part of the stem, but still It can be quite severe and in some parts of Central Europe (e. g. Romania) It Is considered the most Important pest in deciduous forests . THE DAHAGE AND ECONOMIC IMPORTANCE OF THE MICROTINES In few countries only, data are available on the economical importance of the damage caused by the voles. Most often very subjective estimates are given . More accurate estimates seem to be confined to Scandinavia, West Germany and part of France, mostly based on enquiries to foresters and orchardmen; some, however , were verified by direct investiga­ tions. To give an Idea of the significance of Mlcrotine damage, I find it more adequate to present examples from various European countries instead of treating each species separate 1y. In Finland the field vole is the most harmful rodent damaging afforested areas and orchards. Apple trees are the favorite objects for the rodents In winter. In the peak year of voles, 1958, more than 18% of the apple trees In Finnish orchards were severely damaged and In the period 1954-1958, 90,000 young apple plants were killed (Myllym~ki 1959). Based on questionnaires a minimum estimate of the damage caused by field voles to horticultural plants in the 1950 1 s and 1960 1 s ls given by the Agricultural Research Centre (Kanervo et al. 1970). For the 13 years covered by the survey the verified damage to apple trees and other horticultural plants was calculated to about 2 mill ion dollars. In the second largest Finnish orchard in the winter of 1965-66 , 13,500 apple trees in their best yielding years were killed by the voles. The entire damage to horticulture in Finland is estimated at a total of 3. 143 mill ion dollars per year. In Finnish spruce nurseries the percentage of 2-3 year-old seedlings, damaged by the voles in the period 1962-66, varied from 12.2 to 77.5 with an average of 52.5%, and the mortality was 13.4-64.6%, with an average of 41 .9% (Hylly~kl 1970b). The economic losses by vole damage to forestry in the 1960 1 s were at a m1n1mum $120,000, when only verified cases are Included, and the following figures give an Indica­ tion of the importance of the vole damage to seedlings of forest trees in nurseries in 1966 (Kanervo et al. 1970) : 28.3% of 848,000 pine plants damaged, 8.8% of 1 ,329,000 spruce plants damaged, 50.0% of 10,000 birch plants damaged, and in 1968-69, 22,500 grafts were destroyed, or almost 4% of the entire number of grafts in the seed orchards of the Finnish State Board of Forestry. Besides this, field voles damage various crops as barley, oats, and wheat by eating the green plants - as well as peas, sugar beets, and mange!. No estimates of the economic Importance of this damage are given. In the northern parts of Finland (and Sweden) severe losses on stored hay, caused by voles were recorded in the winter 1966-67 followed by epidemics of tularemia, among the local inhabitants. In Norway the field vole ls also by far the most Important rodent. In the peak year 1965-66 spruce plantations were severely damaged, about 3,243 ha of young spruce were destroyed and 6,269 ha more or less damaged. The losses have been estimated at about 1.434 million No.Kr. (appr. $200,000) as an absolute minimum (Wegge 1967), private foresters and nurseries not being included. In a single nursery 22,400 spruce plants had to be renewed in 1965, and 23 , 500 the following winter, at a cost of about 36,000 No . Kr. (appr. $5 , 000), not considering the waste of time and experimental data. As a rule the more severe losses occurred in areas where old grass- covered fields were afforested, like in Finland. In Sweden up to I mill ion ha of abandoned fields were made available for afforestation In the 19G0 1 s and damages from field voles have been of great significance for this project. A preliminary planting experiment In 1959-60 was heavily Influenced by voles, as about 16% 23 of all young plants were killed and 46% more or less damaged in 1962 (B~rring 1963) . The losses were estimated at about $36,700 during this period (Stenmark 1967). The verified damage to horticulture in 1962 was about 70,000-140,000 dollars, and the total estimated damage to fruit trees about 4-8 mill ion dollars in the same year (Stenmark 1963). For the water vole very few data exist, but for the period 1957-62 approximately 42,000 ha of forest plantations were damaged, a loss of about 1,650,000 Sv.Kr.