Mouse Control in Washington Orchards • Protect Valuable Fruit Trees • Cut Economic Loss • Reduce Health Hazards EM 2650 Revised September 1977 In cooperation with the United States Department of Agriculture Issued in furtherance of the Acts of Mav 8 and June 30. 1914 bv the Washington State University Cooperative Exten-sion Service, J . 0. Young. Director Cooperative Extension Service. College of Agriculture, Washington State University. Pullman Mouse Control in Washington Orchards. Raymond E. Hunter, Grant-Adams area Extension agent Ronald B. Tukey, Extension horticulturist. Mice can cause extensive injury and even the Some are found only in other fruit-growing death of fruit trees. They gnaw and peel the regions of the country. Others do not feed on bark from the roots and trunk of the tree at or fruit trees. just below the groundline. These wounds can weaken the tree while also serving as points of It is the short-tailed meadow mouse (Microtus infection from various root rot diseases. When. sp) which causes the problem in Washington severely girdled, the trees die unless State. It is medium-sized, stout ( 1.5-2.0 ounces) bridge-grafted. with small, black, beadlike eyes and small, fur-covered ears. An important designating Preventing injury from mice requires a careful feature is its short tail (one-third of head and and regular monitoring of mouse populations body length), which is well-covered with hair and activity. This is particularly important in the (see Figure 1 ). late summer, fall, and winter when mouse populations are at their peak and alternate food The meadow mouse ranges in color from sources can be depleted. blackish to greyish, or it can be yellowish brown. Its fur is loose, rather long, and heavily Various cultural and management practices may interspersed with black guard hairs, all of which be used to reduce the hazard of injury. These give it a grizzled appearance. The color is darkest include placing mechanical guards around young on the back, which shades into gray, ash or buff trees, keeping the base of trees free of weeds and on the underpart. The tail is dusty-colored on grass, destroying natural covers around the the upper side and slightly paler below. The feet orchard and treating the orchard with do not have black guard hairs. rodenticides. The meadow mouse lives in an environment just There are three rodenticides and four basic above or below the soil surface. Here it forms an methods of applying them. Two of these extensive network of runways. It feeds on the chemicals are applied as baits. They may be succulent stems and roots of grasses, legumes, applied by hand or in a mechanical trail builder. and weeds along these paths. It nests just below They also may be scattered over the orchard the soil surface, in dense cover, often at the base floor. These baits are effective only for short of trees where there is little disturbance and periods of time after application and, therefore, good protection from its natural enemies: are effective primarily against mice already in hawks, owls, shrikes, snakes, badgers, and the orchard. skunks. Ground sprays with the insecticide endrin Its large appetite combined with prolific provide long-term or seasonal control over the breeding causes much of the problem. Each winter. Because of the hazard, these sprays mouse eats its weight in forage each day. In can should not be applied until late fall when harvest ·produce as many .as 8 to 10 litters each year operations are over, and all fruit on the ground with an average of 6 and up to 11 young per has been removed or destroyed. litter. The ne~ females become sexually mature and can begin breeding at just four weeks of age. DESCRIPTION, HABITS AND BREEDING CHARACTERISTICS OF MICE Mouse populations are cyclic both within and between years. Their number is lowest in the There are several different kinds of mice which spring and highest in the fall. Peaks in can be found in orchards and around trees. population occur approximately every four ROD.ENT S: P 1lCTt:lRJAl.. KEY TO SOME COMMON UNITED STATES GENERA Harold George Scott and Margery R. Borom I -------- 1- - I .At- tail almost naked, scaly..._, ,.lit' tail almost nakerl, annulatl"...._ ,.lit' tail hairy..._, large (9-lPlong) small (5-7•1ong) tail long tail short hair short hair long ~firf;(!? ' J II t:nI /lfi~ff ~~ -;- .;z. ~ !A ' ' . -- ·• ·-. - :.,..( - II Rattus Mus DOMESTIC HOUSE MICE , RATS r 1 ..tJIIt" tail bones longer than l/2 borly "'- ..tJI/t" tail bones shorter than 1/ 2 body...._ large (about 16•tong) small (7-9•tong) front feet enlarged front feet not enlarged ~ ........ Neotoma ~y~ Geomy~-East WOOD \\lUTE-FOOTED Thomomy~-West RATS MICE POCKET GOPHEHS ..tJI/t" tail bones longer than borly ..,.,_ .A- tail bones shorter than borly -..::---------------. tail bush) tail not bushy large (about ts•tong) small (about 9•long) very large (about 26•1ong) Sciurus · Qiporlomy~ Citellus Tami as-East THEE 1\ANGAHOO GHOUND Eutamias-WI'st Marmot~ SQUIHHELS HATS SQUIRRELS \.llll''ll :NKS W(XlllCitLCKS U.S . DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE, PUBLIC HEALTH SERVICE, Communicable Di_sease Center, Training Branch, Atlanta, Georgia- 1962 EM 2650 - Page 5 years. These peaks and the ability to multiply so protection for mice. As a result they are least rapidly have often been misinterpreted as a likely to be attacked by heavy mouse migration of heavy mouse populations into the populations. orchard. While such migrations do occur, they are usually of only limited distance from around Most susceptible are trees in permanent sod. The or within the orchard. dense cover and large overhang of branches provide good protection against enemies. SUSCEPTIBILITY OF TREES Control of weeds and sod around the base of the AND ORCHARDS tree eliminated much of this natural protection. Cultivating or otherwise disturbing the soil There is a significant difference in the around the base of the tree discourages nesting susceptibility of trees under differing close to the tree trunk. management practices and of varying ages, species, and rootstocks to serious injury and Managing the sod to provide ample food sources economic loss. These differences may be while at the same time balancing the mouse expressed in a generalized manner as follows: population to this food supply is one of the most effective means of reducing the hazard of Tree Age tree injury. Required is a well-fertilized and irrigated sod throughout the year. A cover which The volume of bark and trunk is important. In stops growing and becomes mature in late contrast to large, mature trees, a young tree has summer and fall can force mice to tum to trees only a limited amount of bark, and a few mice as a source of food. can readily girdle it. Mice prefer the relatively soft and succulent young or inner bark. Thus, MOUSE CONTROL MEASURES older trees with heavy, thick bark are less susceptible to serious injury or loss. There are a number of different practices which should be considered in developing an effective Species and Rootstock mouse-control program. Extensive use of various rodenticides can kill many mice, but they do not There are significant differences in the palata­ assure trees free from mouse damage. bility of bark to mice. The apple tree is preferred over all other species. Pear is much less suscep­ Monitoring Mouse Activity tible than apple, but preferred over stone fruits. Peach and, in some instances, cherry trees can be Foremost in control is the systemic survey of all be attacked while apricot, plum and prune are plantings and areas harboring mice followed by rarely bothered by mice. Dwarfing rootstocks routine monitoring of mouse activity. The field for apple are ,the most susceptible because of mouse develops a characteristic network of apparent differences in palatibility as well as runways. These serve as feeding areas and extend recovery following grafting across the girdle or from the nesting sites. Both are closely wounds. The more dwarfing rootstocks provide associated with dense ground cover. a smaller, more fibrous, root system than standard seedlings. Thus, they usually require In orchards, runways tend to be concentrated in-arching with new rootstocks rather than more heavily under the dripline of the trees. In bridging from large roots to the trunk. As a hedgerow plantings, they extend up and down result, tree loss is usually much higher. the row. Nests are often located near or close to the trunk of the tree. Rarely are the various Soil Management Practices colonies well-distributed in or near the orchard. They are more common or frequent where the Orchards which are clean-cultivated throughout soil is deep, fertile, well-covered with grass and the growing season provide little food source or weeds and well-drained. EM 2650 - Page 6 Once this network is located, it should be the principal means of controlling mice, there watched. Activity is evidenced by small piles of are other practices which may be used to reduce brownish droppings and short grass clippings the hazard of extensive mouse damage to trees. scattered along the path under the canopy of These are important because even the loss or cover. The freshness of these droppings and weakening of a few trees in a planting can be clippings is indicative of recent activity. How very costly in lost production. Further, while closely the vegetation along the sides of these the hazard is greatest from late summer until paths is clipped as well as the width of the path spring, feeding actually can occur almost any is a fair indication of use or population numbers.