A Current Assessment of Vole Damage and Numbers and of Methods Used to Control Voles in Ontario Apple Orchards

A province-wide eva lua t ion o f the amount, d i s t r i b u t i o n , source and c o n t r o l o f damage t o Ontar io apple t rees was i n i t i a t e d . Data wereobtained from 280 responses t o a quest ionnai re d i s t r i b u t e d t o growers across Ontario, and from t r a p censuses i n three widely separa ted areas. Meadow voles (Microtus pennsyl vani cus) damaged o r dest royed 8,423 t rees i n o u r quest ionnai re sample and o ther mamnals damaged another 10,307 t rees . No re1 a t ionsh ip was found between a p p l i c a t i o n o f roden t i c ide and l e v e l s o f damage, b u t roden t i c ide t rea ted b a i t s sharp ly reduced numbers o f voles on our t r a p p l o t s . Orchards wi t h h igh l e v e l s o f damage were on average only oneth i rd as l a r g e as the average orchard i n our t o t a l sample. A l l damage by voles appeared t o be caused by meadow voles and no p ine voles (M. pinetorum) were found. Numbers o f voles va r ied g r e a t l y among our three study areas. Future work w i l l concentrate on fac to rs causing h igh l e v e l s o f damage and on the r e l a t i o n s h i p between d ispersa l o f voles and the e f fec ts o f r o d e n t i c i d e treatment. I n t r o d u c t i o n I n Ontario, the re has been very l i t t l e systematic research i n t o the amount and c o n t r o l o f damage caused by herbivorous mammals i n apple orchards, desp i te abundant evidence from growers t h a t t h i s damage i s extens ive and c o s t l y (Brooks and Schwarzkopf 1981). I n 1981, Ontar io growers sought t o suppor t a research program t h a t would determine the amount o f damage caused by voles and t h a t would develop more economical and e f f e c t i v e c o n t r o l measures than those t h a t e x i s t now. I n September 1981, we i n i t i a t e d a four-year research program. The ob jec t i ves o f the f i r s t phase o f t h i s research were: 1. To i d e n t i f y the mammalian species causing damage t o Ontar io f r u i t t rees. 2. To q u a n t i f y the e x t e n t o f mammalian pest damage i n Ontario. 3. To assess the nature and e f fec t i veness o f cu r ren t management and con t ro l pract ices. 4. Using t rapp ing techniques, t o est imate dens i t i es o f voles i n orchards i n three areas o f the province. From in fo rmat ion obtained dur ing t h i s i n i t i a l phase, we have formulated research plans d i r e c t e d toward the long-term goals of the p r o j e c t . S p e c i f i c a l l y , these goals a re t o develop recommendations t o advise growers on cos t and l a b o r e f f i c i e n t methods t o reduce t r e e damage by rodents and, w i t h i n these constra in ts , t o recommend methods t h a t minimize the use o f tox icants i n c o n t r o l . Methods and M a t e r i a l s 1. Ques t ionna i re I n Se~tember 1981. auest ionnaires were d i s t r i b u t e d bv mai l t o 1100 apple' growers i n 0 n i a r i o . These quest ionnaires provided informa t i o n on: (a) t r e e composit ion ( i .e., number, age, va r ie ty , e tc . ) and s i z e o f the orchard; (b) methods (i.e., t iming, and frequency o f use o f herb ic ides, rodent ic ides, mowing, c u l t i v a t i o n , e tc . ) of h a b i t a t management and rodent pest con t ro l used by growers; ( c ) amount o f damage i n f l i c t e d on t rees by mammalian pests; and (d) general l o c a t i o n o f the orchard, depth o f w i n t e r snow cover and o ther f a c t o r s . Returned quest ionnai res (280) were a1 located t o f o u r regions (F ig . 1 ) . A. Lake E r i e = a l l counties border ing on Lake E r i e B. Centra l Ontar io = a l l counties from Lambton t o York C . Georgian Bay = count ies o f Grey, Simcoe and Wel l ington D. Eastern Ontar io = a l l counties eas t of York. 2. Tra i n Pro rams St:da:d I i ?e-t rappi nq and snap-trappinq techniques ( ~ a v i s 1956, Krebs e t a1. 1969, ~ e n z u l l i e t a1. 1980, '~ tockrahm e t a1. 1981, Webster and Brooks 1981) were used t o i d e n t i f y rodent species res iden t i n apple orchards and t o est imate populat ion l e v e l s o f these species. Trap g r i d s were es tab l i shed i n orchards i n Haldimand-Norfol k muni c i p a l i t y , Grey County, and Pr ince Edward County (F ig . 1 ) . The study orchards were se lec ted because they had experienced damage by voles i n prev ious years. Sampling was conducted i n such widely dispersed t r a p g r ids , because the Ontar io Pest ic ides Advisory Committee (OPAC) expressed i n t e r e s t i n determining whether i t would be f e a s i b l e t o est imate vo le l e v e l s across apple growing areas o f Ontar io on the bas is of samples f rom a s i n g l e g r id . Four 0.21-ha l i v e t r a p g r ids were establ ished i n each o f the th ree sample areas. Trapping comenced i n e a r l y September and ended i n mid-December. Each area was trapped throughout f o u r consecutive 24-h per iods on th ree occasions (Cycles 1, 2, and 3), g i v i n g a t o t a l o f 12 days and 12 n igh ts t rapping on each study g r i d . Ba i ted Sherman l i v e t raps were s e t a t each g r i d marker w i t h 5 m between markers. Traps were locked open f o r 24 h before each four-day Cycle began. Captured animals were marked w i t h numbered ear tags, weighed, sexed and released. Reproductive a c t i v i t y was a lso noted. F ive 0.21-ha g r i d s were establ ished on the three areas f o r snap-