Potential Repellents to Reduce Damage by Herbivores

Economic losses caused by herbivores and other species that inflict damage by browsing or gnawing are substantial. Because lethal approaches to damage reduction are not always practical or desirable, there is an increase in interest in the development of alternative, non-lethal technologies. Repellents may provide a feasible alternative. Here, we present recent studies of three repellent types: 1) anthranilate derivatives (e.g., methyl anthranilate), 2) predator scents (e.g., coyote urine), and 3) bittering agents (e.g., denatonium saccharide). Anthranilate derivatives and predator odors both appear to be promising repellents. Avoidance of the former substances is based on irritant volatiles, and anthranilates may be especially beneficial when the aim is to prevent gnawing damage. Predator odors may be most applicable for protection of vegetation. The effectiveness of these substances appear to be based on the presence of highly volatile, light molecular weight sulfur compounds. Unlike anthranilates or predator odors, bitter substances are largely ineffective as repellents for herbivores. Proc. 16th Vertebr. Pest Conf. (W.S. Halverson& A.C. Crabb, eds.) Published at Univ. of Calif., Davis. 1994. INTRODUCTION Human and wildlife interactions are becoming more frequent, often with detrimental effects to both human resources and to wildlife. Though limited data are available that document losses (Salmon 1988), it is generally accepted that vertebrates cause considerable economic harm (Conover, unpublished data). Although a number of lethal strategies are available to reduce agricultural damage these methods are not always practical or desirable. Non-lethal strategies are being investigated as alternatives or adjuvants. One possible non-lethal approach is the use of repellents. Repellents deter damage by decreasing a plant's desirability to the foraging animal. Deterrence can be achieved through a conditioned aversion or through an unlearned initial avoidance (Mason and Clark 1992). Conditioned food aversions occur when ingestion of novel foods is paired with nausea (Garcia 1989). Thus, any flavor paired with gastrointestinal distress can become an effective repellent. Efficacy of repellents based on conditioned aversions can be limited, however, because individuals need to be trained to avoid these stimuli and because the stimuli must be novel to form a strong aversion. Damage that occurs during training or subsequent sampling may be extensive. This can be especially problematic if the damage is inflicted by a transitory or a migrating species (i.e., deer moving from summer to winter ranges). Repellents that elicit initial avoidance are generally either irritants (e.g., capsaicin) or those that evoke a "fear" response (e.g., predator scents). This type of repellent is especially promising because no training is needed to elicit avoidance behavior. Unfortunately, few compounds have been identified that induce innate avoidances. Those that are available are either broadly offensive to all mammals (Meehan 1988) or show considerable interand intraspecific variability in effectiveness. Here, we present studies of three categories of repellents: 1) irritating acetophenone or anthranilate derivatives (e.g., methyl anthranilate), 2) fear-inducing predator scents (e.g., coyote [Canis latrans] urine); and 3) bittering agents that have been assumed to serve as taste irritants (e.g., denatonium saccharide). A C E T O P H E N O N E O R A N T H R A N I L A T E DERIVATIVES Dimethyl and methyl anthranilate (MA) are aversive to many avian species under laboratory conditions (Kare and Pick 1960, Kare and Mason 1986). Field tests also indicate that these substances inhibit foraging damage by birds to crops (Askam 1992) and at livestock feeders (Mason et al. 1985, Glahn et al. 1989), and they can be effective at alleviating hazards imposed by the mere presence of birds (e.g., airports) (Vogt 1992). They also can be used to relieve non-target hazards that granular pesticides and treated seeds pose to birds (Mason et al. 1993). Ortho-aminoacetophenone (OAP) with similar olfactory properties (to humans) and chemical structure as MA also repels birds (Mason et al. 1991). Potential of these substances as mammalian repellents, however, has been largely ignored. We tested the efficacy of five of these substances to inhibit water intake by mice (Mus musculus) (Nolte et al. 1993b). Water deprived mice were offered water treated with 1.0% concentrations (w/v) of MA, OAP, 2-amino4',5'-methoxyacetophenone (AMAP), 2-methoxyacetophenone (MAP) and veratryl amine (VA) in single choice tests. After adaptation to an 18 h water deprivation schedule mice were given 3 h access to water in 10 ml syringes fitted with sipper tubes on each of four pretreatment days. At the end of each 3 h period, ingestion was measured and the mice were permitted an additional 3 h ad libitum access to water. Water tubes were then removed from their cages, and animals were deprived until the following day. A four-day treatment period immediately followed pretreatment. Treatment period sessions were similar to pretreatment sessions, except that five groups of mice