Elsevier

Crop Protection

Volume 20, Issue 6, July 2001, Pages 535-537
Crop Protection

Short Communication
Preliminary field tests of capsaicinoids to reduce lettuce damage by rabbits

https://doi.org/10.1016/S0261-2194(01)00015-1Get rights and content

Abstract

Capsaicinoids are alkaloids responsible for pungency, the ability to produce an organoleptic sensation of heat, and are present only in Capsicum species. Products containing capsaicinoids are marketed as a plant antifeedant. This study investigated the influence of capsaicinoids on the suppression of feeding by rabbits in southern New Mexico, USA. A two-choice study, treated and untreated lettuce plants, determined the effects of capsaicinoids on free-ranging rabbits. The capsaicinoids were effective, but only when an untreated choice was present. When untreated plants had been consumed, rabbits consumed the capsaicinoid-treated lettuce plants. The use of capsaicinoids to protect plants may have limited use when rabbits have little choice in food sources.

Introduction

Rabbits damage orchards, gardens, ornamental plantings, and agricultural crops (Craven, 1993; Knight, 1993). Desert cottontail rabbits (Sylvilagus audubonii) and black-tailed jackrabbits (Lepus californicus) are becoming an increasingly serious pest in the western United States when crop fields adjoin wild natural areas. In southern New Mexico, intense browsing by rabbits damages spring planted crops of wheat, onions, and lettuce, causing economic loss (personal observation). Currently, rabbit controls include noise-making devices, fencing, poisoning, live traps, body snares and shooting (Marsh et al., 1990; Dolbeer et al., 1994). Fencing has been effective in Europe in keeping rabbits (Oryctolagus cuniculus) from destroying crops, but can be cost-prohibitive for some agricultural sites (McKillop et al., 1998).

Repellents or antifeedants have been advocated as a potential means to reduce damage by rabbits (Sullivan et al., 1992). Sensory stimuli that elicit innate aversive responses without the need for conditioned aversions are promising candidate repellents. Many alkaloids exhibit a bitter or pungent taste to rabbits and a bitter or pungent diet is normally instinctively avoided. The sensation of pain or irritation is a superior olfactory response for repellancy. The capsaicinoids provide an excellent chemical source for this olfactory response (Bryant, 1997). For capsaicinoids to serve as a chemical defense compound they should have a significant effect against rabbits. Several spray-on liquid products are currently being advertised as an effective, safe, and inexpensive method to repel rabbits from plants (Gardener's Supply Co., Burlington, VT, USA).

The active ingredients in many of these repellancy formulations are capsaicinoids, alkaloids that produce the characteristic pungent taste when consumed and are only present in Capsicum species (Bosland, 1992). The capsaicinoids are related vanillylamides of carboxyl acids with capsaicin being the major alkaloid. It is speculated that Capsicum species evolved the capsaicinoids to repel mammalian species from consuming the fruit (Bosland, 1992). The capsaicinoids have been used as a repellent against mammalian wildlife damage to orchards, nurseries, gardens, and ornamental plantings (Swihart and Conover, 1991; Andelt et al., 1994). Fitzgerald et al. (1997) found that chile pungency had the potential for reducing gray squirrel (Sciurus carolinensis) damage and nuisance activities to bird feeders. They tested three chile powders, 8250 Scoville heat units (SHU), 27,500 SHU, and 82,500 SHU, on sunflower hearts to discourage consumption by free-ranging gray squirrels. Scoville heat units (SHU) are a standard measurement of the capsaicinoid amount in chiles (Collins et al., 1995). When all three pungency levels were offered simultaneously with a control, no significant differences in mean consumption was observed among the treated samples. However, they were all aversive to squirrels, even the lowest level of pungency. Andelt et al. (1994) found that a 6.2% capsaicin concentration applied to apple twigs reduced the feeding by captive mule deer. When pellet/corn deer food was treated with 0.62 and 6.2% capsaicin concentration, and presented to captive deer and free-ranging deer at three independent sites, either concentration was more effective than standard repellents being used (Lutz and Swanson, 1997).

The effectiveness of capsaicinoids for protecting plants from rabbits has not yet been demonstrated. Therefore, to evaluate whether capsaicinoids would effectively protect lettuce from damage caused by rabbits, a test was conducted near Las Cruces, New Mexico, USA.

Section snippets

Capsaicinoid source

Among chiles, jalapeños (Capsicum annuum L.) and habaneros (C. chinense Jacq.) are two pod-types that have high capsaicinoid content (Bosland, 1992). Jalapeño chiles range in pungency from 5000 to more than 75,000 SHU (Bosland, P.W., personal observation). The habanero is one of the most pungent chiles and can have a pungency in excess of 300,000 SHU (Bosland, P.W., personal observation).

Whole mature fruits of jalapeño and habanero were prepared by oven drying at 58–60°C for 2–5 days, until the

Results and discussion

Current procedures to reduce crop loss caused by free-living rabbits in the southern New Mexico, USA predominately relies on shooting. Capsaicinoids may provide a more humane method of protecting crops from rabbits. In this study, capsaicinoids tended to be aversive to rabbits, but only provided limited and ephemeral protection from rabbit damage (Table 1). The habanero powder, which had the higher pungency level, provided greater protection (Table 1). The untreated plants were damaged first,

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