Chewcards for detecting animal pests
Rat damage to a chewcard
A chewcard is a multiple animal-species detection device specifically designed to cheaply monitor, or map the distribution, of low density small mammalian pests, principally possums but also rodents and other pest species. It is an interference device, left in the field for a specified period (days or weeks), which identifies animals present by tooth impressions (and foot prints if ink is used) made as they investigate and consume bait material applied to the cards. Presently the standard design is a 9 x 18 cm card made of 3 mm white plastic coreboard, cut so that flutes (internal channels) are 9 cm long (run across the width of the card). Attractant bait is applied to the internal channels at either end of the card (photo 1). Cards can be baited manually by the monitoring crew, or purchased ready baited from a commercial supplier.
Bait application and placement
Smooth peanut butter is the most commonly used bait by do-it-yourselfers, and is universally accepted by small mammal pests. Adding icing sugar or honey to peanut butter (5 parts peanut butter to 1 part sugar/honey) can increase the attractiveness of chewcards to possums. Bait is best applied to blocks of about 20 cards using a 50-mm flat paint scraper. The bait should be soft and smooth. If it has dried out and becomes crumbly when applying it, mix in a small quantity of canola vegetable oil to achieve a workable consistency before applying, or heat in a microwave oven (30-60 seconds for 1 kg of bait). In wet conditions or for long deployments, commercial long-life baits are preferable to peanut butter-based baits. Commercial suppliers provide unbaited cards, cards baited with peanut butter, or cards baited with their own proprietary baits.
Chewcards are often heavily damaged by rats in areas where they are common, leading to concerns that rats may be destroying possum sign. Recent evidence indicates that in most circumstances this is not a significant problem, so earlier recommendations of using rat repellent baits, baiting strategies, or card mounting systems is no longer considered neccessary.
Cards are applied to tree trunks, 30 cm above the ground (lower if hedgehogs are to be monitored). The card is folded in half then attached to a tree using a 50-mm flat-head nail inserted through both halves about 10 mm back from the fold. The card is held open with a 90-degree internal angled as the nail is inserted, before it is tacked to the tree so the right angle is maintained and the top half of the card is horizontal (see photo 2). In open country cards can be mounted on 50-cm long steel/aluminium pegs.
Appling tracking ink to part of the lower inside surface of the chewcard to record the foot prints of card visitors can be a useful addition, particularly for mice, which frequently visit cards without leaving discernable bite marks. When ink is used, a smear of bait applied to the inside fold line is recommended to increase the incidence of tracking.
Chewcards are typically deployed for about 7 nights where pest abundance is expected to be low, but shorter intervals are best where abundance is high. This may be a short as one or two nights for possums, but it is recommended that deployments of at least three days are used where rodent monitoring is important, to allow rats and mice to overcome their natural short-term caution around new objects. When monitoring to detect incursions in pest free areas, or populations that are nearly eradicated, deployments of several months may be used.
Interpreting bite marks
Manaaki Whenua - Landcare Research, together with the Department of Conservation, has developed several tools for interpreting bite marks on chewcards (see files below).
- Chewcards: A guide to the interpretation of animal tooth impressions (powerpoint, PDF) is a visual interpretation of all the bite marks of most New Zealand pest and native species likely interact with chewcards. It is likely to be the most useful tool for beginner-intermediate chewcard users.
- Bitemark key for chew track cards is a more technical tool, without visual aids, that separates potential species based on precise variables (tooth size, shape, arrangement). It includes detailed descriptions of all the bite marks made by all common pest species, as well as the diagnostic characteristics for separating easily confused bite marks.
- The diagnostic-parameter selection document it not a tool for interpreting chewcards on its own, but provides the background data and methods on which the key and power-point information is based.
- The summary table extracts all the essential quantitative details from the diagnostic parameter document for identifying all species likely to interact with chewcards.