Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Chew-track cards / Chew cards

Rat damage to a chew–track card

Rat damage to a chew–track card

Establishment and interpretation

Chew-Track-Cards (CTCs) are a multiple species detection device specifically designed to cheaply 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 about a week, 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 coreflute, cut so that flutes (internal channels) are 9 cm long . Attractant bait is applied to the internal channels at either end of the card (photo 1).

Bait application and placement

Possum bait (5:1:0.5 smooth peanut butter/icing sugar/finely ground dried lucerne pellets by weight) is applied to blocks of about 20 cards using a 50 mm paint scraper. An alternative bait comprising more readily available ingredients (5:1:0.5 smooth peanut butter/icing sugar/dark honey by weight) is probably just as attractive to possums. There is now evidence that Peanut butter based baits do not perform well in wet conditions so an alternative yet to be tested is Connovation’s prefeed paste (smooth in a tube). Place about a tablespoon of bait on the scrapper and work it into a 5-cm section of flutes (channels) at one end of the block to achieve bait penetration of about 1-2 cm. Use longitudinal strokes of the scraper to apply most of the bait. The side cards tend to miss out so finish off with a few transverse strokes at each side edge with the scraper angled about 30 degrees to the block of cards. Check desired depth is achieved at both sides and in the middle of the block. Flip the block over and do the same at the diagonally opposite corner (photo 1). 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 areas where rats are common cards are often heavily damaged by them 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 a rat repellent bait based on a high proportion of lurcerne is no longer considered essential. [Currently the best rat repellent baits is 1:1:0.02 standard possum bait/finely ground dried lucerne pellet/eucaltyptus oil. Where rat repellent bait is used apply the standard possum bait to one part of the card and the rat repellent to the other].

Cards are applied to tree trunks, 30 cm above the ground (lower if hedgehogs are to be monitored). Fold the card in half then push a 50 mm flat-head nail through the top half of the card, about 10 mm back from the fold. Then push the nail through the bottom half about 5 mm from the fold. This offset nail placement helps hold the card in a right-angled position when placed on the tree. Finally nail to the tree with the nail angled up at about 30 degrees (photo 2). Place CTCs at sites of best sign, not necessarily the nearest useable tree (for detection surveys rather than density assessment surveys). In open country cards can be mounted on steel/aluminium pegs.

Bait placement on a CTCA CTC mounted on a tree
Photo 1. Bait placement on a CTC. Photo 2. A CTC mounted on a tree

Interpreting bite marks

Possums

Possums: Upper incisor pair impressions are aprox. 6 mm wide, (two channels wide) with just slight curves on each tooth. A small gap between the two teeth may or may not be present. Lower jaw patterns are variable, from similar to top jaws, to quite sharp point-like marks (depending on the wear pattern). Possums rarely cut through the card but usually extensively chew and crush the card margins. Pull marks producing long broad depressions are often evident where the possum pulls toward the edge of the card. Possums will frequently leave extensive areas of crushed card, about 2 cm wide along the baited margins while leaving few clear tooth impressions (photo 3). One or two of the latter can usually be found with careful inspection. Cards without obvious crushed areas need to be checked for small areas of light biting or scattered individual bite marks (photo 4)

CTC margin crushed by a possumClear impressions of possum upper incisor pairs
Photo 3. CTC margin crushed by a possum Photo 4. Clear impressions of possum upper incisor pairs

Sharper tooth impressions are sometimes attributable to possums. Where the lower jaw incisors are worn on an angle lines of sharp v-shaped marks can be seen on the under side of crushed areas of the card (photo 5). Less commonly, possums chew with their head at a sharp angle to the card giving single-tooth angled impressions (photo 6).

Single upper jaw incisor tooth impressions made by a possum biting at an acute angleSharp impressions made by worn lower-jaw incisors
Photo 5. Sharp impressions made by worn lower-jaw incisors. Photo 6. Single upper jaw incisor tooth impressions made by a possum biting at an acute angle.

Rodents:

Rodents: Rodent bite marks are easily distinguished from those of possums. Both cut out, sometimes large, sections of the card; something possums never do. They sometimes also leave clear incisor pair marks or holes which are much smaller than for possums. Both mice and rats have similar bite marks that are mainly distinguished on size. They leave pairs of incisor marks, nearly straight lined on top and more curved underneath. Incisor pairs are about 1 mm across for mice (less than half the width of the coreflute channels) and about 2 mm across for rats (more than half the width of a coreflute channel). Look for individual bites clear of continuous chewing along card edges. Rats frequently chew large chunks out of the cards leaving a relative cleanly cut edge. Mice usually chew small amounts, sometimes making just small scattered nicks along the edge, or chew short channels between card partitions on just one surface. Continuous mouse chewing along the card edge also tends to be less cleanly cut than for rats, with a short chewed flange attached to the remaining card with numerous light tooth impressions beyond that, as apposed to cleanly cut edges frequently made by rats (see photos 7 and 8) (note: there is some overlap in these specific patterns). A hand lens is often required to positively ID mouse bites.

Rat damage to a CTCMouse gnawing on a CTC
Photo 7. Rat damage to a CTC. Photo 8. Mouse gnawing on a CTC.

Carnivores:

Stoats, ferrets, cats or dog bite marks are seen only occasionally on CTCs baited for possums. They all have widely set long circular sharp canine teeth. These species are separated based on tooth size. Stoat canines are needle like leaving pin-prick like holes spaced 6-8 mm apart. Tooth and bite marks increase in size for the other species.

Hedgehog:

Like stoats the upper jaw marks consist of a pair of sharp canine marks. These tend to be a bit blunter than for stoats and are closer together at about 5 mm apart. They are also slightly elongated rather than circular. The lower jaw produces a mark from a blunt pair of incisors, about 4 mm across, that do not cut through the card. Extensive biting by hedgehogs can lead to crushing of the card that may be confused with possum biting. In this case, the sharp upper canines reveals hedgehog as the species responsible.

Rabbit:

Rabbit chewing on a CTC showing the cleanly cut margins and incisor pair impressionsRabbits produce crushed card margins from extensive biting with broad possum like incisors. However they differ from possums in two respects. Firstly they usually also cleanly cut out sections of the CTC, and secondly, their incisors have a groove down the centre so a canine pair actually looks like a straight line of four closely set teeth (see photo 9).

Capturing footprints

Ink can be added to the margin of the lower inside part of the CTC to capture foot prints of animals interacted with the cards (not recommended for possum only detection surveys). Below are some notes on footprint recognition for some pest species

Tracks:

Tracks are mostly useful for identifying rodents. Both rats and mice leave similar track marks; the size of the marks is the main distinguishing feature.

Mouse:

Toe marks are circular and around or less than 1 mm diameter. A good mouse track is a roughly circular cluster, c. 5 mm diameter, of half a dozen toe/foot pad marks.

Rat:

Toe/foot pad marks are up to 2 mm across and the whole track is 10-15 mm across. Front tracks are roughly circular with one large and two small foot pads. Hind foot is larger, slightly oblong, with 5 toes and up to 5 foot pads. For both, toes are slightly oblong and foot pads irregular. A line drawn between the hind most toes passes through the foot pads (important for distinguishing from stoats where this line crosses in front of the foot pads).

However rodent tracks on CTCs are usually smeared so you don’t have the luxury of the above patterns to look at. Usually you get parallel streaks, and you must rely on the width of the streaks to distinguish rat from mouse (approx. 1 mm wide for mouse and 2 mm wide for rats.

Stoat:

Similar in pattern and size to rats, although toes tend to be a bit bigger and there is hair between the toes and around the pads. On very clear prints dermal ridges can be seen on the pads.

Hedgehog:

Three large foot pads and 4 long heavy (3-4 x 6-8 mm) forward-pointing toes.

Possum:

Five large foot pads (cat like) with relatively small (2-3 mm) circular (hind foot) or elongated (front foot) toe marks up to 20 mm away from the foot pad.