Quick Dips

Dodging the Bait

David Morgan

Just because we may regard the introduced possum as a pest does not mean that it is any less likely to be equipped with survival mechanisms than other animals living in their natural habitat. Recent research by Manaaki Whenua-Landcare Research on possums' behaviour towards 1080 baits shows something of this pest's resilience in the face of massive control efforts.

We have been investigating what happens when possums survive a 1080 operation by finding and eating only small, sub-lethal baits, which occur as a result of breakage during transport, handling, and use.

The present control strategy often involves an initial aerial knockdown operation using 1080 baits, most commonly made from cereals and fruit. The bait also contains green dye, to deter birds, and cinnamon flavour to conceal the 1080. After initial operations, possum populations are kept at low levels by repeated ground-based operations, often using the same type of bait. These maintenance operations have been repeated as often as three-monthly in some locations, as eradication of bovine Tb depends on keeping possum numbers low enough that the disease cannot spread.

There is a growing suspicion that 1080 may be becoming less effective under such intensive possum control, so we set up an experiment to find out more about the possible occurrence of bait shyness.

Possums were trapped in an area of North Canterbury that had never been subjected to 1080 operations, and settled in captivity. Some were offered small pieces of cereal pellet baits, designed to be sub-lethal. They all ate these bait pieces, but when they were offered full-size baits two days later, nearly two-thirds refused the baits. Three months later when baits were again presented to the bait-shy possums, most were still shy.

Confirming that there is a problem was the first part of the study and set us to seeking practical solutions. Next we offered the shy possums some different bait types to see if the shyness could be overcome. Changing the bait flavour from cinnamon to orange did not fool the possums, which still refused to eat it. Nor was a change of toxin from 1080 to brodifacoum successful. Only when the bait type was changed from cereal pellet to carrot were the majority of possums killed. It seems that possums remember the bait base material rather than added flavour or toxin, and most of them remember for at least three months.

To confirm this, a group of shy possums were offered cereal pellets with no toxin, no added flavour, and no green dye. A group of possums that had never encountered baits before were offered the same type of bait. While only one out of 15 of these quot;naivequot; possums rejected bait, six of the shy possums would not eat it. When green dye was added to the baits in an experiment with a second group of shy possums, nine out of 15 refused it compared with, again, only one of 15 naive possums.

We now intend to test these findings in a field experiment. As recognition of the bait base, rather than additives, seems to be the reason possums survive repeated 1080 baiting operations, a study will be set up where extensive bait shyness is first deliberately created by presenting sub-lethal cereal baits. After three months, the effectiveness of alternative baits will be compared with cereal baits.

Our research team has also been developing new bait types, such as synthetic gels and pastes. If the early predictions prove correct, new bait types may become an essential part of the control toolbox if we are to achieve the goal of wiping out bovine Tb.

David Morgan works for Manaaki Whenua-Landcare Research, Lincoln