In the US authorities have been trialling a new bait called ContraPest to tackle rat infestations in New York and some East Coast cities. The bait stops female rats from having pups by making them infertile by triggering early menopause and affects male rats by impairing sperm production. There are no side effects and the rats die of natural causes. More humane than using poisons.
The New York subway trial halved the resident rat population in three months. On the East Coast a trial at a farm made numbers fall by a third in three months. In another urban trial population growth was supressed during the peak breeding season so the population expanded at only one third the expected rate.
ContraPest is being developed by SenesTech, an Arizona based biotechnology company and they are optimistic about it's prospects.
One breeding pair of rats can produce 15,000 pups a year, so if you can reduce that by a third in the first few months of using this bait, that is 5000 fewer rats. Part of the logic of using fertility control rather than poisoning is that killing rats simply means that others move in and take their place; fertility control maintains a small population of existing rats that guard their territory from newcomers.
One advantage is that there is no risk of secondary poisoning to non-target species. The flavoured liquid is ingested by the rat, the chemicals are then broken down by the rats' metabolism, preventing them from getting into predators bodies or the wider environment.
One contradictory point is that rats that don't take the bait may end up having bigger, healthier litters because there is less competition for food.
SenesTech are also looking at adapting the formulation for the control of mice and feral pigs; possibly feral deer, dogs and cats too!