Hairy caterpillars cause horse abortions

Now here’s something we’ll bet you never knew: in Australia about one-third of spontaneous abortions in thoroughbred racehorses are caused by hairy caterpillars.

Odd though it sounds, it’s true. And, odd though it sounds, it’s about to get odder.

The caterpillars belong to a species known officially as the bag-shelter moth (Ochrogaster lunifer), and unofficially as the itchy-grub.

The species is common across coastal and inland Australia. It gets its official name from the behaviour of the caterpillars, which gather in large numbers either at the base or in the canopies of food trees – the position depends on geography – and collectively wrap themselves in silk, creating what looks like a purse or bag.

The caterpillars have another peculiar habit. In summer and autumn they set off in long nose-to-tail processions. Mischievous children have been known to manipulate the line such that front of the lead caterpillar comes into contact with the rear end of the last one. The insects are then doomed, it seems, to walk in a circle until starvation or dehydration intervenes.

Such children, however, generally only pull off the trick once. Each O. lunifer is covered in approximately 2.5 million hairs. Coming into contact with them prompts severe itching and sometimes painful dermatitis.

A pregnant race horse that has a close encounter, however, can suffer much more devastating consequences. Accidentally eating one or more as they tromp along the ground leads to some of the hairs being digested and passed around the horse’s body. If they lodge in the placental membrane they are likely to cause inflammation and the onset of a condition called equine amnionitis, resulting in the loss of the foetus.

According to researchers at the University of Queensland, the cumulative cost of caterpillar-caused abortions runs into millions of dollars every year.

To try to reduce this, a team led by biologist Meron Zaluki has drawn up a series of guidelines for horse-owners, setting out how to reduce the risk of horse-bag shelter moth encounters. These include locating and safely removing eggs and trademark silk purses from fields and disposing of them safely.

“It’s important to be careful and to wear protective equipment when handling caterpillar material as the hairs can cause skin irritation and potentially get into eyes,” says Zaluki.

Other researchers are currently investigating strategies for biological control of the moth, including using predatory wasps to attack caterpillar eggs. And what could possibly go wrong with that?