Hairy and dangerous

By Tim Low June 1, 2017
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These are the caterpillars that horse breeders dread.

Each is armed with two million finely barbed hairs. Should a pregnant mare ingest some of these hairs when they fall on grass, they can penetrate the intestinal wall, allowing bacteria into the bloodstream and infection of the placenta. You would not think caterpillars would matter to horses, but their hairs are a dire problem in the Hunter Valley, where they’ve caused hundreds of thoroughbred foals to be aborted. The hairs also cause people problems such as extreme itching, rashes and swollen eyes and faces.

These so-called processionary caterpillars are famous for going about single file, up to 200 or more at a time. They set out in a line when they run out of food after stripping bare a wattle tree, or when they need a haven for winter. They have to be well-armed with hairs because, by doing everything in groups, they are very noticeable to hungry birds and lizards. They are not, however, safe from cuckoos, which can safely swallow them, hairs and all.


Ochrogaster lunifer nest (Image: Mark Marathon)

On Fraser Island I once found a complete ring of these caterpillars going round a small tree on a journey of futility. They weren’t there some hours later so the ring must have broken. The processions often fragment on rough ground when a member loses contact with the one in front. Long hairs at the rear of each caterpillar guide the one behind. Followers that lose a connection are reluctant to become leaders, so stalled groups take some time to get going again with a new leader. Sometimes they form a solid mass all bunched up together trying to follow each other, with no one taking the lead.

They browse leaves at night and rest each day at the base of a tree or in its branches inside a big woven communal nest. These silken bags are very hazardous to handle because they abound in shed hairs. Old nests that dismember onto pastures are dangerous to horses.

The caterpillars eventually become bag-shelter moths (Ochrogaster lunifer), which live for only a few days, just long enough to mate and lay eggs. They can’t feed because they have no mouths.

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“These so-called processionary caterpillars are famous for going about single file, up to 200 or more at a time.”(Image: Christopher Watson)