Australasian Science: Australia's authority on science since 1938

The Very Hungry Caterpillar Zombie

By Magdeline Lum

Quirky experiments and research findings

As dawn rises over a dew-covered European forest, an army of gypsy moth caterpillars head downwards searching for cover in crevices or in the soil. The Sun’s rays fall on the leaves of the trees, revealing a splinter group of caterpillars heading upwards. The urge to climb to the highest leaf is great.

When the summit is reached, a grisly spectacle is revealed as the caterpillars liquefy. Their bodies melt and, as the Sun travels higher into the sky, pink rain falls, coating the leaves and branches.

This pink rain contains millions of baculoviruses, the hidden force driving the caterpillars to climb. The condition that impels the caterpillars to climb to their deaths has been observed for over a century and is called wipfelkrankheit, which is German for “tree top disease”.

It is only just recently that scientists have begun to unravel the details of how a baculovirus, in this case, Lymantria dispar nucleopolyhedrovirus (LdMNPV), is able to cause a gypsy moth caterpillar to behave unlike a gypsy moth caterpillar.

Prof Kelli Hoover from Penn State University led a research group to investigate the mechanism that LdMNPV utilises to control the behaviour of its moth hosts while reprogramming the cells in the caterpillars to produce virus particles. This is due to one of its genes, egt. The researchers removed the egt gene from LdMNPV, and then infected one group of gypsy moth caterpillars with wild-type LdMNPV and another group with the egt-deletion strain.

In both cases the caterpillars died but the difference was where they died. Caterpillars infected with wild-type LdMNPV died at the top of their containers whereas caterpillars infected with LdMNPV with the egt gene removed died at the bottom of the containers.

The researchers also selected a group of caterpillars infected with LdMNPV with egt reinserted and found that this group climbed to the top of their containers before coming to a lofty end.

The researchers concluded that the egt gene inactivates the caterpillar hormone, 20-hydroxyecdysone. This hormone controls moulting, and when levels increase the caterpillar climbs to a high position.

It is thought that once the virus has inserted its genes into the caterpillar’s cells, the moulting hormone increases, triggering the caterpillar to climb. However, the group surmises that egt lowers these levels just enough so that the caterpillars climb without triggering a moult, so they keep feeding.

The research has been published in Science.