How a virus arcs a caterpillar into a zombie destined to climb to its death

The cotton moth caterpillar climbed higher and higher, its tiny body constantly peeling from leaf to leaf. When it reaches the top of the plant, it dies, facilitating the spread of the insect-targeted virus there.

The virus behind this deadly climb manipulates a gene linked to the caterpillar's vision. As a result, insects are more attracted to sunlight than usual, say researchers online on March 8 in Molecular Ecology.

The virus involved in the absorption of this caterpillar is a type of baculovirus. These viruses could have evolved with their insect hosts 200 to 300 million years ago, said Xiaoxia Liu, an entomologist at the China Agricultural University in Beijing. Baculoviruses can infect more than 800 species of insects, mainly caterpillars, moths, and butterflies. Once infected, the host develops "treetop disease," which is forced to climb before dying, leaving the infected corpses reared for prey by predators.

The sneaky tricks of the virus have been known for more than a century, Liu said. But how they turn caterpillars into zombies destined to rise to death on their own is not understood.

Previous studies have shown that infected caterpillars exhibit greater 'phototaxis,' meaning they are more attracted to light than uninfected insects. Liu and his team confirmed this effect in the laboratory using a cotton moth caterpillar (Helicoverpa armigera) infected with a baculovirus called HearNPV.

The researchers compared the positions of infected and uninfected caterpillars in a glass tube surrounding a climbing net under an LED light. Uninfected caterpillars migrate up and down the net but return to the bottom before hatching. This behavior makes sense because this species develops into adults underground in the wild. But infected hosts will end up dead at the top of the network. The higher the light source, the higher the infected host rises.

The team moved to a horizontal plane to ensure that the host responded to light and not gravity by placing the caterpillars in a hexagonal box with lighted sidewalks. On the second day after infection, host caterpillars crawled in the morning about four times more often than uninfected ones.