Scientists from the Agricultural University of China have found that a deadly virus that infects caterpillars alters the activity of light-related genes to drive insects to the tops of trees. The study was published in the journal Molecular Ecology.
Nucleopolyhedroviruses infect some insects, including the caterpillars of the bollworm. Before the caterpillars die, they climb the treetops – this is believed to contribute to the more active spread of the virus. Previous studies have shown that caterpillars infected with HearNPV tend towards light sources. The researchers placed the infected caterpillars in glass tubes and placed a light source at the top, middle, and bottom of the tube. The higher the source, the higher the infected caterpillars climbed. The control group of healthy caterpillars did not respond to light.
Additional experiments with completely blind caterpillars confirmed that it was light, not gravity, altitude, or other factors, that caused the reaction – the caterpillars that could not recognize the light source did not react to it.
“Because sunlight illuminates plants from above, positive phototaxy (the ability to navigate and move towards a light source) is likely a reliable mechanism for the death of infected larvae in host plants at high altitude,” said the study’s authors.
They also studied genetic differences between infected caterpillars and healthy ones. They found six genes involved in the response to light, whose activity changes under the influence of the virus. Three of these have proven to be the most important: HaBL to detect short wavelength light, HaLW to detect long wavelength light, and TRPL, which converts light into electrical signals. When these genes were turned off in infected caterpillars, the insects were less likely to approach a light source or die.