Researchers at a leading agricultural university in China have uncovered a striking effect of a deadly virus that targets caterpillars: it rewires the host’s genetic activity related to light, nudging infected insects to climb toward the treetop where the sun shines. The discoveries appear in Molecular Ecology, adding a new layer to our understanding of how certain pathogens manipulate hosts and influence disease spread in natural ecosystems.
Nucleopolyhedroviruses, a group of viruses that can infect a range of insects including the bollworm’s caterpillars, have long been known to alter host physiology in ways that aid transmission. In this study, researchers observed that as the infection progresses, caterpillars tend to ascend toward the upper canopy. This behavior seems to play a role in intensifying the virus’s reach, since higher leaves and flowers are more exposed to sunlit areas where plant-feeding insects congregate and viruses can spread through feeding damage and environmental exposure. Earlier work noted a tendency for HearNPV-infected caterpillars to move toward light sources; the current experiments build on that foundation by offering a controlled demonstration of the behavior in a lab setting. The scientists used glass tubes with light sources placed at distinct heights to test how infected larvae responded to varying illumination—an approach that clarified the sheer influence of light on their movement. In contrast, a control group of uninfected caterpillars did not show the same inclination toward light, underscoring a virus-driven manipulation rather than a simple reaction to environmental cues.
To confirm that the response was truly tied to light and not to gravity or the mere presence of height, the team conducted additional trials with caterpillars that were entirely unable to sense light. These blind insects did not exhibit the same climbing behavior, reinforcing the conclusion that illumination, rather than altitude or other factors, drives the observed pattern of movement. The evidence suggests a precise, pathogen-induced alteration in the nervous system circuitry of the caterpillars, which intercepts natural light sensing to steer them toward sunlit zones in trees where the virus can spread more effectively before the host perishes.
Commenting on the implications, the researchers note that the bright, overhead blaze of sunlight aligns with a probable adaptive advantage for the infected larvae. Positive phototaxis, the tendency to move toward light, appears to be a consistent and reliable mechanism that places infected caterpillars in high-exposure environments, increasing opportunities for viral transmission as the canopy becomes a bustling arena of feeding and predation risk. The study further delves into the genetic underpinnings of this response, revealing six genes that shift their activity in the presence of the virus and the host’s altered physiology. Among these, three stand out as pivotal: HaBL, which detects short-wavelength light; HaLW, which senses long-wavelength light; and TRPL, a gene involved in converting light cues into neural signals. When researchers turned these genes off in infected caterpillars, the insects showed a diminished likelihood of approaching light and, correspondingly, a reduced rate of mortality linked to the sensor-triggered behavior. The findings illuminate how a pathogen can hijack sensory pathways to optimize its life cycle, offering insight into the intricate interplay between host biology and viral strategies. The broader impact touches on pest management in agricultural settings, where understanding such behavioral manipulations could inform timing and methods for controlling outbreaks while minimizing unintended consequences on non-target species. Overall, the work contributes to a more nuanced view of how light perception intersects with disease dynamics in insect populations, suggesting new avenues for research into host-pathogen interactions and the ecological consequences of phototactic responses in tree-dwelling pests.