They Smell the Corn

Most organisms can tell the difference between night and day simply by noticing the sun's heat and light. However, some rely on other clues. The nocturnal caterpillars called Mythimna separate use plant chemicals as their alarm clock.


Junji Takabayashi and his colleagues from Kyoto University in Japan tested how corn odors and sunlight influence the caterpillars. The caterpillars come out when the evening arrives and hide during the day, mostly in order to avoid predatory wasps.

Surprisingly, changing the lighting conditions had no effect on the caterpillar behavior, indicating that these insects don't use the sun at all. Scientists have no idea why they don't use light cues like most other organisms and developed instead a much more complicated mechanism.

To test the influence of plant scents, researchers collected gas from the plants during the day and at night. (It was discovered a few years ago that corn plants release a different set of chemicals into the air in the morning and evening.) They then exposed one set of caterpillar larvae to the daytime fumes and another set to the evening fumes and observed the larvae for several hours.

The creatures were almost 50% more likely to go into hiding after being exposed to the "day" fumes than they were when exposed to the "night" fumes. Night fumes made the caterpillars come out of hiding to feed on leaves, their typical evening activity.

These combined experiments show that the insects use plant scent alone as an alarm clock, says Takabayashi. The team is now trying to identify the exact chemical compound that the caterpillars respond to. They hope that this will provide clues on why these creatures use such a strange mechanism to find out whether it's day or night.