The air is clean
Plants emit scents to attract insects. They usually want to be pollinated. But sometimes it is also a cry for help that is overheard.
Nature tells hungry animals when it's time to eat in many ways. In some cases it's simply the change between day and night: while some retire to sleep at dusk, some hungry night owls are just really waking up.
Many caterpillars only nibble on tender young leaves at night. Lights out, it seems, signaling them: It's safe now. This is because, according to the widespread assumption, they avoid a sophisticated defensive measure by their involuntary host. After all, many plants react to the uninvited guests with a ruse: they emit scents to attract parasitic wasps, for example. They don't follow the greens' call for help because they have an appetite for the treat themselves, no: the well-being of their offspring is important to them. The insect plants its eggs directly in the voracious caterpillars so that the little ones can sit right next to the meat pots. The subtenants soon spoil the appetite of the leaf eaters for good. This solves the problem for the plant.
However, the scent of the corn plant varies greatly. During the day - when a particularly large number of parasitic wasps are out and about - the plant emits significantly more attractants than at night. Should these - or other substances from the chemical repertoire of the plant - tell the caterpillars when it is safe to feed them large amounts? And not the twilight?
Kaori Shiojiri and his colleagues from Kyoto University sensed the inner alarm clock of the caterpillars of the Asian traveling owl (Mythimna separata). If the animals were really nocturnal, the researchers thought that they would have to retreat to prepared hiding places in the light. But this assumption was far from the truth: the voracious caterpillars happily ate the researchers' lab food, around the clock - light and darkness left them completely unimpressed.
However, that all changed when the researchers put live corn plants in front of the caterpillars. Without a single caterpillar feeding on it, after eight hours of light there were 20 percent more larvae in their hiding places than before. In the dark, on the other hand, a third more caterpillars went in search of food. So the day-night activity had to be related to the plants. Darkness alone was not enough to lure the caterpillars out of their lair.
In order to measure the pure effect of the vapors, the researchers separated all the general conditions for the caterpillars - i.e. light, darkness, grazed and ungrazed maize plants - from each other. They conducted air from caterpillar-free or already nibbled corn plants that were in the dark to caterpillars in the light and vice versa.
Lo and behold, the gluttonous animals let themselves be fooled: Even when it was dark – actually safe – the animals hid as soon as the air flowed around them from illuminated corn plants. The caterpillars reacted particularly strongly to the chemical cocktail of light-exposed plants that had already been visited by other hungry colleagues.
It's not light and dark that dictate the caterpillars' nocturnal activity, but the plant itself acts as the clock. And why not - the overheard scent message, which also contains the dangerous call for help during the day, offers similarly reliable indications of when it is a good and bad idea to leave the hiding place. The researchers now want to find out exactly which substances caterpillars and parasitic wasps react to. Then they might not only be able to get larvae of the Asian burrowing owl to escape before they cause major damage in the corn fields.
