The parasitic wasp Cotesia glomerata laying eggs into a Large White (Pieris brassicae) caterpillar. Image credit: Hans Smid (CC BY 4.0)
Many caterpillars are major pests in agriculture, feeding on a variety of crops. They constantly face threats from both predators and the toxic defenses of the plants they eat. While scientists have long studied how predators find their prey, much less is known about how prey – like caterpillars – manage to survive and defend themselves.
Plants have evolved various strategies to fend off insects. When attacked by herbivores, some plants release specific chemical signals known as herbivore-induced plant volatiles. These scents act as a distress call, attracting natural enemies of the herbivores, such as parasitoid wasps, which use the caterpillars for reproduction, killing the herbivores in the process.
In turn, caterpillars have evolved different defense strategies. They have a sophisticated sense of smell, which may help them detect not only the scent of host plants but also the presence of predators or other caterpillars nearby.
Despite how common and important caterpillars are in farming systems, it is still not fully understood how they choose where to feed or how they avoid being eaten. To explore this, Wang et al. studied how much caterpillars rely on their sense of smell to survive and find food.
In insects, the ability to smell relies on a group of sensory proteins, most of which need a key gene called Orco to function. Without this gene, an insect’s ability to smell is severely impaired. Wang et al. used a gene editing tool known as CRISPR/Cas9 to ‘turn off’ the Orco gene in caterpillars of the large cabbage white butterfly (Pieris brassicae), effectively disabling their sense of smell. The researchers then compared the development, survival performance, and behavioral preferences of these mutant caterpillars with normal ones.
This revealed that caterpillars without a working sense of smell gained less weight and were less successful at finding suitable food sources. They were also more likely to be killed by parasitoid wasps. Behavioral experiments showed that caterpillars with an intact Orco gene avoided plants where other caterpillars were under attack by detecting the warning signals in the air. In contrast, mutant caterpillars could not recognize these danger cues and were unable to find suitable host plants or select enemy-free spaces.
The study of Wang et al. shows that caterpillars use smell not only to find food, but also to avoid danger. These insights could help farmers develop new, environmentally friendly ways to manage pests. For example, by planting companion plants that produce specific scents, or by breeding crops that naturally repel pests, we might be able to steer caterpillars away from valuable crops without relying on chemical pesticides.
