Every gardener knows the frustration of finding their prize crop covered in aphids. But the pest's impact is even greater on agriculture, causing enormous financial damage. Harald Wolf, from the University of Ulm, Germany, explains that most insecticides incapacitate their victims by blocking receptors in the central nervous system. So when Hartmut Kayser, a biochemist from Syngenta,told Wolf about his work on the insecticide pymetrozine, Wolf was keen to discover the insecticide's target in the locust's control circuitry. Confident that pymetrozine would also inactivate a component of the central nervous system, the team began probing the insect's middle leg control circuitry, and were astonished to find that the chemical targeted sensory cells instead(p. 4451).
Wolf explains that pymetrozine is a widely used insecticide that destroys plant sucking aphids and white flies. As soon as the pests are exposed to the insecticide, the insects are unable to continue feeding. The insecticide has a similar effect on locusts, while also forcing the middle leg to extend slowly as the insecticide takes effect. Given that the insecticide has such a dramatic effect on the well understood control system in the insect's middle leg joint, Wolf decided to study the insecticide's effects on locusts to see if he could pinpoint the toxin's target.
Jessica Ausborn and Wolfgang Mader began systematically testing components of the middle leg's femur-tibia joint control system to see which were affected by a dose of pymetrozine, expecting to find that the central nervous system was the target. But they were amazed when they tested the first component in the control system, the chorodontal organ, that they need look no further; `[it] turned out to be the primary site of pymetrozine action' says Wolf. The insecticide was affecting mechanosensory cells that detect the joint's position and movement, and not a component of the central nervous system. Wolf remembers telling colleagues about this unexpected discovery at a journal club meeting later the same day; `no one believed it' he says. Wolf needed more proof.
Undertaking an extensive battery of control experiments to check the insecticide's effects throughout the joint control circuit, Ausborn finally convinced Wolf that the chorodontal cells, and no other mechanosensors, were the target when she tested the chorodontal organ directly and found that it stopped responding after a dose of pymetrozine; `That's when we opened the bottle of champagne' Wolf says.
Having satisfied himself that pymetrozine targets chorodontal sensory cells Wolf is excited that pymetrozine could prove to be a powerful tool in the neurophysiologist's experimental arsenal, offering the potential to knock out the function of a single class of sensory receptors, the chorodontal mechanoreceptors, without surgical intervention. Of course he can't rule out the chance that pymetrozine may also target other tissues throughout the insect's body, but it's certainly effective on the locust chorodontal organ. Wolf is also keen to identify the molecular target of this unconventional insecticide, which could prove essential for the development of new insecticides, but he admits that this could be a long way down the line.