Chemical Used as Defense Against Pests, Used by Moths to Lay Eggs

Isothiocyanates, produced by cruciferous plants as a defense against pests, are also key in telling diamondback moths where to lay their eggs.

Researchers from the Nanjing Agricultural University in China, together with the Max Planck Institute for Chemical Ecology in Germany, showed that the chemical mostly used as a defense mechanism by plants also works as an oviposition cue for the moth species.

The details and results of their study are published in the journal Current Biology.

Finding Molecular Basis in Diamondback Moth's Host Selection

Isothiocyanates (ITC) are broken down products of sulfur-containing metabolites known as glucosinolates. The ITC is released in the event of plant tissue disruption, either when animals chew through them or from other mechanical factors. Researchers cited a previous study noting ITC's potency in repelling herbivores, as well as its toxic properties. However, there are specialist herbivores that use the ITC as a cue for finding host plants.

The researchers behind the study then identified the diamondback moth, Plutella xylostella, as a crucifer specialist that has two olfactory receptors specially designed to detect ITCs. This sensory capability for a diamondback moth drives its preference for the thale cress, also known as mouse-ear cress, or Arabidopsis thaliana.

"We wanted to know whether the moths use isothiocyanates as odor cues to locate their host plants. In fact, behavioral experiments showed that three isothiocyanates are key signals for female moths to locate and lay eggs on cruciferous plants," said Shuang-Lin Dong, leader of the study from Nanjing Agricultural University.

In understanding the mechanism of how the P. xylostella evolved to work around this plant defense, the researcher found that two receptors, OR35 and OR49, responded to the isothiocyanates defined as important for the moth's oviposition. Further tests showed that the two receptors did not respond to other scents or even sex pheromones from other moths, supporting the finding that these two receptors were evolved to detect oviposition cues.

Evolving Past Defense Mechanisms

"We were surprised that even two receptors are specifically tuned to the isothiocyanates. The two receptors, however, detect the isothiocyanates with different sensitivities," Shuang-Lin Dong added. This disparity, they hypothesized, is to allow the female moths to locate prospective hosts from a distance through the more sensitive receptor, while the other will provide fine-tuned information regarding ICT concentrations in the area.

Marcus Knaden, the corresponding author from the Max Planck Institute, explains that evolution has allowed plants to develop defense mechanisms against herbivores. The exchange of chemical signals is an important part of the interaction between plants and insects.

"However, there will be always someone who misuses the communication for its own benefit, like in our case the diamondback moth," Knaden added. He further explained that finding out how these "cheaters" in plant-insect interactions occur could help researchers improve control against global crop pests.

In an additional statement, Shuang-Lin Dong explains that they can use ICT and similar attractive chemicals to trap the pests. They could also develop chemical agents that block or disrupt the response of insects to these substances and interfere with their ability to locate their host plants.

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