The notorious cabbage moth is turning the tables: In the case of the pest, the antibodies in the cabbage family not only fail, they also become an attractant, researchers report. They uncovered the way in which the defensive substances of these plants serve the female cabbage moths as odor signals so that they can lay their eggs on the leaves. There is potential in the results for developing pesticide-free control strategies, the scientists say.
Cauliflower and Co. as well as rapeseed, mustard, horseradish …: Many representatives of the cabbage family (Brassica) are important crops and food crops for humans. Not only do they taste good to us, however: some insect species also target their leaves. The cabbage plants are not at the mercy of them, however. If insects bite into their leaves, they form so-called isothiocyanates, which are bad for attackers and whose smell also scares them off. But with one of the worst pests in the cultivation of cabbage plants, these phytochemicals fail: The cabbage moth (Plutella xylostella) has developed resistance to this defense system in the course of evolution and can thus successfully attack the representatives of the cabbage family. Without control measures, your caterpillars can cause great damage to agriculture.
Antibodies act as an attractant
There have already been previous studies which showed that the pests not only tolerate the antibodies well, but even find them attractive. The scientists led by Shuang-Lin Dong from the Agricultural University in Nanjing have now investigated to what extent the cabbage moth actually uses isothiocyanates as scent indicators to find its host plants, and above all, which molecular mechanisms the concept is based on. First, the scientists carried out tests to determine the attractiveness of isothiocyanates. “Through behavioral experiments with female cabbage moths, we were able to show that three specific isothiocyanates are key signals for oviposition,” reports Shuang-Lin Dong.
The scientists then investigated which sensors in the insects are responsible for the perception of these substances. To do this, they first recorded all olfactory receptors that are specifically formed in the female butterflies. They then expressed the genes of these receptors in the egg cells of frogs in order to be able to carry out experiments with the sensors created. “With this method we were able to determine which of the individual olfactory receptors react to which scent. It turned out that the receptors OR35 and OR49 respond specifically to the three isothiocyanates that we had previously identified as crucial for oviposition, ”says co-author Markus Knaden from the Max Planck Institute for Chemical Ecology in Jena. The experiments showed, however, that the two receptors did not react to other plant fragrances or the moths’ sexual attractants.
Potential for crop protection
To further check whether the two sensors actually play a role in the perception of the female cabbage moth, the researchers genetically engineered lines of the insects that were no longer able to develop one or both receptors. The tests with these so-called knockout mutants showed that if one of the two olfactory receptors of the moths was inactivated, they laid significantly fewer eggs on the plants that are normally preferred and smell of isothiocyanates. If both receptors were switched off, the moths were ultimately no longer able to perceive the normally attracting antibodies. Presumably, OR35 and OR49 have developed in a targeted manner in order to precisely detect the isothiocyanates as egg-laying signals, the scientists sum up.
“It seems surprising that there are two receptors. The receptors sense the isothiocyanates with different degrees of sensitivity, ”says Shuang-Lin Dong. The more sensitive receptor could therefore ensure that female butterflies can localize plants from afar, while the other helps to detect higher concentrations of the substances on the plant, ”explains the scientist. The researchers are now planning further investigations to check whether other pests also use special receptors to use the defense substances of plants as a source of information for themselves.
The information about the ingenious strategies of the cabbage moth could now help to improve the control of the widespread crop pest: “Our results offer two approaches,” says Shuang-Lin Dong: “On the one hand, we could use the identified isothiocyanates or similar attractive substances as attractants . On the other hand, we could try to use other substances to manipulate the perception of isothiocyanates in such a way that the moths can no longer find their host plants, ”says the scientist.
Source: Max Planck Institute for Chemical Ecology, specialist article: Current Biology, doi: 10.1016 / j.cub.2020.08.047