Built-in Emergency Call
Spider mites are not exactly friends of gardeners and farmers. Predatory mites are more likely - they eat up the unloved pests in abundance. Some plants naturally attract hungry beneficial insects. Others can be helped with genetic engineering tricks.
By the time the white, felty threads appear on the underside of the leaves, it's usually too late: the common spider mite (Tetranychus urticae) has struck. Her web, with which she protects herself from the inclemency of the weather, is not the real problem. Rather, their unbridled appetite for plant juices causes numerous leaves to wither, damaging the entire plant. Fruit trees, potatoes, beans, wine, hops and a number of ornamental plants are on their menu. The economic damage caused by the small arachnids is significant.
Some people would like to use chemicals straight away, but Mother Nature recommends a different recipe here: predatory mites like to eat their spinning relatives. The predatory mite Phytoseiulus persimilis, originally from Chile, creates five adult or twenty juvenile spider mites per day and is therefore often used for biological pest control.
What is right for the farmer can only be cheap for nature. Because some plants are able to defend themselves against a spider mite infestation by sending out a chemical emergency call: With certain scents, so-called terpenes, they attract hungry predatory mites, which then greedily pounce on the spider mites. Too bad not all crops have this built-in call for help.
This can be remedied, thought Iris Kappers. Together with her colleagues from the Dutch University of Wageningen, she tried to genetically retrofit plants with the SOS signal. Initially, no useful plant was used as a "guinea pig", but the Arabidopsis thaliana thale cress, which is highly valued by geneticists and plant physiologists.
The researchers took the gene for the enzyme sesquiterpene synthase from strawberries and introduced it into Arabidopsis mitochondria. Because conveniently, these cell organelles produce the substance farnesyl pyrophosphate, with which the enzyme can build the desired terpenes.
The upgrade was successful. Using gas chromatography, the researchers were able to demonstrate that large amounts of terpenes flowed from the leaves of their genetically modified plants. The assassin bugs were also convincing: given a choice, 388 of them crawled to the transgenic herb, while only 197 got lost on the wild form.
It's like calling the fire department all the time
(Marcel Dicke) After this proof-of-principle experiment, the researchers want to try the same thing with crops that are also not immune to spider mites. However, the scientists concede that retrofitting them with genetic engineering is still quite imperfect for practical use. Because the scents smell extremely tempting for assassin bugs, but less so for human noses. Their use should therefore initially be limited to plant parts that are not intended for immediate consumption.
Also, nature uses the chemical cry for help only when necessary; however, the transgenic plants constantly produce terpenes – which sooner or later will dull the weapon. "It's like constantly calling the fire department," explains co-author Marcel Dicke. "At some point she won't come anymore."
