Tomatoes and peppers get light spots, apples and pears get dark spots. Cucumbers writhe, sometimes particularly badly. "Like pig's tails," says Christine Dieckhoff. She heads the Biological Plant Protection department at the Augustenberg Agricultural Technology Center (LTZ) in Karlsruhe and deals with the consequences of, for example, green rice bugs biting fruit and vegetables. Raspberries would be inedible: "It tastes like a bug." Introduced decades ago from East Africa, the green rice bug has been spreading rapidly in Germany since the mid-2010s - probably also because of climate change. Also the marbled stink bug, which came from China and a wide range of fruit, vegetables and host plants in arable farming such as asparagus , corn and potatoes are becoming more common. For farmers, this means economic damage - whether because of crop failure or because the products can no longer be sold. In countries like Italy, the total damage was recently estimated at several hundred million euros a year. So it's no wonder that the German Farmers' Association is on the alert: "We assume that the pressure from pests and diseases will increase significantly in the future," says General Secretary Bernhard Krüsken.
Climate change and above all the year-specific weather played an important role, explains Sandra Krengel-Horney from the Julius Kühn Institute (JKI), the Federal Research Institute for Cultivated Plants. "Many pathogens can form more generations under warmer conditions and migrate into the stands earlier." Even adult aphids, for example, could hibernate in mild winters and colonize stocks more quickly in the following year and possibly transmit viral diseases. New species could become established, and the distribution could also shift further north.
This has consequences, as Dieckhoff from the LTZ emphasizes. Farmers relied on forecasting models. But they would have to be revised. "We are realizing that we no longer know pests that we actually know well." In the meantime, there is a corn borer breed in Germany that produces two generations a year.
The proportion of harmful species is in the lower percentage range, says Olaf Zimmermann, who is responsible for pest biology at the LTZ, among other things. "We're on the lookout for many, but we don't know what pops up." In view of the resources, the experts could actually only work against the most important pests.
In addition to one or two species of cicadas from the Mediterranean region, he is currently particularly interested in the Japanese beetle. "It's the hottest new thing right now," says Zimmermann. The first specimens have already been found in the southwest and on the Swiss border.
The beetle can cause severe feeding damage, especially on fruit trees, strawberries, beans, corn, vines, roses and many other shrub and tree species. The grubs, i.e. larvae, feed mainly on grass roots and can destroy entire meadows and pastures in large numbers.
What to do? Introduced species usually come without opponents, as Zimmermann says. "There we are blank." He could imagine training sniffer dogs with the Japanese beetle. But that takes a few years.
A natural enemy of the marbled stink bug is the samurai wasp - an East Asian animal that is two millimeters in size and places its eggs in the bug's clutches. The brood of the parasitic wasp then eats the offspring.
Zimmermann describes such egg parasites as a "golden solution" because stink bugs that can reproduce do not even hatch from the eggs. And the good news: just like the stink bug, the samurai wasp has now made it to Germany.
From the point of view of entomologists, this is a good thing - because the parasitic wasps should not be deliberately placed in a greenhouse infested with bugs, for example, as Zimmermann explains. The laws have been supposed to be amended for years, but nothing has happened so far.
The farmers' association calls for a wide range of active ingredients to protect the plants, "in order to be able to secure quantities and qualities in an emergency and also to avoid resistance," as General Secretary Krüsken puts it. In addition, the association sees potential in resistance breeding in the medium term in order to obtain more resistant varieties.
"In arable farming, fruit growing and viticulture, there are already many varieties that are resistant to fungal pathogens," explains JKI expert Krengel-Horney. "Unfortunately, things have been different so far with regard to harmful insects." Resistance is being worked on in many cultures, but breeding takes time. "It is not certain whether we will always be able to keep up with the rapid changes caused by climate change with established breeding methods." It will also not be possible to completely do without pesticides.
In the fight against marbled stink bugs in the USA, the chemical club that may be used there did not bring the hoped-for success, says Dieckhoff. The only thing that helps is netting affected plants and collecting the bug eggs. Even a spread of the samurai wasp in this country will not eradicate the bugs. "We can't get rid of them," Dieckhoff states. The aim is therefore to achieve a balance between wasps and bugs.