Christine Dieckhoff advises that those who take up the fight with Halyomorpha halys and Nezara viridula should rather wear gloves. “They’re not called stink bugs for nothing.” And the head of the Biological Plant Protection department at the Agricultural Technology Center (LTZ) Augustenberg in Karlsruhe doesn’t think much of the supposed tip from Internet forums to suck the animals away. “You can’t get rid of the stench.”
Halyomorpha halys, the marbled stink bug, and Nezara viridula, the green rice bug, migrated from Asia and Africa, respectively. The rice bug was first registered in Germany in 1979, reports Dieckhoff. Because it was cold enough, the animals could not hibernate. “It’s different now.”
The marbled stink bug has previously caused major damage to fields, fruit and vegetables in the United States. At the beginning of the 2000s it came to Switzerland in packaging material and is now also on the rise in Germany, says Dieckhoff – via the Rhine plain and transport routes. Conurbations like Munich and Berlin are popular. She feels comfortable where people have built sheltered places to spend the winter, such as house roofs and barns. And in the spring, the first plants to eat thrive on balconies.
More precisely: for piercing and sucking. That leaves damage. Sometimes just spots on the peppers, for example, which a farmer can no longer use for trade. Sometimes the cucumber tissue rots. “Some also say it tastes like bugs,” says Dieckhoff. Tomatoes, beans and all kinds of fruit are also on the menu. “The list of host plants that are not suitable is shorter.” According to the LTZ, the bug caused harvest losses of more than half a billion euros in South Tyrol in 2019 alone.
And according to Dieckhoff, the bugs migrate with the fruits, which ripen one after the other throughout the year. First berries, then apples, for example. “That makes it difficult to fight them.” Bugs can also fly. “They are not outstanding fliers, but good fliers.” And even the nymphs, the young animals, are good on their feet.
They have no natural enemies in this country. And since there are no effective chemical pesticides, allotment gardeners basically only have to collect bugs and eggs and use a dense net. However, this must be done in good time so that the bugs are not trapped in their paradise. Dieckhoff advises that it must be close-meshed and close to the ground.
Or you rely on biological protection, natural enemies. And like the bugs themselves, they can come from abroad. In 2020, researchers at the LTZ Augustenberg detected the samurai wasp Trissolcus japonicus in Germany for the first time, in the Heidelberg area. The two millimeter small animal, which comes from East Asia, specializes in placing its eggs in the clutches of bugs. The brood of the parasitic wasp then eats the offspring. According to Dieckhoff, the two species have been so well attuned to each other for thousands of years that there is no danger to native bugs or even other animals such as ladybirds.
The expert also lists some worms and ants as predators for the bug eggs. In turn, some caterpillar flies laid their eggs on the adult bugs. From there, the larva bores into the bug and hollows it out from the inside. “That sounds a bit like an alien,” admits Dieckhoff. “But it’s biology.”
Dieckhoff and colleagues are now investigating to what extent these natural helpers can be a building block in the fight against so-called harmful bugs as part of a project. The Federal Ministry of Agriculture is funding the project with more than 650,000 euros. Also involved are the Julius Kühn Institute as the federal research institute for cultivated plants based in Dossenheim in northern Baden, and Katz Biotech AG from Baruth/Mark in Brandenburg, which specializes in the production and use of beneficial insects in biological plant protection.
The experts want to investigate the occurrence of harmful bugs and their antagonists over the course of the season in various fruit-growing crops and locations. The projects also include breeding, collecting images of damage and advice for farmers and gardeners.
Dieckhoff explains that the aim is not to eradicate the bugs. This does not work simply because the parasitic wasps depend on the bugs as hosts. “So they will never completely destroy a population.” But the wasps could push back the bugs to an extent that is not a problem. “Every system, including a garden, can tolerate bugs,” says Dieckhoff. “Right now that’s just completely out of whack with invasive species.”