Which insecticide kills honeybees

Insect death : Neonicotinoids in honeydew kill beneficial insects

Insecticides are sprayed on crops to ward off pests. It has been well researched whether useful insects are also harmed when they come into contact with the toxins, for example when they are pollinated via pollen and nectar. Now, for the first time, a research team has observed how the neonicotinoids, which are frequently used in agriculture, affect hoverflies or parasitic wasps when they get caught in the "honeydew" - the secretion of lice and cicadas that suck the sugar juice from the sieve tubes of plants.

Excretions from thousands of lice collected and fed

Miguel Calvo-Agudo from the University of Valencia in Spain and colleagues sprayed or watered citrus trees sometimes with pure water, sometimes with neonicotinoid-mixed water. It is known that insecticides of this type, such as thiamethoxam or imidachlorpid, can also harm beneficial insects, which is why they are now limited to greenhouses in the European Union. However, the effect if they land in the honeydew in typical concentrations has never been investigated. For this purpose, Calvo-Agudo's team collected the excretions of citrus bugs - a painstaking undertaking. These insects suck the sugar juice from the sieve tubes of the citrus trees and release most of it as honeydew. For many insects, such as hover flies, wasps, ants and bees, this half-digested sugar juice is an important source of food.

The scientists then fed the honeydew collected from the abdomen of thousands of lice to a species of hoverfly (Sphaerophoria rueppellii) and an ichneumon wasp (Anagyrus pseudococci) and observed the effects: more than half of the animals died within three days if the honeydew came from the trees treated with neonicotinoids, while only five to 16 percent of the animals that got unpolluted honeydew died in the same period, the researchers sum up Study in the journal "PNAS" together. This means that insect species that do not come into contact with nectar or pollen are also threatened by neonicotinoids. Exposure via honeydew must therefore be taken into account in future risk assessments of neonicotinoids.

Insecticides that interfere with biological plant protection

The study is "extremely important," says Randolf Menzel, emeritus neurobiologist at the Free University of Berlin, "because it has demonstrated a new and very important way of absorbing insecticides by non-target insects." It is true that this route has been suspected time and again, "but there has not yet been an exact study." Since a large number of non-target insects ingest honeydew and the neonicotinoids remain in the trees for years, Menzel considers it "urgently necessary to study the damaging effects of contaminated honeydew in more detail. "This should not only measure the survival rate, but also the effects on the behavior, immune defense and reproduction of insect species that feed on honeydew.

Ultimately, such research could also serve plant protection. Because parasitic wasps are considered to be the "most important antagonists of aphids", the economically most important pests of cereal plants, says Teja Tscharntke from the Department of Agroecology at the Georg-August University in Göttingen. Since these wasp species mostly feed on honeydew, "poisoned honeydew prevents biological pest control." Tscharnke suspects that the effects of contaminated honeydew are "widespread". Because even if the neonicotinoids are not immediately fatal, they could reduce lifespan or impair the ability to find sexual partners. "The entire spectrum of negative effects beyond direct mortality has been investigated far too little and is not taken into account when approving pesticides, but it plays a major role in the functioning of ecosystems and food webs," says Tscharntke. Contaminated honeydew may also be the cause of the effects of neonicotinoids on honey bees: "Honey bees can consume a lot of honeydew, and pesticides can be found in the majority of honey jars sold," says Tscharntke. “The popular forest honey is largely based on the sugar that honeybees have obtained from scale insects and other sucking insects in the forest. Correspondingly, the use of insecticides in the forest should also lead to poisoning of honey bees who consume honeydew. "

For the animal ecologist Johannes Steidle from the Institute of Biology at the University of Hohenheim, the study is a "milestone", among other things because it shows how organisms can be killed by neonicotinoids even though they are not the target of the treatment. Predatory and parasitic insects are particularly important ecologically because they are the enemies of herbivores, says Steidle: "What happens when these enemies are missing can be seen in the box tree moth and the chestnut leaf miner. Both have hardly any natural enemies in Central Europe and regularly defoliate their herbivores . " The same is to be feared for many other plant species if the natural enemies cease to exist. "So neonicotinoids lead into a future that is not green, but rather bare." (with SMC)

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