Neonicotinoids can also be harmful to beneficial insects through honeydew. It was already known that this group of insecticides could reach bees via nectar and pollen, but this happens only when the crops are blossoming. Honeydew, however, is available all year round.
Aphids suck the sap from plants to feed. As it contains more saccharides than they need, they excrete the surplus in the form of honeydew: the sticky stuff on your bike’s saddle after leaving it beneath a lime tree. Honeydew is an important food source for many beneficial insects such as bees, ants, parasitic wasps, hoverflies and other predators of herbivorous insects. © Shutterstock
Neonicotinoids are the most widely used group of insecticides worldwide. They are used to combat harmful insects that eat plants, for example. ‘Recent studies have shown that insect populations are declining rapidly’, says Marcel Dicke, professor of Entomology. ‘An important question is how much of a role the insecticides play in this. The debate regarding side effects on beneficial insects mainly concerns bees, but it also affects many other species of beneficial insects. The effects of these insecticides are probably much further reaching than thought previously.’
The researchers discovered that beneficial insects are also exposed to neonicotinoids via honeydew, which is a sweet fluid produced by aphids, mealybugs and whiteflies, amongst others, and is an important food source for many insects. The study was conducted by researchers of WUR, the Instituto Valenciano de Investigaciones Agrarias and the Universitat de València and is published today in the prestigious journal Proceedings of the National Academy of Sciences of the United States of America (PNAS).
The researchers treated orange trees with two different commonly used neonicotinoids. They then introduced mealybugs, which produce honeydew, on the leaves. ‘If the plant has been treated with neonicotinoids, the mealybugs will ingest it through the saps, and the neonicotinoids will then also end up in the honeydew’, Dicke explains. Chemical analyses showed that the Neonicotinoids were indeed present in the honeydew, and the researchers observed that parasitic wasps and hoverflies perished after they had ingested the honeydew.
‘This already happened with treatment at half the concentrations of those normally used’, Dicke adds. According to him, this could mean that the effect also occurs long after the plants have been treated. ‘At a high concentration, many of the mealybugs perish before they can produce a lot of honeydew. So the effect will initially not be as noticeable. But neonicotinoids remain in the plant and even in the soil for a long time, and a lingering effect can be expected once the concentrations in the plants decrease.’
In 2018, the European Commission banned three neonicotinoids from use in open crops. This decision was made following the many studies that showed that these substances are harmful to pollinating insects such as bees. Dicke: ‘There is ongoing discussion regarding this, because farmers argue that it could be applied outside of the blossoming period or on crops that do not bloom. Until now, the deaths of beneficial insects through exposure to honeydew had not been included in the risk assessments, because people had not realised that this could also play a role.’
Neonicotinoids are used around the world on crops that contain many honeydew producing insects, such as aphids and mealybugs. ‘It is important to develop new methods to protect crops’, Dicke says. ‘I would suggest measures such as straticulture with crop diversity, which creates a diverse insect stock and the presence of natural predators, or the use of insects for biological control. That transition is not always an easy one, and it requires a different mindset, but there are plenty of possibilities.’