A common pesticide may be causing more collateral damage than thought. According to a new study, neonicotinoids can kill beneficial insects such as honey bees, hoverflies, and parasitic wasps by contaminating honeydew, a sugar-rich liquid excreted by certain insects.
Researchers already knew neonicotinoids could harm honey bees and other beneficial insects when applied to important crops such as cotton, potato, and citrus. A 2017 study, for example, found the chemicals can poison bees, causing symptoms like paralysis, vomiting, or death when they eat contaminated nectar or pollen, or even crawl over sprayed surfaces. Yet neonicotinoids still account for more than 20% of the world’s insecticide market.
In the new study, scientists wanted to see whether the chemicals could harm these and other insects more indirectly. They looked to the invasive mealybug (pictured), a 6-millimeter-long insect that eats plants typically contaminated with pesticides. As they nosh, the bugs excrete a sticky substance called honeydew, which pollinating insects like hoverflies and parasitic wasps consume.
The scientists applied two of the most commonly used neonicotinoid insecticides (thiamethoxam and imidacloprid) to clementine trees grown in a greenhouse. They added the chemicals to the soil in one group and sprayed it on leaves in another, mimicking the ways farmers control pest infestations today. The team sprayed a third group of trees with distilled water as a control. Then they infested the trees with mealybugs and fed their resulting honeydew to hoverflies and parasitic wasps.
All of the hoverflies that ate honeydew from trees sprayed with thiamethoxam died within 3 days, while just 10% of the control group died, researchers report today in the Proceedings of the National Academy of Sciences. In the soil-treated trees, nearly 70% of the hoverflies died from the same chemical, compared with about 14% in the control group. More than half of the wasps also died after eating honeydew from the soil-treated and sprayed trees (with thiamethoxam), whereas less than 20% died in the controls.
The study suggests honeydew could be another way beneficial insects are exposed to deadly insecticides. This can devastate more insects across the food web than nectar contaminated with insecticides could, the team says, because honeydew is more abundant, especially in agricultural fields.