While pesticides protect crops from hungry animals, nuisance insects, or even microbial infections, they also affect other important organisms, including bees and earthworms. And today, research published in ACS Environmental Science and Technology Letters This suggests that insects are affected by relatively small amounts of chemicals that may be released from pesticide-treated seeds. Exposure to sublethal doses of these pesticides and fungicides resulted in weight loss and mitochondrial DNA (mtDNA) damage in insects.

Insecticide treatments can be introduced at several different stages of a plant’s life, either by pre-sowing cover or by spraying already established crops. Often times, different chemicals are applied at the same time to maximize their effectiveness. Neonicotinoids, also known as neonics, are a common class of pesticides used in the United States and other countries today, although many of them are banned in the European Union. Recent research has shown that these pesticides and many fungicides persist in groundwater and soil, where earthworms may encounter them. One way to monitor the health of infected insects is through changes in organism weight and mtDNA damage. Unlike DNA in a cell’s nucleus, mtDNA cannot repair itself, and thus may help identify the less obvious, “off-target” effects of a particular environmental toxin. Chensheng (Alex) Lu and colleagues wanted to use this approach to investigate how realistic doses and combinations of neonics and difenoconazole (DIF) fungicides affected the moth.

In laboratory experiments, the team exposed groups of juvenile earthworms (Eisenia fetida). For individual pesticides, and combinations of neonics and DIF, at concentrations that mimic the residues left behind on pesticide-treated seeds. After 30 days, worms were weighed and examined for mtDNA damage. While all insects survived, the weight of earthworms in an insecticide-treated soil decreased by 30 to 80 percent during this period compared to a control group living in untreated soil. Furthermore, insects exposed to one of the four tested neonicotinoids and DIF at the same time gained significantly less weight than those exposed to the same compound. There was also a significant increase in mtDNA damage as a result of pesticide exposure. Because mitochondria generate most of the energy inside cells, damage to their DNA can disrupt cellular functions and other metabolic processes. The researchers say the findings establish a link between neonics and fungicide compounds that are potentially present in the environment and the health of insects, which may inform the unanticipated risks of using neonics in seed treatments.

The authors acknowledge funding from the National Natural Science Foundation of China.