No Evidence of Toxicity to Birds Ingesting Neonicotinoid-Coated Wheat Seeds During Controlled Laboratory Study

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Scientists determined what happens to the neonicotinoid insecticide, imidacloprid, on coated wheat seeds once ingested by Japanese quail (Coturnix japonica)—a model species for free-range, seed-eating, upland game birds. Imidacloprid was found to be rapidly adsorbed, metabolized, and excreted, and resulted in no overt signs of toxicity during a controlled laboratory study.

Neonicotinoids are the most widely applied class of insecticides worldwide. Imidacloprid, a neonicotinoid that is typically applied to corn, soybeans, and wheat in the form of seed coatings, is a perceived environmental hazard to birds because birds may be exposed to the insecticide through ingestion of pesticide-coated seeds during planting season.

Previous controlled laboratory studies have demonstrated that neonicotinoids adversely affect the genomic, cellular, endocrine, immunological, growth, reproductive, and neurobehavioral end points in birds. Imidacloprid and other neonicotinoids have occasionally been detected in free-ranging birds and implicated in some bird mortality events in the environment. However, data on how neonicotinoid insecticides are adsorbed, distributed, metabolized, and eliminated have not been defined in birds. This knowledge gap makes it difficult to understand the toxicity that may be associated with neonicotinoid-coated seed consumption by free-ranging birds.

The U.S. Geological Survey (USGS), partnering with academic scientists, initiated a controlled laboratory study to investigate what happens to imidacloprid in seed coatings and toxicological endpoints (deoxyribonucleic acid [DNA] damage, hepatic gene expression, endocrine responses and overt signs of intoxication, lethality) upon ingestion in a model species—Japanese quail.

Scientists performed a series of laboratory tests on Japanese quail that were raised from a colony at the USGS Patuxent Wildlife Research Center.  Adult male Japanese quail were orally dosed with untreated wheat seeds and wheat seeds coated with an imidacloprid formulation (at approximately 3 and 9 percent of the reported median lethal dose for Japanese quail, respectively) for 1 or 10 days. The number of seeds fed to the quail was based on the capacity of their gastrointestinal track and exposure period reflects a range of potential environmental exposures (for example, one instance of ingesting a few seeds for one day to ingestion of many seeds on 10 consecutive days).

Captive Japanese quail (Coturnix japonica)

Captive Japanese quail (Coturnix japonica), and wheat seeds with (brown) and without (cream) pesticide coatings used to treat insect pests. 

(Credit: Yu Chen, USGS, Patuxent Wildlife Research Center. Public domain.)

Scientists reported rapid imidacloprid absorption (within 1 hour) into blood, and distribution to brain, muscle, kidney and liver followed by rapid clearance to below laboratory detection limits at both dose levels and exposure durations in all quail tissues within 24 hours.

Doses of imidacloprid that approached 9 percent of the median lethal dose for Japanese quail resulted in no overt signs of toxicity.  Furthermore, there was no evidence of enhanced expression of genes associated with metabolism of contaminants, oxidative DNA damage, or alterations in concentrations of corticosterone and thyroid hormones. The scientists explain that the observed tolerance to imidacloprid present in this laboratory study likely reflects rapid clearance of imidacloprid in Japanese quail.

These findings indicate that the hazard associated with ingestion of imidacloprid-treated seeds to free-ranging quail could be low; however, this conclusion does not explain the findings of previously reported field mortality incidents in France and South Africa.  Based on the findings in this study, it is possible that reported mortality events in previous studies were associated with birds that either consumed very large quantities of coated seeds and (or) have greater sensitivity to imidacloprid than predicted by this controlled exposure study using Japanese quail.

This study builds on other studies completed by the integrated science teams of the USGS Contaminant Biology and Toxic Substances Hydrology combined programs that determined the occurrence of neonicotinoids in the environment, how they are transported, how organisms such as birds, fish, and insects are exposed, and their effects on target and non-target organisms.

USGS, as a non-regulatory science agency, provides information on the sources, occurrence, potential exposure pathways and ecotoxicology of a wide variety of environmental contaminants and pathogens.   These integrated field and laboratory studies are used by a variety of stakeholders within DOI as well as other government agencies, industry, academia, and NGOs to safeguard health while balancing other resource management priorities.

This research was funded by the integrated science teams of the combined USGS Contaminant Biology and Toxic Substances Hydrology combined programs.

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