Comparative Sublethal Toxicity of Three Neonicotinoids in Red-Winged Blackbirds

Abstract

Seed-eating birds that use agricultural areas could be exposed to neonicotinoids through several pathways, including ingestion of treated seeds. Previous studies in birds have found sublethal exposure to neonicotinoids can reduce body mass and fat stores, but the underlying mechanism is not clear. We hypothesized that neonicotinoids reduce body mass principally by acting as an appetite suppressant rather than increasing energy consumption. Red-winged blackbirds (Agelaius phoeniceus) were exposed to an oral dose of one of three neonicotinoids at concentrations that could realistically be consumed by seed-eating birds (0, 10, 20, or 30 mg/kg bw of imidacloprid, thiamethoxam, or clothianidin) or were given a moderate or high food-restricted diet (n = 9 or 10 birds per treatment) and were then monitored for 3 days. Imidacloprid caused neurotoxic symptoms and rapid reductions in food consumption and body mass that persisted for >48 h. Food restriction had similar effects on body mass and biochemical indicators of energetic state (plasma triglycerides, uric acid, and β-hydroxybutyrate), which suggests appetite suppression was the main driver of mass loss in imidacloprid-exposed birds. Single doses of clothianidin and thiamethoxam altered food consumption and biochemical markers, but effects were less severe than for imidacloprid, and neither compound caused overt neurotoxicity. This study confirms that acute neonicotinoid exposures can affect behavior, appetite, mass, and energetic state in seed-eating songbirds, with potential consequences for survival.

Keywords: appetite suppression; birds; fattening index; neonicotinoids; neurotoxicity; relative toxicity.

1

Cumulative proportion of captive red-winged blackbirds exposed to a vehicle control (sunflower oil) or low, medium, or high doses (10, 20, or 30 mg/kg bw) of imidacloprid exhibiting symptoms over time from dosing (A) and the scores for severity of symptoms based on neurotoxicity evaluations at 1 h and 6 h postdosing (B). Neurobehavioral abnormalities were scored from 0 (normal, active) to 5 (virtual paralysis). Boxes indicate interquartile range (IQR), middle lines indicate median, whiskers show the minimum and maximum values within 1.5× IQR, and circles represent outliers (>1.5× IQR from box).

2

Percent change in (A) daily food consumption and (B) body mass in red-winged blackbirds exposed to a vehicle control (sunflower oil), imidacloprid (IMI), clothianidin (CLO), thiamethoxam (THX), or food restriction (moderate or high), relative to predosing levels. Changes are presented for measures taken 24 h postdosing compared to immediately prior to dosing, with the red dashed line representing no change. Asterisks indicate significant differences between predosing and postdosing measures within a treatment group (*p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001). Boxes indicate interquartile range (IQR), middle lines indicate median, whiskers show the minimum and maximum values within 1.5× IQR, and circles represent outliers (>1.5× IQR from box).

3

Change in (A) fattening index and (B) uric acid in red-winged blackbirds exposed to a vehicle control (sunflower oil), imidacloprid (IMI), clothianidin (CLO), thiamethoxam (THX), or restricted food (moderate or high). Changes are presented for the difference between blood samples taken at 6 h postdosing compared to blood samples taken at the same time the previous day (18 h predosing), with the red dashed line representing no change. Asterisks indicate significant differences between predosing and postdosing measures within a treatment group (*p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001). Boxes indicate interquartile range (IQR), middle lines indicate median, whiskers show the minimum and maximum values within 1.5× IQR, and circles represent outliers (>1.5× IQR from box).