Toxicity and Sublethal Effects of Autumn Crocus (Colchicum autumnale) Bulb Powder on Red Imported Fire Ants (Solenopsis invicta)

Abstract

Autumn crocus (Colchicum autumnale L.) is a medicinal plant as it contains high concentrations of colchicine. In this study, we reported that the ground powder of autumn crocus bulb is highly toxic to invasive Solenopsis invicta Buren, commonly referred to as red imported fire ants (RIFAs). Ants fed with sugar water containing 5000 mg/L of bulb powder showed 54.67% mortality in three days compared to 45.33% mortality when fed with sugar water containing 50 mg/L of colchicine. Additionally, the effects of short-term feeding with sugar water containing 1 mg/L of colchicine and 100 mg/L of autumn crocus bulb powder were evaluated for RIFAs' colony weight, food consumption, and aggressiveness, i.e., aggregation, grasping ability, and walking speed. After 15 days of feeding, the cumulative colony weight loss reached 44.63% and 58.73% due to the sublethal concentrations of colchicine and autumn crocus bulb powder, respectively. The consumption of sugar water and mealworm (Tenebrio molitor L.) was substantially reduced. The aggregation rates decreased 48.67% and 34.67%, grasping rates were reduced to 38.67% and 16.67%, and walking speed decreased 1.13 cm/s and 0.67 cm/s as a result of the feeding of the two sublethal concentrations of colchicine and autumn crocus bulb powder, respectively. Our results for the first time show that powder derived from autumn crocus bulbs could potentially be a botanical pesticide for controlling RIFAs, and application of such a product could be ecologically benign due to its rapid biodegradation in the environment.

Keywords: Colchicum autumnale; autumn crocus; colchicine; invasive species; pest control; red imported fire ant (RIFA).

Figure 1

Autumn crocus (Colchicum autumnale) plants at the time of flowering (A), underground bulbs (B), sliced bulb (C), ground bulb powder (D), HPLC chromatogram of colchicine standard at 10 mg/L (E) and autumn crocus bulb powder sample (F). The fourth (far right) peak in (F) had the same UV-Vis spectra as the peak of colchicine (E).

Figure 2

Mortality rates of S. invicta ants after three days of feeding with 10% sugar water containing different concentrations of colchicine (A) and autumn crocus bulb powder (B). The ‘0′ on the x-axis was the control, i.e., 10% of sugar water solution without any addition. Data are presented as mean ± standard error (S.E.). Different letters above bars indicate significant differences in mortality due to concentration effects within a treatment (A or B) at p < 0.05 level based on Tukey’s honestly significant difference (HSD) test (n = 3).

Figure 3

Toxicity of a sublethal concentration of colchicine at 1 mg/L and autumn crocus bulb powder at 100 mg/L in 10% sugar water to S. invicta ants over 15 days of feeding. Data are presented as mean ± S.E. Different letters at each observation day indicate significant differences among treatments at p < 0.05 level based on Tukey’s HSD test (n = 3).

Figure 4

Accumulated colony weight loss (mean ± S.E.) of S. invicta colonies over 15 days of feeding with 10% sugar water, the sugar water containing 1 mg/L colchicine, and 100 mg/L autumn crocus bulb, respectively. Different letters at each recording day indicate significant differences among treatments at p < 0.05 level based on Tukey’s HSD test (n = 3).

Figure 5

The amount of sugar water (A) and T. molitor (B) consumed by S. invicta workers over 15 days of feeding. Data are presented as mean ± S.E. Different letters at each sampling day indicate significant differences among treatments at p < 0.05 level based on Tukey’s HSD test (n = 3).

Figure 6

The mean aggregating rate (A), grasping rate (B), and walking speed (C) of small workers of S. invicta fed with sugar water and sugar water containing 1 mg/L colchicine or 100 mg/L autumn crocus bulb. Data are presented as mean ± S.E. Different letters at each sampling day indicate significant differences per parameter among treatments at p < 0.05 level based on Tukey’s HSD test (n = 3).