The Insecticide Imidacloprid Decreases Nannotrigona Stingless Bee Survival and Food Consumption and Modulates the Expression of Detoxification and Immune-Related Genes

Abstract

Stingless bees are ecologically and economically important species in the tropics and subtropics, but there has been little research on the characterization of detoxification systems and immune responses within them. This is critical for understanding their responses to, and defenses against, a variety of environmental stresses, including agrochemicals. Therefore, we studied the detoxification and immune responses of a stingless bee, Nanotrigona perilampoides, which is an important stingless bee that is widely distributed throughout Mexico, including urban areas, and has the potential to be used in commercial pollination. We first determined the LC₅₀ of the neonicotinoid insecticide imidacloprid for foragers of N. perilampoides, then chronically exposed bees for 10 days to imidacloprid at two field-realistic concentrations, LC₁₀ (0.45 ng/µL) or LC₂₀ (0.74 ng/µL), which are respectively 2.7 and 1.3-fold lower than the residues of imidacloprid that have been found in honey (6 ng/g) in central Mexico. We found that exposing N. perilampoides stingless bees to imidacloprid at these concentrations markedly reduced bee survival and food consumption, revealing the great sensitivity of this stingless bee to the insecticide in comparison to honey bees. The expression of detoxification (GSTD1) and immune-related genes (abaecin, defensin1, and hymenopteacin) in N. perilampoides also changed over time in response to imidacloprid. Gene expression was always lower in bees after 8 days of exposure to imidacloprid (LC₁₀ or LC₂₀) than it was after 4 days. Our results demonstrate that N. perilampoides stingless bees are extremely sensitive to imidacloprid, even at low concentrations, and provide greater insight into how stingless bees respond to pesticide toxicity. This is the first study of its kind to look at detoxification systems and immune responses in Mexican stingless bees, an ecologically and economically important taxon.

Keywords: decline; detoxification; neonicotinoid; pesticide; pollinator; stingless bee.

Figure 1

Kaplan–Meier survival curves of the effect of imidacloprid (Imd) and dimethoate (reference toxic chemical) insecticides on Nannotrigona perilampoides stingless bees. Bees (n = 15 bees per cage, n = 4 cages per treatment) were fed with Imd LC₁₀ or LC₂₀, or a control solution for 10 days. Different lowercase letters indicate the significant differences between the respective sublethal concentrations of Imd and control on survival of bees (n = 240; Cox proportional hazard, p < 0.05). For statistical details, see Table 3.

Figure 2

Effects of exposure to LC₁₀ or LC₂₀ of the insecticide imidacloprid (Imd) on average daily syrup consumption (reported in µL/bee/day, mean ± Standard Error of the Mean, SEM) by Nannotrigona perilampoides stingless bees. Bees (n = 15 bees per cage, n = 4 cages per treatment) were fed with Imd (LC₁₀ or LC₂₀), or a control solution for 10 days. Symbols on the box plot represent maximum and minimum values (whiskers: ┬ ┴) and mean values (-). Different lowercase letters indicate significant differences between the respective Imd concentration and control on food consumption (ANOVA, p < 0.05).

Figure 3

Fold-change in the abundance of transcripts of innate immune (a–c) and detoxification-related (d) genes in adult Nannotrigona perilampoides stingless bees. Bees (n = 15 bees per cage, n = 4 cages per treatment) were fed with sugar syrup spiked with Imd (LC₁₀ or LC₂₀), or a control solution for 10 days. Symbols on the box plots show the minimum and maximum values (n = 6 bees) (whiskers: ┬ ┴) and mean values (-), with jittered data points. For statistical details, see Table 4.