A conserved class of queen pheromones? Re-evaluating the evidence in bumblebees (Bombus impatiens)

Abstract

The regulation of reproductive division of labour is a key component in the evolution of social insects. Chemical signals are important mechanisms to regulate worker reproduction, either as queen-produced pheromones that coercively inhibit worker reproduction or as queen signals that honestly advertise her fecundity. A recent study suggested that a conserved class of hydrocarbons serve as queen pheromones across three independent origins of eusociality. In bumblebees (Bombus terrestris), pentacosane (C25) was suggested to serve as a queen pheromone. Here, we repeat these studies using a different species of bumblebee (Bombus impatiens) with a more controlled experimental design. Instead of dequeened colonies, we used same-aged, three-worker queenless groups comprising either experienced or naive workers (with/without adult exposure to queen pheromone). We quantified three hydrocarbons (C23, C25 and C27) on the cuticular surfaces of females and tested their effects on the two worker types. Our results indicate differences in responses of naive and experienced workers, genetic effects on worker reproduction, and general effects of hydrocarbons and duration of egg laying on ovary resorption rates. However, we found no evidence to support the theory that a conserved class of hydrocarbons serve as queen pheromones or queen signals in Bombus impatiens.

Keywords: Bombus impatiens; bumblebees; hydrocarbons; pheromones; reproduction; social insects.

Figure 1.

Average terminal oocyte size (mm) in two types of workers assigned to seven different treatments. Workers (n = 569, 18–69 workers per treatment group) were sampled from source colonies (n = 7) as either naive (when they were less than 24 h old) or as experienced (at the age of 3 days or older from young colonies where queen reduces worker reproduction). Workers were then kept in groups of three for 10 days during which they were exposed daily to either hexane (control solvent) or low dose (20 ng day⁻¹) or high dose (2000 ng day⁻¹) of tricosane (_C23), pentacosane (_C25) or heptacosane (_C27). Chemicals were applied daily onto a cotton ball in a volume of 5 µl. Data are presented as means ± s.e.

Figure 2.

Cumulative number of eggs and larvae laid by three-worker groups in 10 days. Egg laying was examined in two types of workers assigned to seven different treatments (n = 190 groups, 6–23 groups per treatment). Workers were sampled from source colonies (n = 7) as either naive (when they were less than 24 h old) or as experienced (at the age of 3 days or older from young colonies where queen reduces worker reproduction). Workers were then kept in groups of three for 10 days during which they were exposed daily to either hexane (control solvent) or low dose (20 ng day⁻¹) or high dose (2000 ng day⁻¹) of tricosane (_C23), pentacosane (_C25) or heptacosane (_C27). Chemicals were applied daily onto a cotton ball in a volume of 5 µl. Data are presented as means ± s.e.

Figure 3.

The relation between latency to egg laying and the percentages of resorption in worker ovaries, regardless of treatment or worker type. Workers (n = 534) were sampled from seven source colonies as either naive (when they were less than 24 h old) or as experienced (at the age of 3 days or older from young colonies where queen reduces worker reproduction) and were subjected to one of the seven treatments. The proportion of resorption was negatively correlated with the timing of first egg laying in the cages.