Olfactory attraction of the hornet Vespa velutina to honeybee colony odors and pheromones

Abstract

Since the beginning of the last century, the number of biological invasions has continuously increased worldwide. Due to their environmental and economical consequences, invasive species are now a major concern. Social wasps are particularly efficient invaders because of their distinctive biology and behavior. Among them, the yellow-legged hornet, Vespa velutina, is a keen hunter of domestic honeybees. Its recent introduction to Europe may induce important beekeeping, pollination, and biodiversity problems. Hornets use olfactory cues for the long-range detection of food sources, in this case the location of honeybee colonies, but the exact nature of these cues remains unknown. Here, we studied the orientation behavior of V. velutina workers towards a range of hive products and protein sources, as well as towards prominent chemical substances emitted by these food sources. In a multiple choice test performed under controlled laboratory conditions, we found that hornets are strongly attracted to the odor of some hive products, especially pollen and honey. When testing specific compounds, the honeybee aggregation pheromone, geraniol, proved highly attractive. Pheromones produced by honeybee larvae or by the queen were also of interest to hornet workers, albeit to a lesser extent. Our results indicate that V. velutina workers are selectively attracted towards olfactory cues from hives (stored food, brood, and queen), which may signal a high prey density. This study opens new perspectives for understanding hornets' hunting behavior and paves the way for developing efficient trapping strategies against this invasive species.

Figure 1. Experimental setup.

A. Schematic drawing of the cubic wooden cage (60×60×60 cm) used for the experiments. The front part was equipped with a mesh sleeve for introducing hornets into the cage. The base contained a 5 cm deep mobile drawer (in grey) and a stainless steel plate used to securely deliver tested stimuli. Inside the cage and attached to the roof, a digital camcorder allowed the recording of hornets’ behavior. B. Experimental procedure used in the first experiment. Each of the 20 replications was divided in 2 consecutive sessions separated by a 10 minute interval. Within a session, 5 stimuli among the 10 tested (numbered circles) were placed 5 cm apart in the mobile drawer, and delivered together with a control (ctrl circle). The sequence of stimuli presentation and exact location in the drawer were fully randomized. The second experiment was similarly designed but contained 4 sessions of 5 stimuli among the 20 tested.

Figure 2. Hornet attraction towards hive products and other food samples.

A. Distribution of the number of visits on each sample for groups of 10 hornets. Bold lines show the median number of visits per session (20 replications, N  = 200 hornets). The boxes represent the first and third quartiles (25–75%), while whiskers represent the distribution from 10 to 90%. The thick vertical dashed lines separate the different sources emitting these molecules (from left to right: honeybee, fish, meat). B. Distribution of the cumulative duration that groups of 10 hornets spent on each sample. Bold lines show the median duration by session (20 replications, N  = 200 hornets). Grey bars: samples; white bars: control. Comparison between samples and control were done with Wilcoxon tests (***: p<0.0001; **: p<0.001; *: p<0.01; (*) 0.01<p<0.05).

Figure 3. Hornet attraction towards chemical substances.

A. Distribution of the number of visits on each stimulus exhibited by groups of 10 hornets. Bold lines show the median number of visits per session (20 replications, N  = 200 hornets). The boxes represent the first and third quartiles, while whiskers represent the distribution from 10 to 90%. B. Distribution of the cumulative duration that groups of 10 hornets spent on each stimulus. Bold lines show the median duration by session (20 replications, N  = 200 hornets). Grey bars: stimuli; white bars: control. Comparison between odor stimuli and control were done with Wilcoxon tests (***: p<0.0001; **: p<0.001; *: p<0.01; (*) 0.01<p<0.05).