A Hitchhiker's Ride: The Honey Bee Louse Braula Coeca (Diptera: Braulidae) Selects its Host by Eavesdropping

Abstract

The bee louse Braula spp. had until recently a distribution coincident with its host the honey bee. The adult fly usually attaches to a worker honey bee and steals food from its mouth. However, not all worker bees carry Braula spp. and the mechanism used by Braula spp. to select hosts is not well understood. Using choice remounting bioassays and chemical analyses, we determined host selection and the cues used by B. coeca, a species associated with the African honey bee Apis mellifera scutellata. Braula coeca successfully remounted bees from which they were initially removed and preferred their mandibular gland pheromones (MDG) over those of bees not carrying them. The bee lice did not show any preference for the cuticular hydrocarbons of both types of workers. Chemical analyses of the MDG extracts, revealed quantitative differences between the two categories of workers, with workers carrying B. coeca having more of the queen substance (9-oxo-2(E)-decenoic acid) and worker substance (10-hydroxy-2(E)-decenoic). Braula coeca showed a dose response to the queen substance, indicating its ability to use host derived kairomones as cues that allowed it to benefit from trophallactic dominance by individuals that have a higher probability of being fed by other workers.

Keywords: Cuticular hydrocarbons; Honey bee parasite; Kairomones; Mandibular gland secretions; Trophallaxis.

Fig. 1

Steps involved in the sampling of honey bee workers carrying or not carrying the bee lice Braula coeca from honey bee hives and how they were kept in the laboratory prior to bioassays. Collection (A), separating bees carrying and those not carrying bee louse (B), removal of bee lice on bees (C) and placing into rearing Eppendorf containing moist cotton wool (D) and maintaining bees and bee lice in incubators (E) before bioassays. Broken vertical lines indicates the different sampling steps followed. Illustration created in https://biorender.com

Fig. 2

Petri dish bioassay arena set-up used to determine how Braula coeca choses its host between worker bees that carried the lice and those that did not carry it (A) when collected from the hive. A similar set-up (B) was used to test the different odor sources used by B. coeca for host selection. Middle line was a line drawn on the bottom of the Petri dish. Illustration made in https://biorender.com

Fig. 3

Remounting of honey bee workers previously carrying (HBr) and those that did not carry (HB) the bee lice Braula coeca (A). Open bars represent successful remounting while checked bars represent unsuccessful mounting. Preferences (B) of B. coeca to extracts (odors) of Mandibular glands (MDG) open bars, cuticular hydrocarbons (CHCs) from HBr blue, CHCs from HB (orange), and solvent controls (checked bars). DCM = dichloromethane, NR = non-responding B. coeca and P = p values. Numbers inside the bars represent the B. coeca that responded to the treatment

Fig. 4

A Amount of mandibular gland pheromones 9-oxo-2(E)-decenoic acid (9-ODA), 9-hydroxy-2(E)-decenoic acid (9HDA), 10-hydroxy decanoic acid (10-HDAA) and 10-hydroxy-2(E)-decenoic(10-HDA) in µg from head extracts of honey bee workers carrying Braula (open bars) and those not carrying (checked bars). B Total Ion Chromatogram (TIC) of Cuticular Hydrocarbons (CHC, 1 = Eicosanol, 2 = n-Tricosene, 3 = n-Pentacosane, 4 = n-Hexacosane, 5 = n-Heptacosane, 6 = Hentriacontane, 7 = n-Dotriacontane, 8 = Pentatriacontene) from Braula coeca (black), honey bee not carrying B. coeca (pink), honey bee carrying B. coeca (blue) and hexane blank (brown), Fig. 4C The amount of CHCs from Braula, honey bee and honey bee carrying B. coeca

Fig. 5

Responses of Braula coeca to different doses of the queen substance 9-ODA (A) and worker substance 10-HDA (B). Checked bars represent solvent control, open bars represent 9-ODA and brown bars, 10-HDA. Numbers in the bars represent individual B. coeca responding to the test odors