Feeding by Tropilaelaps mercedesae on pre- and post-capped brood increases damages to Apis mellifera colonies

Authors: PHOKASEM, PATCHARIN - Chiang Mai University; De Guzman, Lilia; KHONGPHINITBUNJONG, KITIPHONG - Mae Fah Luan University; Frake, Amanda; CHANTAWANNAKUL1, PANUWAN - Chiang Mai University

Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2019
Publication Date: 9/10/2019
Citation: Phokasem, P., De Guzman, L.I., Khongphinitbunjong, K., Frake, A.M., Chantawannakul1, P. 2019. Feeding by Tropilaelaps mercedesae on pre- and post-capped brood increases damages to Apis mellifera colonies. Scientific Reports. 9:01-12.
DOI: https://doi.org/10.1038/s41598-019-49662-4

Interpretive Summary: Managed European honey bee colonies in Asia are threatened with tropilaelaps mites more so than varroa mites. We examined the types of injury inflicted by Tropilaelaps mercedesae on different stages of Apis mellifera in Northern Thailand. Here, we report for the first time that these mites feed on unsealed larval stages of honey bees. While varroa mites inflict 1-2 large wounds only, tropilaelaps mites cause multiple wounds on honey bee hosts. Phoretic tropilaelaps mites are known to have short survival. Thus, the ability of tropilaelaps mites to feed on unsealed brood may increase their survival when no suitable host for infestation is available. Transmission of mite-borne virus such as Deformed wing virus (DWV) was also enhanced by mites’ feeding on early larval stages. DWV and Black queen cell virus were detected in all 4th instar larvae and PP analysed. Because of the high mobility of tropilaelaps mites, a single mite can repeatedly feed on an unsealed larva or feed on multiple larvae. Hence, unsealed larvae may serve as replication and transmission hub for mite-borne viruses. Tropilaelaps mites also feed indiscriminately on developing bees inflicting significant injuries on the abdomen, wings, legs, mouthparts and antennae. These feeding injuries were also evident on adult bees. Feeding injuries on unsealed brood may accelerate death of bees infested with tropilaelaps mites while developing into adulthood.

Technical Abstract: Loss of honey bee colonies or reduced colony productivity is the costliest economic effect of parasitic mites. In much of Asia, Tropilaelaps mercedesae is the major cause of Apis mellifera colony mortality more so than Varroa destructor. Tropilaelaps mites have the reproductive advantage over varroa mites due to the successful reproduction by unmated, young females even without undergoing a phoretic phase. Here, we report for the first time that tropilaelaps mites fed on both pre- and post-capped stages of honey bees. These mites are short-lived on adult bees. However, in Asia where brood production is year-round, feeding on pre-capped brood may extend their survival outside capped brood cells. Among pre-capped stages, 4th instar larvae (L4) sustained the highest number of wounds (4.6 ± 0.5) while 2nd-3rd larval instars had at least two wounds. Consequently, wounds were evident on uninfested capped brood [5th-6th larval instars (L5-L6) = 3.91±0.64 wounds; prepupae (PP) = 5.25±0.73 wounds]. Tropilaelaps mite infestations resulted in 6- and 3.4-fold increases in the number of wounds in PP and L5-L6, respectively. When wound-inflicted PP metamorphosed to white-eyed pupae, all wound scars disappeared with the exuviae, decreasing the number of wounds to =10 on all pupal stages. Transmission of mite-borne virus such as Deformed wing virus (DWV) was also enhanced by mites’ feeding on early larval stages. DWV and Black queen cell virus were detected in all L4 and PP analysed. However, viral levels were more pronounced in scarred L4 and infested PP. Elevated DWV levels in L4 and mite-free PP without wound scars suggest the presence of fresh wounds incurred before cells were capped. About 98% and 94% of stained L4 and PP samples had fresh wounds, respectively. The wide variation in DWV levels of the infested PP may indicate that a majority of them were repeat hosts incurring both pre-and post-capped injuries. The remarkably high numbers of wounds and viral load on scarred or infested developing bees may have caused significant weight loss and extensive injuries observed on adult bees, which include the abdomen and different organs such as wings, legs, proboscis and antennae that seat important sensory organs. Together, the survival of infested bees was significantly compromised. This study demonstrates the ability of tropilaelaps mites to inflict profound damages on A. mellifera hosts. Effective management approaches need to be developed to mitigate tropilaelaps mite problems.