The ectoparasitic mite Varroa destructor is the main sanitary pest of honey bees worldwide (Nazzi and Le Conte, 2016; Noël et al., 2020; Traynor et al., 2020). This mite reproduces in brood cells by feeding on the hemolymph of larvae and pupae, and has a phoretic phase where it feeds on bees fat bodies (Rosenkranz et al., 2010; Nazzi and Le Conte, 2016; Ramsey et al., 2019). In addition to the direct damage caused by V. destructor it also acts as a vector of different RNA viruses and favors their replication by suppressing the bees immune response (Yang and Cox-Foster, 2005; Beaurepaire et al., 2020). The virus mostly associated with V. destructor is the Deformed Wing Virus (DWV) (De Miranda and Genersch, 2010; Wilfert et al., 2016). The infestation of worker pupae by V. destructor decreases honey bees’ longevity (Amdam et al., 2004; Aldea and Bozinovic, 2020), increases the titers of the DWV (De Miranda and Genersch, 2010; Beaurepaire et al., 2020), and suppresses the immune response exposing honey bees to infection by other organisms (Yang and Cox-Foster, 2005; Annoscia et al., 2019). At the colony level, infected colonies reduce their population and productivity (Currie and Gatien, 2006; Emsen et al., 2014) and produce fewer swarms (Fries et al., 2003). In many regions of the world, colonies that do not receive acaricides often collapse (Rosenkranz et al., 2010). The synthetic molecules widely used to control V. destructor are the organophosphate coumaphos, the pyrethroids tau-fluvalinate and flumethrin, and the formamidine amitraz (Rosenkranz et al., 2010). However, mites can develop resistance to all of them and residues can persist in hive products (Elzen et al., 2000; Maggi et al., 2011; Mitton et al., 2022).

Integrated pest management is a practice that is in growing development to address diseases in animals, allowing for a reduction in the use of conventional zootherapeutics. Regarding honey bees colonies, it is considered the most effective approach to mitigate the damage caused by Varroa and minimize the use of acaricides (Imdorf et al., 2003; Noël et al., 2020; Jack and Ellis, 2021). Among the resistance behaviors exhibited by honey bees against Varroa, grooming holds particular significance (reviewed by Pritchard, 2016). Autogrooming is the ability of parasitized bees to dislodge mites by themselves, while allogrooming refers to this behavior with assistance from other bees (Boecking and Spivak, 1999; Pritchard, 2016; Mondet et al., 2020). Since, in a standard Langstroth hive with wooden bottom boards, the 50% of the mites fallen are still alive and can reinfest new bees (Lobb and Martin, 1997), the effectiveness of grooming behavior in controlling Varroa could be further increased if hives are equipped with bottom boards that prevent detached mites from reinfesting the colony (Harbo and Harris, 2004; Araneda and Calzadilla, 2011).

The aim of this study was to compare the effect of different bottom boards in V. destructor population and in colony strength.

Several studies have been conducted to explore alternative methods for Varroa control, aiming to include them in integrated pest management and minimize the use of acaricides (reviewed in van der Steen and Vejsnæs, 2021). However, despite these efforts, maintaining mite populations below the damage threshold for bee colonies remains challenging and costly. This study focused on the potential use of tubular, screen, and sticky bottom boards to limit the growth of mite populations to prevent detached mites from re-establish contact with the bees.

In this study, the tubular bottom boards and the screen bottom boards perform similarly to the conventional bottom boards, failing to significantly control the mite population growth. These results contrast with those found by Araneda and Calzadilla (2011) who reported that the screen bottom board and the tubular bottom board exhibited an effectiveness (ratio between varroas fallen in one month in the evaluated bottom boards and total varroas fallen in the following month after applying an acaricide) of 20% and 16%, respectively. Pettis and Shimanuki (1999) showed that Varroa infestation level did not experience noteworthy reductions in colonies equipped with modified screen bottom boards, which is in line with the results obtained in this study.

In contrast to this research, where no significant differences were found in bee population and brood area, the studies by Harbo and Harris (2004) reports that the introduction of screen bottom board in the colonies led to an increase in brood. Sammataro et al. (2004) found that screen bottom boards are most effective in reducing Varroa infestation when used in colonies with selected hygienic queens. Recently, Liu et al. (2020) conducted a meta-analysis from several studies that included 145 colonies and found that the Varroa population in colonies with screen bottom boards is significantly lower compared to those with conventional bottom boards.

The use of the sticky bottom board seems to be a promising strategy to be implemented in integrated pest management programs, since it allowed to limit Varroa infestation levels by nearly 50% in comparison to the conventional boards. Pettis and Shimanuki (1999) also evaluated sticky bottom boards, but only obtained a 28% reduction in mite infestation in an intermediate sampling, without preventing infection levels above the economic threshold. However, the limitations of these sticky bottom boards must be considered since in December the colonies reached a significant level of infestation (6%), enough to begin to affect them.

The difference in effectiveness in limiting Varroa growth between the three analyzed bottom boards could be due to the percentage of surface area where the detached mites would exit the hive. In tubular bottom board this space is approximately 10% of the surface, so many of the detached mites will fall on the tubes and might reinfest the bees. The sticky and screen bottom boards have a 3 mm mesh where the wires occupy a smaller proportion of the surface. In this case the difference between both bottom boards may not lie in the use of an adhesive to retain the fallen mites, but in the surface of the mesh, which in the screen bottom boards is approximately 60% of that of the sticky bottom boards. The results obtained in colonies with screen bottom boards in the last sampling, where Varroa infestation does not increase, is striking. Perhaps with a high mite load the differences between the screen and the sticky bottom boards will be reduced. These factors justify future studies to optimize the design of the devices.

This study shows significant differences in Varroa infestation levels, particularly when mite populations were high. A decline in colony strength parameters (bee and brood population) was observed across all groups in comparison to the treated group of colonies. A phenomenon resembling the “Parasitic Mite Syndrome,” characterized by scattered brood and bee loss as described by Rosenkranz et al. (2010), was evident in certain colonies. It is conceivable that if the trial had extended over a longer period, many colonies would have eventually depopulate and collapse due to this syndrome as Varroa infestation increased.

Two of the evaluated bottom boards, the tubular and the screen bottom boards, allow the entry of air from the environment, which could lead to higher consumption of honey reserves to maintain the appropriate temperature in the brood nest during the autumn and winter. However, this phenomenon was not observed, so the use of these bottom boards does not imply a cost in honey reserves. The greater consumption of reserves of the colonies treated with acaricides with conventional bottom boards in relation to the other colonies is an unexpected result that it is difficult to explain since this group of colonies did not differ from the others in bee population and brood area. On the other hand, the colonies of the different groups did not differ in honey production. Therefore, the use of the three bottom boards evaluated does not negatively affect the consumption of reserves or honey production.

Previous studies have shown that mite-resistant bee populations exhibit higher grooming and hygienic behaviors compared to mite-susceptible colonies (Guzman-Novoa et al., 2012; Hunt et al., 2016; Locke, 2016; Mendoza et al., 2020; Russo et al., 2020; Mendoza et al., 2022). The benefit of these bottom boards, especially the sticky ones, can be enhanced when populations of bees selected for high grooming behavior or Varroa resistance characteristics are obtained. The combination of these strategies can be synergistic, thus achieving effective mite control with minimal acaricide use. Therefore, further investigation of these designed bottom boards is needed to assess their effectiveness under such conditions.

In conclusion this study shows a significant effectiveness of the sticky bottom board in limiting Varroa population growth, particularly when mite infestation levels are high. The use of an adhesive to retain detached mites offers a practical and promising approach to managing high Varroa levels and complement other strategies for the mite control in integrated pest management programs.

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

The manuscript presents research on animals that do not require ethical approval for their study.

PJ: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing. EN: Conceptualization, Investigation, Validation, Visualization, Writing – review & editing. JA: Investigation, Methodology, Validation, Visualization, Writing – review & editing. VR-B: Investigation, Methodology, Validation, Visualization, Writing – review & editing. BB: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Validation, Visualization, Writing – original draft, Writing – review & editing. CI: Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing.