Evaluation of the predatory mite Stratiolaelaps scimitus’s efficiency as a biological control agent against Varroa destructor

 

Biologie  |  Environnement

 

Flavien Flückiger, 2006 | Travers, NE

 

Alarming devastation of managed Apis mellifera colonies continues to persist, thus giving reason to finding viable solutions. Varroa destructor, an ectoparasite commonly known as varroa mite, indeed is an important factor in honeybee health decline. Although miticide treatments are used to combat varroa mite, they are increasingly problematic. Therefore, understanding the efficiency of an alternative bio-control agent, Stratiolaelaps scimitus, is of interest. Here, using S. scimitus, we looked for the most suitable substrate for breeding beneath the hives and a berry-orchid potting soil mixture yielded the best results. Furthermore, despite the constant presence of the predatory mite, we observed an increase in the number of varroa mites per hive. However, we were unable to distinguish the injuries caused to varroa mites by S. scimitus from those caused by bees themselves, suggesting that the method requires adaptations, leaving us with doubts about the efficacy of S. scimitus. Lastly, the data provide useful insight of S. scimitus as a biological tool against V. destructor while casting doubt about the current effectiveness and we suggest optimizing several parameters to achieve favorable results in future studies.

Problématique

In the last decade, heightened numbers of bee colonies losses have been reported (i.e. approx. 40%). Indeed, a shocking reality, and one with numerous consequences. Varroa mites attack and weaken honeybees, thus emphasizing the need to find an effective method for beekeepers. Miticides, such as formic and oxalic acid, are used, but they pose problems (e.g. temperature effectiveness threshold of 30°C). Consequently, we sought to test a method using S. scimitus as a control agent. First, an optimal substrate for breeding S. scimitus needed to be established. Second, we had to define the characteristic wounds made by S. scimitus on varroa mites. Thirdly, we wanted to confirm the method in real-hive scenarios.

Méthodologie

Previous research with S. scimitus sprinkled them directly onto frames (Rondeau, 2018), we however placed them underneath the hives in an adapted substrate to maintain a perennial population. First, to find the substrate, we compared their populations by extracting specimens from samples using a Macfadyen machine. Then, in a laboratory, we placed individuals from each species in boxes to represent all the potential interactions. Post-introduction, we identified the characteristics of the wounds, particularly those of S. scimitus on varroa mites, under a binocular magnifier. Finally, we tried out the method with hives to observe and count the varroa falls and monitor the evolution of predatory mite populations.

Résultats

A mixture of two-thirds berry potting soil and one-third orchid potting soil was the best mixture for the S. scimitus breeding, reaching 48.1 individuals/gram of substrate. On the other hand, no wounds were found on any specimens in the laboratory boxes. In the field experiments, despite the presence of S. scimitus, we observed an increase in the varroa numbers, and in some hives, up to 15 varroa mites fell per day. Additionally, the most common of all the wounds we noticed were missing and/or partially missing legs. Finally, we confirmed the continuous presence of S. scimitus in all hives.

Discussion

Here, we can validate the substrate determination method, and attest to the presence of substantial population increases of S. scimitus post-substrate manipulation, a positive point to underscore, as more predators are likely to go into varroa mites infested hives. Secondly, we can’t conclude anything about the interactions, but we might get different results if we used hungry S. scimitus specimens who would attack to feed. Thirdly, the increase in the amount of varroa in hives leaves us to question the effectiveness of the method. Although we observed wounds on the varroa mites collected from the hives, we were unable to deduce their origin, as we didn’t manage to characterize the types which are committed by S. scimitus in the laboratory. Lastly, the continued presence of S. scimitus in the hives shows that there have always been predator attacking the varroa mites.

Conclusions

To summarize, we report a viable substrate for S. scimitus breeding. However, the damage assessment method needs to be modified, particularly to distinguish the kinds of injuries sustained by either predatory mites or bees themselves. Although our results are inconclusive for the test in field conditions, they do not precisely define the effectiveness of this biological control method, thus leaving the door for the potential of S. scimitus open.

 

 

Appréciation de l’expert

Dr. Andrew Brown

Cette étude nous plonge dans une problématique à laquelle font face les apiculteurs d’aujourd’hui. Par son travail, Flavien a testé l’utilisation d’un agent de contrôle biologique : Stratiolaelaps scimitus, afin de combattre une espèce nuisible pour les abeilles. Méticuleux et systématique, Flavien a démontré quel était le substrat d’élevage optimal pour S. scimitus, tout en testant avec succès ses propres méthodes dans des conditions réelles pour évaluer l’efficacité de ce traitement. Ce travail de maturité parvient à articuler et mettre en œuvre une étude très intéressante dans le but d’assurer l’avenir des abeilles.

Mention:

très bien

Prix spécial «International Summer Science Institute (ISSI)» décerné par l’Institut Weizmann

 

 

 

Lycée Denis-de-Rougemont, Neuchâtel
Enseignant: Prof. Alexandre Aebi