An IPM Series: How Reduced Pesticide Efficacy Occurs

Thursday 12 October 2023

Chemical treatments have an important role in integrated pest management (IPM) for treating various pests and diseases of honey bees. In Atlantic Canada, beekeepers need chemical treatments to manage Varroa mites, Nosema (Vairimorpha) and brood diseases. However, there is an ongoing reliance on synthetic treatments, such as Apivar®, when managing these pests. This week’s blog will explain the impact of misusing synthetic treatments, and the benefits of an IPM approach to treatment.

An IPM Series: How Reduced Pesticide Efficacy Occurs

Apivar® (3.3% amitraz) is a synthetic miticide widely used to manage Varroa mites. Currently, Apivar® is the only recommended synthetic miticide for Varroa mites in Atlantic Canada. There were other products available with active ingredients fluvalinate (Apistan®) and coumaphos (Checkmite+®), but, due to extensive and exclusive use of these products, widespread reduced efficacy occurred (Rinkevich, 2020). That is why it is essential to maintain the efficacy of Apivar® by only using the treatment when the population level is above the economic threshold, and alternating treatment of Apivar® with other non-synthetic treatments.

Reduced pesticide efficacy is a phenomenon by which organisms can survive higher doses or concentrations of a toxic substance which previously resulted in high levels of mortality (Van Leeuwen and Dermauw, 2016; Whalon et al. 2008). There are various mechanisms of reduced efficacy, including: enhanced detoxification (Field et al. 2001), target-site insensitivity (Rinkevich et al. 2013; Fournier, 2005; González-Cabrera et al. 2013), and reduced cuticular penetration (Balabanidou et al. 2018). The mechanism of reduced efficacy to pyrethroids (Apistan®) in Varroa destructor is well known. It is associated with mutations at the residue L925 of the major target site for pyrethroids—the voltage-gated sodium channel (González-Cabrera et al. 2018; González-Cabrera et al. 2016; Hubert et al. 2014). On the other hand, the precise mechanisms causing the reduced efficacy to coumaphos, and potentially amitraz, in V. destructor are still unknown (Maggi et al. 2009; Maggi et al. 2011). Overall, it is possible that reduced efficacy can develop because synthetic products persist at high concentrations in wax and represent a constant exposure (Traynor et al. 2016).

Amitraz has been used to control Varroa mite populations for more than 20 years in the USA. Since that time there have been many reported incidents of reduced efficacy of amitraz (Elzen, 2000; Maggi, 2010; Kamler, 2016). However, Varroa has largely maintained susceptibility to amitraz despite a long and extensive use history (Rinkevich, 2020). Since amitraz is currently the only synthetic active ingredient still viable to treat Varroa mites, it is important to continually test the efficacy of amitraz against mite populations worldwide.

Since 2017, the ATTTA team has tested the efficacy of Apivar® against Varroa mites in the Maritimes through field-based studies. Although testing the efficacy of miticide products is important, it is also crucial to test the efficacy of active ingredients through lab-based studies. This past summer, ATTTA tested the efficacy of amitraz against populations of mites in Nova Scotia and New Brunswick. ATTTA previously had no baseline data on the efficacy of amitraz in Atlantic Canada by use of these methods, which made this research even more important. Mites were collected by using drone frame, where for each apiary, 1 drone frame was placed in 3 different hives. A total of 6 beekeepers were included in the study, and data was collected from 206 mites in total. The amitraz bioassay was adapted from a study done by Rinkevich (2020). This bioassay tested the efficacy of amitraz at 5 different concentrations to determine the lethal concentration of 50% of the mites. Mites were exposed to the treatment for 24hrs, and then mortality was assessed. After calculating the LC50, a resistance ratio was calculated by comparing the Maritime population to an amitraz-sensitive USDA Lab population.

Setting up amitraz bioassay (ATTTA©2023).

The results of the baseline 2023 data indicate that there is a reduced efficacy of amitraz for the small number of mites tested (n = 206; RR = 35.4). However, this was the first year for this study, and multiple factors could impact the results. Overall, the study was conducted on a small number of mites, from only 6 beekeepers, and there was not a consistent sample size for each concentration of amitraz tested. ATTTA plans to continue to assess how the efficacy of amitraz changes year to year, and the goal is to test a larger number of mites from more Maritime beekeepers. All the Tech Transfer Teams in Canada, supported by the Canadian Association for Professional Apiculturists, are working towards a standardized approach to assessing reduced efficacy of amitraz and Apivar®.

It is critically important to extend the effectiveness of amitraz-based products to control Varroa mites by practicing IPM. Past use of synthetic products has demonstrated that extensive and exclusive use of synthetic products results in reduced efficacy. There are also many reported incidents of reduced efficacy of amitraz in the global beekeeping industry. Therefore, it is the job of all beekeepers to manage Varroa mites using an IPM plan. This includes testing for Varroa mites at least monthly (pre- and post-treatment), and only treating when levels are above the economic threshold. Beekeepers should also implement cultural and physical controls to reduce the need for chemical treatment. Finally, when chemical treatment is warranted, beekeepers should alternate between synthetic and non-synthetic products.


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