Miticidal activity of fenazaquin and fenpyroximate against Varroa destructor, an ectoparasite of Apis mellifera

Abstract

Background: The Varroa mite (Varroa destructor) is an ectoparasite that can affect the health of honey bees (Apis mellifera) and contributes to the loss of colony productivity. The limited availability of Varroacides with different modes of action in Canada has resulted in the development of chemical resistance in mite populations. Therefore, an urgent need to evaluate new potential miticides that are safe for bees and exhibit high efficacy against Varroa exists. In this study, the acute contact toxicity of 26 active ingredients (19 chemical classes), already available on the market, was evaluated on V. destructor and A. mellifera under laboratory conditions using an apiarium bioassay. In this assay, groups of Varroa-infested worker bees were exposed to different dilutions of candidate compounds. In semi-field trials, Varroa-infested honey bees were randomly treated with four vetted candidate compounds from the apiarium assay in mini-colonies.

Results: Among tested compounds, fenazaquin (quinazoline class) and fenpyroximate (pyrazole class) had higher mite mortality and lower bee mortality over a 24 h exposure period in apiariums. These two compounds, plus spirotetramat and spirodiclofen, were selected for semi-field evaluation based on the findings of the apiarium bioassay trials and previous laboratory studies. Consistent with the apiarium bioassay, semi-field results showed fenazaquin and fenpyroximate had high efficacy (>80%), reducing Varroa abundance by 80% and 68%, respectively.

Conclusion: These findings suggest that fenazaquin would be an effective Varroacide, along with fenpyroximate, which was previously registered for in-hive use as Hivastan. Both compounds have the potential to provide beekeepers with an alternative option for managing Varroa mites in honey bee colonies. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Keywords: Varroa mite; apiarium; fenazaquin; fenpyroximate; honey bee.

Figure 1

Mean (±SE) bee mortality (%) during 24 h exposure to different AIs in apiariums. A group of Varroa‐infested worker bees was exposed to a piece of plastic strip covered with dilutions of AIs (0.05 mg/apiarium, 0.5 mg/apiarium or 5 mg/apiarium) in apiarium cages under laboratory conditions during 24 h. Additional sets of apiariums were treated with different dilutions of amitraz (positive control) or left untreated (negative control). Each bar represents average bee mortality for replicates of each dilution (n = 3) or control treatments (n = 3). Vertical lines on each bar indicate ± standard error (SE). Means with the same letter among dilutions were not significantly different (P > 0.05).

Figure 2

Mean (±SE) Varroa mite mortality (%) during 24 h exposure to different AIs in apiariums. A group of Varroa‐infested worker bees were exposed to a piece of plastic strip covered with dilutions of AIs (0.05 mg/apiarium, 0.5 mg/apiarium or 5 mg/apiarium) in apiarium cages under laboratory conditions during 24 h. Additional sets of apiariums were treated with different dilutions of amitraz (positive control) or left untreated (negative control). Each bar represents average mite mortality for replicates of each dilution (n = 3) or control treatments (n = 3). Vertical lines on each bar indicate ± standard error (SE). Means with the same letter among dilutions were not significantly different (P > 0.05).

Figure 3

The scatter plot of Varroa mite and bee mortality in apiarium assay. Average cumulative Varroa mite mortality (%) was plotted against average cumulative bee mortality (%) in the apiarium assay. The symbols of red circle, blue triangle, and green square show distribution of treatments within each cluster. Each symbol represents one treatment (n = 9). Dashed lines show the area for each cluster.

Figure 4

Daily Varroa mite mortality rate in experimental mini‐colonies. Mean (±SE) daily mite mortality rate (mites/total mites/day) in mini‐colonies were treated with different doses (500 mg/mini‐colony, 1000 mg/mini‐colony or 1500 mg/mini‐colony) of FPs [Envidor (spirodiclofen), Kontos (spirotetramat), Fujimite (fenpyroximate)] or AI (fenazaquin). Additional sets of three mini‐colonies were treated with Apivar (500 mg amitraz per strip, positive control) or left untreated (negative control). Each symbol represents average daily mite mortality for replicates of each dilution (n = 3) or control treatments (n = 3). Vertical lines on each symbol indicate ± standard error (SE). Means with the same letter among dilutions were not significantly different (P > 0.05).

Figure 5

Variability in the rates of daily mite mortality in treated bees. Mean daily mite mortality rate (mites/total mites/day) in mini‐colonies treated with different doses: 500 mg/mini‐colony (solid triangle), 1000 mg/mini‐colony (solid square) or 1500 mg/mini‐colony (solid circle) of the FPs [Envidor (spirodiclofen), Kontos (spirotetramat), Fujimite (Fenpyroximate)] or AI (fenazaquin). Green arrows represent the time points of treatment application. Each symbol (solid square, circle or triangle) indicates the average daily mite mortality rate for all replicates in each dose (n = 3). Each time point of sampling represents the n ^th day of sampling (i.e. first, second, third, and so on).

Figure 6

Daily Varroa mite drop in treated bees. Mean (±SE) daily mite drop (mites/day) in colonies that were exposed to different doses (500 mg/mini‐colony, 1000 mg/mini‐colony or 1500 mg/mini‐colony) of FPs [Envidor (spirodiclofen), Kontos (spirotetramat), and Fujimite (fenpyroximate)] or the AI (fenazaquin). Additional sets of three colonies were treated with Apivar (500 mg amitraz per strip, positive control) or left untreated (negative control). Each symbol represents average mite drop for replicates of each dilution (n = 3) or control treatments (n = 3). Vertical lines on each symbol indicate ± standard error (SE). Means with the same letter among dilutions were not significantly different (P > 0.05).

Figure 7

Changes in mean abundance for Varroa mites in mini‐colonies. Mean (±SE) abundance of Varroa mites (%) in pre‐ and post‐treatment. Varroa‐infested bee colonies were exposed to different doses (500 mg/mini‐colony, 1000 mg/mini‐colony or 1500 mg/mini‐colony) of FPs [Envidor (spirodiclofen), Kontos (spirotetramat), Fujimite (fenpyroximate)] or the AI (fenazaquin). Additional sets of three colonies were treated with Apivar (500 mg amitraz per strip, positive control) or left untreated (negative control). Vertical lines on each point indicate ± standard error (SE). Each point represents the average Varroa mite abundance for each dilution (n = 3) or control treatments (n = 3) at each point of sampling (pre‐ or post‐treatment). The dashed line labelled ET represents the recommended autumn economic threshold (3%) for Varroa treatment. Asterisks indicate a significant reduction in mean abundance of Varroa mites (≤3%) for each dilution (P < 0.05).

Figure 8

Efficacy of tested miticides. Mean (±SE) efficacy (%) of Apivar or different dilutions (500 mg/mini‐colony, 1000 mg/mini‐colony or 1500 mg/mini‐colony) of Envidor (spirodiclofen), Kontos (spirotetramat), Fujimite (fenpyroximate), or fenazaquin. Each bar indicates average efficacy for replicates of each dilution (n = 3) or Apivar (n = 3). Vertical lines on each bar indicate ± standard error (SE). Means with the same letter among dilutions were not significantly different (P > 0.05).