Why timing matters: Seasonal performance of Formic Acid against Tropilaelaps and Varroa
The spread of Tropilaelaps mercedesae into previously mite-free regions, including its recent establishment in mainland Europe, represents a significant and emerging threat to beekeeping. Unlike Varroa destructor, Tropilaelaps reproduces more rapidly, causes greater damage to developing brood, and spends very little time on adult bees. Instead, it moves quickly from one brood cycle to the next, invading cells just before capping. This behaviour makes it far less exposed to many conventional treatments that primarily target mites on adult bees.
In newly affected regions, traditional Varroa control methods, including natural brood breaks and commonly used miticides, are proving insufficient. While brood interruption can suppress Tropilaelaps, it is labour-intensive and often impractical under commercial conditions. As a result, there is an urgent need for treatments that can reliably target mites within sealed brood.
Formic acid is unique in this respect. As a vapour, it can penetrate wax cappings and reach mites inside brood cells, making it particularly well suited to controlling Tropilaelaps. In this study, conducted in western Georgia under late summer and autumn conditions representative of temperate beekeeping, two formic acid treatments - Formic Pro® and Muraviinka® - were evaluated in Apis mellifera colonies managed in single modified Dadant hives. Mite mortality, brood infestation, brood-stage survival and colony strength were assessed.
Both treatments delivered rapid and near-complete control of Tropilaelaps mercedesae in both seasons, reducing brood infestation to near-zero levels. Formic Pro® achieved complete mortality more quickly, reaching this point within approximately 2.5 days in August, while Muraviinka® required between 2.5 and 5 days. In October, both treatments achieved near-complete mortality within 2.5 days.
For Varroa destructor, the response was more variable. While mite mortality increased significantly, control remained incomplete and strongly dependent on season, with significant effects observed only in October. Brood-stage analysis showed higher survival of Varroa in younger brood, particularly following Muraviinka® treatment, whereas Tropilaelaps survival was not influenced by brood age.
Short-term exposure to formic acid had no overall negative effect on colony strength, although adverse effects were observed in some heavily Varroa-infested colonies. It is also important to consider that reinfestation from surrounding colonies can occur rapidly, which may mask true treatment efficacy if assessments are not carried out soon after application.
These findings confirm that formic acid is highly effective against Tropilaelaps mercedesae under conditions typical of temperate apiculture, but provides more variable control of Varroa destructor. Efficacy is closely linked to timing, environmental conditions, and colony status, meaning treatments must be applied carefully as part of a broader integrated pest management strategy. As Tropilaelaps continues to spread, effective control will depend not only on the treatment used, but on a clear understanding of mite biology and precise application.