Abstract
Oxalic acid dihydrate is used to treat varroosis of Apis mellifera. This study investigates lethal and sublethal effects of oxalic acid dihydrate on individually treated honeybees kept in cages under laboratory conditions as well as the distribution in the colony. After oral application, bee mortality occurred at relatively low concentrations (No Observed Adverse Effect Level (NOAEL) 50 µg/bee; Lowest Observed Adverse Effect Level (LOAEL) 75 µg/bee) compared to the dermal treatment (NOAEL 212.5 µg/bee; LOAEL 250 µg/bee). The dosage used in regular treatment via dermal application (circa 175 µg/bee) is below the LOAEL, referring to mortality derived in the laboratory. However, the treatment with oxalic acid dihydrate caused sublethal effects: This could be demonstrated in an increased responsiveness to water, decreased longevity and a reduction in pH-values in the digestive system and the hemolymph. The shift towards stronger acidity after treatment confirms that damage to the epithelial tissue and organs is likely to be caused by hyperacidity. The distribution of oxalic acid dihydrate within a colony was shown by macro-computed tomography; it was rapid and consistent. The increased density of the individual bee was continuous for at least 14 days after the treatment indicating the presence of oxalic acid dihydrate in the hive even long after a treatment.
Highlights
Oxalic acid dihydrate (OAD) is one of the most important organic acids used for the control of Varroa destructor
Our aim was to define the no observed adverse effect level (NOAEL) and the lowest observed adverse effect levels (LOAEL) for OAD, including a safety margin for the dosage used in practical beekeeping
The described LD10 (48 h) of 176.68 μg/bee was derived from a combinatory effect of OAD and is much more toxic to bees than application on the cuticula, which was relatively well tolerated
Summary
Oxalic acid dihydrate (OAD) is one of the most important organic acids used for the control of Varroa destructor. The European Group for Integrated Varroa Control developed OAD for the final application stage in beekeeping [2,3]. Three different application methods of OAD exist: trickling, spraying and evaporation. There are principal points to be considered concerning the medical treatment of honey bee colonies: the tolerability of the ingredient to bees, and the toxicity to mites, as well as its distribution in the colony, which again is directly affects the toxicity and efficacy of a substance. The trickling method of OAD combines high efficacy against V. destructor and low bee mortality. The tolerability in the bee colony has been documented in a concentration of 3.5% (w/v) OAD and a dose of 30–50 mL per colony for Central Europe [4]. Approval as a veterinary drug has been given in many countries worldwide over recent years, for Germany in 2006 [5]
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