Pesticides in Beeswax: How Toxic Are They to Honey Bee Larvae?

Chemosphere, Article Number 132214

Pesticides accumulate in beeswax and pose a risk to the development of bee larvae. In an in vitro study, Agroscope evaluated the toxicity of pesticides in beeswax

Losses in honey bee (Apis mellifera) colonies are associated with many causes. Scarcity or insufficient quality of food, parasites and brood diseases, the quality of the queen and pesticides may all be contributory factors. The Varroa destructor mite and the viruses associated with it represent a major threat to honey bees. To control the parasite, beekeepers use veterinary medicinal products (VMPs) that can accumulate in the hive. In addition, bees can bring pesticides from agriculture into the colonies along with foraged pollen and nectar. Lipophilic pesticides – the class to which many VMPs and plant-protection products (PPPs) belong – accumulate mainly in the beeswax. One example is coumaphos – a pesticide authorised in some countries as a VMP for varroa mite control, or as a PPP. 

Pesticides in recycled beeswax

According to good beekeeping practice, the wax combs are replaced on a regular basis. Beekeepers melt the old combs to create new wax foundation sheets, which they place into the hives as templates for the bees to build new combs. During the recycling process, contaminants such as coumaphos remain in the wax and hence are also contained in the newly manufactured wax foundations. This can have an adverse effect on the health of the honey bees.

In vitro study confirms toxicity of pesticides in wax

Although there are many studies on pesticides in beeswax, little is known about what concentrations pose a risk to honey bee development. Agroscope therefore conducted an in vitro study to determine the risk of pesticides in the wax to honey bee larvae. Coumaphos – a pesticide very frequently found in beeswax – served as the model substance.

In an initial test series, we examined the emergence rate of bees exposed at the larval stage to contaminated beeswax. Coumaphos concentrations in beeswax of up to 20 mg/kg showed no effect on the emergence rate. This was comparable to the control. With higher concentrations of coumaphos (30 to100 mg/kg), however, lower emergence rates were observed. Coumaphos concentrations of 30 mg/kg and upwards in the beeswax therefore have an adverse effect on the development of honey bee larvae.

Pesticides migrate from wax to larval jelly

In a second step, we tested whether coumaphos can pass from the beeswax to an artificial larval diet, and which concentrations are toxic to the larvae. Coumaphos migrated from the beeswax into the diet to approximately one fifth of the initial concentration in wax. Additional test series showed that coumaphos had a five times more toxic effect on the bee larva when supplemented into the diet as compared to exposure through wax. The reduced emergence rates at coumaphos levels in the wax of 30 mg/kg and upwards can therefore be explained by the oral exposure of the larvae.

Lower residue levels in Swiss beeswax

The latest analyses of Swiss beeswax show that residues due to varroatreatments are lower than in previous years. Nowadays, Swiss beekeepers mainly use organic acids for mite control that do not accumulate in the beeswax. This reduces the exposure of honey bee larvae to lipophilic varroatreatment agents.