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Lethal and sublethal of dimethoate and λ-cyhalothrin on bumble bee workers in lab and semi-field design

Pollination is one major ecosystem service in decline, while the demand for pollination services is still increasing. Worldwide 87 of 115 of the leading global food crop species are pollinated by insects and around 35 % of the world's harvest depends on pollinators. In Europe 84 % of cultivated species are pollination dependent. One main group among the pollinating insects are bees, including the most prominent honey bee Apis mellifera, the group of solitary bees and bumble bees. Especially the contribution of bumble bees to pollination is often underestimated, although nowadays it is almost impossible to imagine commercial pollination services without bumble bees. They are often used in orchards, greenhouses where natural pollination is restricted and for plants that need special pollination, such as tomatoes.

General bee decline and subsequently decreasing pollination is of rising concern. Main reasons for the high honey and bumble bee mortality in the last years are rising pesticide application, habitat fragmentation/destruction including disappearing field margins, which are important habitats mainly due to farming intensification and infection with parasites. Bumble bees are often considered less threatened by pesticide applications due to their higher weight/surface ratio and therefore higher tolerance to pesticides. At the same time some pesticides (e.g. pyrethroids) are sprayed in high concentrations in the morning or evening which avoids contact with honey bees but bumble bee foraging is likely to coincide with the application due to their robustness to low temperatures.

Aside from mortality, also sublethal effects have a high impact on the economy and ecology of pollination services since they, among others, reduce foraging ability which results on the one hand in less yield (economical importance) and on the other hand species diversity is decreased (ecological importance). Until now studies only demonstrated sublethal effects that influenced bumble bees e. g. reduced foraging activity. The present study aims to investigate possible effects of insecticide use on the pollination efficiency of bumble bees and therefore the effect on quality and quantity of yield. In semi-field design using small tents (ca. 14 m²) Vicia faba will be treated with the insecticides lambda-cyhalothrine or dimethoate and then pollinated by Bombus terrestris micro-colonies for 30 days. The bean pods are then allowed to ripen and are harvested to investigate quality and quantity of the pods.

But knowledge gaps do not only exist for sublethal effects, also lethal effects (LC50) are not assessed very well for bumble bees. Representative species for insects is only the honey bee although it is not possible to extrapolate risk assessment from honey to bumble bees due to differences in potential risks. These differences in risks include higher exposure of bumble bees to pesticides due to their foraging habitats in and around crops as well as their smaller colony size. Also the queen is more likely to be exposed to pesticides when building up her colony which could impact the colony strength. This is the reason why the present study does also investigate the lethal effects of the insecticide Perfekthion (active ingredient: dimethoate) on B. terrestris in a laboratory ring test, where EC50 values will be generated for oral and contact toxicity. For the acute contact test additional treatments will be performed with different wetting agent to test for their performance and for impacts on the toxicity of dimethoate. Performance is here defined as suitability to help the active substance to penetrate the insects thorax quickly. The toxicity of wetting agents is until now only addressed by very few papers while no study was conducted on performance of different wetting agents which makes this topic important to investigate.


Carsten Brühl

Eva Eschenbach