Bees are more than honey-makers. They pollinate crops and hence are key elements in our food production. Honeybees, wild bee species such as bumblebees and solitary bees, butterflies, wasps, and flies, all provide an invaluable work of pollination. In fact, a third of the food that we eat depends on them! Nowadays beekeepers around the world continue to observe the mysterious disappearance of bees as well as declines in honeybee colony sizes. The use of insecticides in agriculture could be one of the main factors responsible for it.

Neonicotinoids are pesticides widely used to coat the seed and make it resistant to attack from insects in the soil. However, these pesticides may represent a consistent risk to bees because, after seed germination, the compound accumulates in the stem, flowers, and leaves of the plant, from where it can be transferred to bees. To protect bee populations, the European Union prohibited the use of three neonicotinoids (read also the Break: A dangerous habit: bees prefer pesticide-contaminated nectar). Pesticide-users (e.g.farmers and agricultural stakeholders) criticized the measure, claiming that the effects of the insecticide have mostly been measured on bees artificially fed with it and that, the evidence for neonicotinoids representing a menace for bees in real fields, is weak.

Thus, the key question is: do neonicotinoids influence honeybees and wild bees in real-world agricultural landscapes?

A Swedish research team decided to take on the challenge and investigate the effects of neonicotinoids under natural field conditions. The study was done in 16 independent landscapes, surrounding 16 oilseed rape fields. The 16 landscapes were grouped by ecological similarity, resulting in 8 groups of two. For each pair, one of the fields was treated with a fungicide that is harmless to bees (as a control), and the other with the same fungicide plus a product containing neonicotinoids.

Researchers measured: a) The density of wild bees (amount of wild bees per unit of area), b) the nest building activity of a solitary wild bee species, c) The colony development capacity of the wild but social bumblebee, and finally d) the colony strength (number of adult bees) of the European domesticated honeybee.

The team found that wild bee density was lower in fields sown with neonicotinoid-coated seeds, demonstrating that the presence of neonicotinoids compounds has a negative effect in the number of these bee species.

To analyze the nesting capacity of solitary bees, scientists distributed one trap nest with 27 bees in each field and monitored their nest building activity. Solitary bees in normal conditions each build their own nest. Consistently, bees living in insecticide-free fields did so, whereas bees did not build new nests in any of the eight fields treated with the pesticide, thus evidencing again the harmful effect of neonicotinoids.

The colony development capacity of bumblebees was also negatively affected by neonicotinoids. Bumblebees are social bees and form colonies of one queen and tens or hundreds of workers. During their development, bumblebee colonies grow in weight and worker force to then switch to producing new queens and males. This change in working pattern results in a decline in colony weight. For this study, six commercially reared colonies were deposited in each field. Bumblebee colonies at control fields grew in weight and worker force as it was expected while the rate of weight gain of colonies at fields with insecticide-coated seeds was lower.

In addition, it is known that bumblebees have an annual life cycle where the new queens born at the end of the season hibernate and form new colonies the following spring. At the end of this experiment, fewer queen and worker/male cocoons were produced at fields treated with neonicotinoids compared to control ones, which will result in less new colonies the following spring.

The last finding was perhaps the most surprising. Neonicotinoid seed treatment did not affect the colony strength of the domesticated honeybees. These results are in line with a previous study reporting that honeybees seem to be better at detoxifying after neonicotinoid exposure compared to bumblebees. Yet, the possibility of neonicotinoids having negative long-term effects in honeybees was not studied here and hence cannot be excluded.

In summary, the Swedish researchers succeeded in demonstrating that the use of neonicotinoids as a seed coating treatment in oilseed rape farming represents indeed a risk for wild bees in real world landscapes. Concomitantly, they question the use of domesticated honeybees as representatives for wild bees in pesticide toxicity experiments. And lastly, the authors also underline how studies performed in laboratory conditions could not be precise enough in realistically predicting the effects of pesticides. They propose studies of communities and ecosystems as an alternative to predict the real-world consequences of pesticides.

Next read: Honeydew: the sweet that can become toxic by Miguel Calvo-Agudo , Alejandro Tena

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