Are Bees Victims of Their Intelligence?
During foraging, honey bees exhibit remarkable cognitive abilities. However, pesticides and heavy metals disrupt neuronal communication, impair foraging behaviour, and ultimately place the entire colony at risk.
Since 1 September 2018, French farmers have no longer been authorised to use the main neonicotinoids in their crops. This represents good news for bees and all other pollinating insects, both wild and managed, as these so-called “bee-killing” pesticides have been implicated for decades in the global pollinator crisis.
Despite this step forward, the survival of bees (in the broad sense, and not limited to honey bees) is far from guaranteed. On the one hand, new generations of pesticides (1), which are already replacing neonicotinoids in several countries, appear to cause similar damage. On the other hand, it is now well established that it is the combination of multiple stressors, rather than a single factor, that threatens bees: pesticides, the decline of wild flowering plants, certain pollutants, the emergence of new parasites and pathogens, the introduction of invasive predators, habitat destruction, and so forth.
But why exactly are bees at risk, and how can they be protected? A close analysis of their behaviour reveals that their lifestyle requires particularly advanced, yet fragile, cognitive abilities, which may represent a major source of their vulnerability to environmental stressors. These different stressors affect brain development and function, thereby impairing the bees’ ability to perform a range of cognitive tasks essential for foraging and, consequently, for survival.
Small brain, remarkable achievements
By foraging from flower to flower to collect pollen and nectar, bees contribute to the reproduction of plants that produce the fruits and vegetables consumed by humans. This long co-evolution between plants and insects has constrained bees to adopt a specific lifestyle based on detailed knowledge of their environment. Foragers exploit fragmented resources that may be distributed over several hundred metres or even several kilometres around the nest (the hive in the case of honey bees). Unlike animal species that lack a fixed nest or rely on locally abundant resources (such as grasshoppers), bees depend for their survival on sophisticated cognitive abilities that enable them to locate flowering plants around the nest and return the collected food to the colony.
These abilities are all the more impressive given the small size of the bee brain (approximately one million neurons within a pinhead-sized structure, compared with about 100 billion neurons in humans). To forage efficiently, bees must first acquire visual knowledge of their environment and form spatial memories in order to orient correctly and find their way back to the nest. They must then learn and memorise the shape, colour, odour, texture, and even the electromagnetic signature of nectar-producing flowers. Even more intriguingly, bees are capable of developing efficient routes linking food sites and the nest by selecting the shortest path. Finally, the well-known dance by which honey bees communicate the quality and location of floral resources to their nestmates should be recalled. All of these abilities rely on neuronal processes that are both highly sophisticated and fragile, and whose proper functioning allows foragers to fulfil their role and ensure the survival of the colony or, in the case of solitary species, that of their offspring.
When the environment targets neurons
Several factors implicated in bee decline have recently been identified for their direct effects on bee brain function and behaviour. This is the case for parasites such as the mite Varroa or the fungus Nosema, which act by modifying the expression of specific genes in the brain. Pesticides and heavy metals (originating from industrial pollution) disrupt neuronal communication and activity levels. Given that the brain can function efficiently only if a certain level of activity is maintained, the link between these cellular effects and impaired foraging behaviour is unavoidable.
Bees may also suffer from nutritional deficiencies when they inhabit environments with reduced floral diversity, such as large monoculture landscapes. The brain of a nutritionally deprived bee larva does not develop optimally, which later reduces foraging efficiency. In adults, a deficiency in essential fatty acids (such as omega-3) prevents the olfactory learning required for discriminating between plant species. Such effects can trigger a cascade leading to food shortages for the entire colony and ultimately to colony collapse.
What can be done to protect our pollinators?
Public research, by assembling irrefutable evidence of toxicity, has played a major role in the exclusion of neonicotinoids from the market, 25 years after the first warnings issued by beekeepers. Our responsibility as specialists in insect behaviour is to identify additional sources of stress and their combined effects on the brain and cognition of different bee species, particularly wild bees that are vital for the pollination of local plants. A comparative approach is required to identify the species most exposed and most sensitive to stressors. Such studies should make it possible to define tolerance thresholds for combinations of stress factors and potentially to identify protective strategies, such as nutritional supplementation or well-defined combinations of flowering plants that ensure a balanced diet.
What can already be done today? As central components of agricultural ecosystems, bees are essential to human societies. Profitable yet pollinator-friendly agriculture must be promoted by developing alternatives to intensive practices based on scientific evidence, as recommended in France by the National Institute for Agricultural Research (Inra). Citizens can also contribute to change by increasing food availability through the planting of melliferous flowers on balconies and in gardens, and by favouring moderate, local, and environmentally responsible consumption. The impact of civil society initiatives at the European level aimed at stricter regulation of neonicotinoids demonstrates that strong citizen mobilisation, grounded in robust scientific arguments, can be effective.
► Read also: Small Brain, High Performance
(1) www.sciencemag.org/news/2018/08/new-pesticide-may-be-harmful-bees-old-one