Omega-3 deficiency impairs honey bee learning

Deficiency in essential omega-3 polyunsaturated fatty acids (PUFAs), particularly the long-chain form of docosahexaenoic acid (DHA), has been linked to health problems in mammals, including many mental disorders and reduced cognitive performance. Insects have very low long-chain PUFA concentrations, and the effect of omega-3 deficiency on cognition in insects has not been studied. We show a low omega-6:3 ratio of pollen collected by honey bee colonies in heterogenous landscapes and in many hand-collected pollens that we analyzed. We identified Eucalyptus as an important bee-forage plant particularly poor in omega-3 and high in the omega-6:3 ratio. We tested the effect of dietary omega-3 deficiency on olfactory and tactile associative learning of the economically highly valued honey bee. Bees fed either of two omega-3–poor diets, or Eucalyptus pollen, showed greatly reduced learning abilities in conditioned proboscis-extension assays compared with those fed omega-3–rich diets, or omega-3–rich pollen mixture. The effect on performance was not due to reduced sucrose sensitivity. Omega-3 deficiency also led to smaller hypopharyngeal glands. Bee brains contained high omega-3 concentrations, which were only slightly affected by diet, suggesting additional peripheral effects on learning. The shift from a low to high omega-6:3 ratio in the Western human diet is deemed a primary cause of many diseases and reduced mental health. A similar shift seems to be occurring in bee forage, possibly an important factor in colony declines. Our study shows the detrimental effect on cognitive performance of omega-3 deficiency in a nonmammal.Omega-3 and omega-6 fatty acids are two families of polyunsaturated fatty acids (PUFAs). Fatty acids (FAs) are important in structuring membrane lipids, and, because these PUFAs cannot be synthesized by higher animals, they must be acquired in the diet (1). Alpha-linolenic acid (ALA) (C18:3n-3) and linoleic acid (LA) (C18:2n-6) are the major omega-3 and omega-6 FAs, respectively. ALA is found in seeds, oils, and pollen. Some fish and other sea life also contain longer chain omega-3 FAs, eicosapentaenoic acid (EPA) (C20:5n-3) and docosahexaenoic acid (DHA) (C22:6n-3). Long-chain omega-3 PUFAs are major constituents of mammalian brain, and deficiency in these PUFAs, coupled with a high omega-6:3 ratio, is associated with many diseases and neurological disorders (2, 3). Because long-chain PUFAs occur in very low concentrations in insects (4), and Drosophila have been found to lack the necessary enzymes to synthesize them (5), insects have not been considered good models for studying the effect of omega-3 deficiency on cognitive performance. Nevertheless, a few studies have addressed this issue in insects, mainly in Drosophila, concluding that, although human and fly brain differ in long-chain FAs, lipids and lipid signaling are to a large extent conserved and important for the neuronal health of Drosophila (6).Bees provide crucial pollination services that support our food security, enrich our diet’s nutritional value, and are highly valued economically (7). These services are threatened worldwide by declining populations of pollinators, including the all-important honey bee. Malnutrition is emerging as one of the leading suspected culprits for declining bee populations, and for the plight of the honey bee in particular (7–10). Bees require nectar, their main carbohydrate source, and pollen, which provides proteins, lipids, vitamins, and minerals (11). Malnutrition may be due to low pollen quantity, quality, or diversity, a condition that is aggravated in agricultural monocultures (12–14), and in greenhouses (15). Malnourished bees have smaller hypopharyngeal glands (HPGs) (a source of queen and worker jelly) (9, 16), are more susceptible to deformed wing virus (16), are less tolerant to parasitism by Nosema ceranae (9), are more vulnerable to pesticides (17), have a compromised immune system (18), and have a shorter lifespan (19). Whereas diet quality is affected by amino acid content and composition, proteins alone cannot explain some of the effects of diet on bee health and colony functioning and deficits in additional nutritional factors: specifically, lipids are suspected (9). ALA and LA are generally considered essential fatty acids (eFAs) for most insects (20, 21), including bees (22).Pollens of different plant species vary greatly in lipid concentration and in the composition of FAs, including ALA and LA (23). In a diverse habitat, colonies tend to collect pollen from a variety of sources (24). But in disturbed habitats and extensive agricultural monocultures, the breadth of the diet is reduced (25), and bees may suffer from a deficiency of eFAs. Proper functioning of a honey bee colony relies on adequate production of young bees and on integration of many behaviors requiring sophisticated cognitive abilities.In the present study, we tested the effect of omega-3 dietary deficiency on the development of honey bee HPG and on the performance of bees in olfactory and tactile learning. Colonies were fed one of four artificial diets, two rich in omega-3 and two poor in omega-3. We found that omega-3–poor diets mainly reduced omega-3 levels in the body, and only slightly in the brain, and reduced HPG size. Omega-3 dietary deficiency greatly reduced performance in both olfactory and tactile associative learning assays. Our results show the influence of dietary omega-3 on cognitive performance in a model insect. Furthermore, we show a low omega-6:3 ratio of many wild flowers and of pollen collected by honey bee colonies, but a higher omega-6:3 ratio of some increasingly dominant cultivated plants, specifically almond and Eucalyptus. In a Petri dish experiment, olfactory associative learning of bees fed 1 wk on Eucalyptus pollen was greatly reduced. The reduction in omega-3 in modern human diet is deemed the most important global factor responsible for increased incidence of disease (2, 26). Likewise, our findings suggest that omega-3 deficits in bee nutrition, due to the limited diversity of pollen availability in transformed landscapes, may play a major role in decreased bee health and colony collapse disorder (CCD).