Mono-Culture Landscapes, Sunflowers, and Honeybee Health

Introduction

Landscapes can affect parasite cause and distribution in wild and agricultural animals. Honeybees are threatened by loss of floral resources and by parasites, principally the mite Varroa destructor and the viruses it carries. Existing mite control relies heavily on chemical treatments that can adversely affect bees. Alternative, pesticide-free control methods are needed to mitigate infestation with these ectoparasites (live on the exterior). Many flowering plants provide nectar and pollen that confer resistance to parasites. Enrichment of landscapes with antiparasitic floral resources (heads composed of many florets) could therefore provide a sustainable means of parasite control in pollinators. Floral rewards of Asteraceae plants can reduce parasitic infection in diverse bee species, including honey and bumble bees.  Studies have found that increased sunflower pollen availability correlated with reduced Varroa mite infestation in landscapes and pollen-supplemented colonies. At the landscape level, each doubling of sunflower crop area was associated with a 28% reduction in mite infestation. In field trials, late-summer supplementation of colonies with sunflower pollen reduced mite infestation by 2.75-fold relative to artificial pollen. Sadly, United States sunflower crop acreage has declined by 2% per year since 1980.

Nutrition

Nutritional resources have profound effects on host-parasite interactions. The nutrition of free-ranging animals is shaped by foraging opportunities in local landscapes, which vary in resource quality, quantity, and medicinal potential. In agriculturally intensive landscapes, foraging patterns of animals—including insects—are heavily influenced by human land use change. Plant-pollinator communities appear particularly vulnerable to both parasites—which spread easily given the broad foraging ranges, high activity levels, and human-assisted migrations of social bees—and to land use changes, which govern the floral food resources and habitat upon which survival and reproduction depend.

Resistance and tolerance of bees to parasites is shaped by nutrition derived from floral nectar and pollen. While nectar and the resulting honey provide a sugar-rich source of energy, dietary pollen provides amino acids and lipids that support overall development, tolerance of parasites, and immune system activity. Beyond supplying macronutrients, specific floral and other plant resources can enhance resistance to infections. Plant resin-derived propolis has a variety of immunomodulatory (regulate immune functions) and antimicrobial effects, protecting colonies from the abiotic (nonliving) environment, predation, and parasitic infection. Nectar and pollen also supply a diverse assortment of plant secondary metabolites, including many compounds with effects on insect parasites, immunity, and infection. At a broader scale, the structure of floral landscapes and the presence of specific species that affect parasite transmission can affect the prevalence of infection in bee communities. Loss of antiparasitic plant resources could exacerbate susceptibility to infection, whereas increased pollinator plantings could enhance disease resistance in wild and managed bees.

Current Varroa treatments are largely based on chemical acaracides (pesticides)—typically applied multiple times per year—that can suppress honeybee immune function, render contaminated hive products unsuitable for human consumption, and lose efficacy as mites evolve resistance. Pollen can improve tolerance of honeybees to infestation at the individual and colony levels but does not fully reverse many of the physiological effects of mite feeding, such that the benefits of resource-rich landscapes can be overwhelmed by the negative effects of mite parasitism. Although adequate pollen stores can improve behavioral resistance of colonies by augmenting removal of infested brood, studies that show the effects of land use and floral resource availability on colony health have yet to implicate specific plant species in levels of mite infestation. Identification of such resources could inform apiary placement, land use policy, and supplementation of colonies.

Pollen

Pollen represents an essential component of bees’ nutrition whose properties go well beyond the supply of essential amino acids or metabolic energy. It appears that the apolar components of this food can provide important tools for the maintenance of the honeybee’s homeostasis including energetic and water balance and allow the coexistence with the rich cohort of symbionts inhabiting the hive. These results are especially intriguing given recent research demonstrating that bumble bee foraging preferences are shaped by protein:lipid ratios in the pollen of flowering plant species, indicating that bees may be carefully selecting appropriate macronutrient ratios for their diets.

Pollen provides bees with protein, minerals, lipids, and vitamins. All animals need essential amino acids, which must be obtained externally and cannot be synthesized by animals. Honeybees also need the same 10 amino acids as other animals (e.g., humans). These amino acids are obtained from pollen only, because honeybees do not have any other sources of protein. Pollen collection by a colony ranges from 10-26 kg (22-57lbs) per year. When honeybees are provided with insufficient pollen, or pollen with low nutritional value, brood rearing decreases and workers live shorter lives. These effects ultimately affect colony productivity. Shortages of pollen during rainy seasons can cause colony decline or collapse.

Pollen is mixed with glandular secretions to produce “bee bread,” which is consumed by young bees, considered the “social stomach” for protein digestion (because foragers cannot digest pollen directly, but still need protein. Rearing one larva requires 25-37.5 mg protein, equivalent to 125-187.5 mg pollen. Newly emerged bees have undeveloped hypopharyngeal and mandibular glands. Hypopharyngeal glands are paired glands inside worker’s head, consisting of a long central duct with many “grapes” (acini) attached. The glands will only develop after consuming a lot of pollen for the first 7-10 days. The glands first secrete the protein-rich component of royal jelly in young bees, but then secrete invertase, which is used to convert sucrose to simple sugars (fructose and glucose), in foragers. Mandibular glands are simple, sac-like structures attached to the base of each mandible. The glands secrete lipid-rich components of the royal jelly in young bees but produce an alarm pheromone (2-heptanone) in foragers.

Self-Medication

Animal self-medication is receiving increasing attention due to its profound implications for host-parasite interactions, including the effects on parasite transmission and the evolution of parasite virulence and host defenses. It has also been suggested that the interference of humans with the ability of animals to self-medicate can increase disease risk in managed species, such as in agricultural systems. Among the growing number of animal pharmacists, honeybees occupy an important position in that their use of plant resin with antibiotic properties is well known from ancient times and may even represent the first documented example of such an important aspect of ethology (study of animal behavior).

Bees use native or processed hive products in two alternative pharmacological ways defined as pharmacophagy and pharmacophory. Pharmacophagy relates to all defense mechanisms resulting from the direct consumption (e.g. honey, pollen, royal jelly) to decrease the disease or increase honeybee health whereas pharmacophory refers to the nonedible hive products (e.g. propolis, resin). This in-hive pharmacy provides three major types of natural medicine (honey, pollen/bee bread and propolis) for self-medication usage. Propolis, not consumable by bees, can only be seen as indirect hive medicine, but nonetheless harbors a very high pharmacophoric (biological) activity. Plant resins are widely used in bee societies as honeybees and other bees share a common spectrum of diseases and predators. Comparative studies (single type resin vs. mixtures) revealed that single resins may have different effects, and mixtures are more effective indicating functional complementarity for repellent effects against pests. Resins of different plant species not only target different organisms; they also act synergistically.

Sunflowers

One plant species of particular interest for bee health is sunflower (Helianthus annuus, Asteraceae). Its abundant production of nectar and pollen is critical for the reproduction of numerous wild bee species in the plant’s native North American range. Sunflowers are also cultivated plants that constitute major oilseed crops in Europe and Asia—which combine for 68% of global production—as well as the Americas, making important contributions to late-season pollen and honey storage in honey bee colonies co-located in agricultural landscapes. However, despite frequent visitation in some agricultural systems, sunflower is not necessarily an optimal or preferred resource for honeybees. Like that of many Asteraceae, the pollen of sunflower is low in protein compared to other bee-visited plants, and exclusively sunflower-based diets do not support development of honey and bumble bees when no additional pollen is provided. This has led to the conclusion that Asteraceae pollen is inappropriate, at least as a sole food source, for bees that lack special physiological adaptations to its secondary metabolites and nutritional deficiencies.

Varroa

Although significant effects of sunflower pollen on endoparasites or viruses was not found in laboratory or field settings, sunflower pollen was associated with reduced levels of Varroa mites in honeybee colonies. Specifically, studies found negative associations between sunflower cropland—even as a minor proportion of total land cover (median 0.32%)—and Varroa infestation, and, in a field experiment, reduced mite levels in colonies supplemented with sunflower pollen. It is well established that floral resources from specific plants have the potential to shape both the nutritional status and parasite loads of pollinators. Results, while largely correlative, suggest that one floral species could mitigate the impacts of a formidable parasite of managed honeybees worldwide, whereas our analyses of crop records suggest that honeybee access to sunflower cropland is declining.

The mechanisms underlying the association between the availability of sunflower and Varroa infestation remain unresolved but could reflect diet-related differences in quality or toxicity of brood food, pupae, or adults upon which Varroa feed. Both nutrient limitation and secondary metabolite content have been posited as explanations for poor development of many bee species on Asteraceae pollen, as well as for the resistance of Asteraceae specialists to parasitoids (within the host). First, the low protein content of sunflower—40% below typical levels for bee-visited plants—could result in fewer brood, or brood of poor nutritional quality for Varroa. For example, larvae of sunflower pollen-fed bumble bees were 80% smaller than those of Brassica-fed bees. Second, Varroa may be sensitive to the secondary metabolites in sunflower pollen or nectar. Mites could be directly exposed to secondary metabolites in brood food or by feeding on pupae or adults that have sequestered metabolites in fat body, Varroa’s main food source. Sequestration of secondary metabolites occurs in other insects (e.g., monarch butterflies) and can enhance resistance to parasites, predators, and parasitoids. The detection of caffeine in Varroa excrement after feeding on pupae provides empirical evidence for exposure of mites to compounds consumed by bees. Third, pollen of sunflower and other Asteraceae is distinguished by an unusual sterol content, including an abundance of Δ-7 sterols that are toxic to some herbivores and a relative lack of 24-methylenecholesterol, the main sterol found in honeybees and Varroa. Honeybees obtain sterols from pollen, with somatic sterol composition reflecting that of the diet. Incorporation of sunflower sterols into the tissues of pupae or adults could therefore affect Varroa survival or reproduction.

Studies indicate that sunflower cropland, an increasingly scarce resource in the United States, is associated with lower levels of Varroa mites, and suggest that sunflower pollen can reduce infestation of bee colonies. If sunflower pollen can be used to effectively manage Varroa mites, the timing of sunflower pollen production—which peaks in late summer (in temperate regions), just as mite levels begin to rise towards their peak in October and November—is ideal for reducing infestation during the critical late-season time frame.

Conclusion

Honeybees can obtain all their nutrients naturally if bees are in a natural setting. Unfortunately, modern agriculture has necessitated large scale mono-cropping which can be harmful to honeybees. This is mainly because each plant species has a specific nectar or pollen characteristic. Much like humans, a lack of variety in foods can cause problems. Many studies have shown poly-floral pollen diets are superior to a single species of pollen, with perhaps one exception (rape seed pollen alone can be excellent). We urgently need to understand the implication of each mono-culture crop on honeybees. For example, how much stress do bees experience when feeding exclusively on almond nectar and pollen for 3-4 weeks? How long do they need to (or can they?) recover after the stressful period? Are there “supplemental” crops available to reduce or eliminate such a stress? By understanding these questions and providing solutions to them, we will be able to make bees as healthy as possible.

Reference Material

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