High toxin concentration in pollen may deter collection by bees in butterfly-pollinated Rhododendron molle.

BACKGROUNDS AND AIMS: The hypothesis that plants evolve features that protect accessible pollen from consumption by flower visitors remains poorly understood. METHODS: To explore potential chemical defenses against pollen consumption, we examined the pollinator assemblage, foraging behaviour, visitation frequency and pollen transfer efficiency in Rhododendron molle, a highly toxic shrub containing Rhodojaponin III. Nutrient (protein and lipid) and toxic components in pollen and other tissues were measured. KEY RESULTS: Overall in the five populations, floral visits by butterflies and bumblebees were relatively more frequent than visits by honeybees. All foraged for nectar but not pollen. Butterflies did not differ from bumblebees in the amount of pollen removed per visit, but deposited more pollen per visit. Pollination experiments indicated that R. molle was self-compatible, but both fruit and seed production were pollen limited. Our analysis indicated that the pollen was not protein-poor and had a higher concentration of the toxic compound Rhodojaponin III than petals and leaves, which compound was undetectable in nectar. CONCLUSION: Pollen toxicity in Rhododendron flowers may discourage pollen robbers (bees) from taking the freely accessible pollen grains, while the toxin-free nectar rewards effective pollinators, promoting pollen transfer. This preliminary study supports the hypothesis that chemical defense in pollen would be likely to evolve in species without physical protection from pollinivores.

Defence compounds in pollen: why do they occur and how do they affect the ecology and evolution of bees?

Pollen plays two important roles in angiosperm reproduction, serving as a vehicle for the plant's male gametes, but also, in many species, as a lure for pollen-feeding animals. Despite being an important food source for many pollinators, pollen often contains compounds with known deterrent or toxic properties, as documented in a growing number of studies. Here we review these studies and discuss the role of pollen defensive compounds in the coevolutionary relationship between plants and bees, the preeminent consumers of pollen. Next, we evaluate three hypotheses that may explain the existence of defensive compounds in pollen. The pleiotropy hypothesis, which proposes that defensive compounds in pollen merely reflect physiological spillover from other plant tissues, is contradicted by evidence from several species. Although plants may experience selection to defend pollen against poor-quality pollinators, we also find only partial support for the protection-against-pollen-collection-hypothesis. Finally, pollen defences might protect pollen from colonisation by antagonistic microorganisms (antimicrobial hypothesis), although data to evaluate this idea are scarce. Further research on the effects of pollen defensive compounds on pollinators, pollen thieves, and pollen-colonising microbes will be needed to understand why many plants have chemically defended pollen, and the consequences of those defences for pollen consumers.

Bumblebee Rejection of Toxic Pollen Facilitates Pollen Transfer.

Many bees are effective pollen collectors; however, pollen grains collected by bees for larval food are lost for plant sexual reproduction. Recognition of these conflicting interests between bees and flowers is essential for understanding of reproduction for both bees and flowers [1-3]. Plant defense compounds in pollen may function to reduce pollen waste by deterring ineffective pollinators [4-6], but this hypothesis remains unexamined. Here, we provide evidence that secondary metabolites in pollen function as chemical defense by deterring some bees from gathering pollen. In two Dipsacus species, a defense compound, dipsacus saponin [7], occurs in pollen but not in nectar. We observed that bumblebees disliked grooming bitter-tasting pollen with a high saponin content. Manipulation of saponin concentrations in nectar and measurements of corbicular pollen showed that the bumblebee species differed in their tolerance to saponin. Those species susceptible to saponin groomed little Dipsacus pollen into their pollen loads, and their ungroomed pollen was observed to be effectively delivered to stigmas. By rewarding bees with edible nectar, but not pollen, plants solve the conflict of pollen partitioning between sexual and reward functions. Ungroomed toxic pollen on the bee body promotes pollen transfer efficiency, facilitating pollination.

Pollen mixing in pollen generalist solitary bees: a possible strategy to complement or mitigate unfavourable pollen properties?

Generalist herbivorous insects, which feed on plant tissue that is nutritionally heterogeneous or varies in its content of secondary metabolites, often benefit from dietary mixing through more balanced nutrient intake or reduced exposure to harmful secondary metabolites. Pollen is similarly heterogeneous as other plant tissue in its content of primary and secondary metabolites, suggesting that providing their offspring with mixed pollen diets might be a promising strategy for pollen generalist bees to complement nutrient imbalances or to mitigate harmful secondary metabolites of unfavourable pollen. In the present study, we compared larval performance of the pollen generalist solitary bee species Osmia cornuta (Megachilidae) on five experimental pollen diets that consisted of different proportions of unfavourable pollen diet of Ranunculus acris (Ranunculaceae) and favourable pollen diet of Sinapis arvensis (Brassicaceae). In addition, we microscopically analysed the pollen contained in the scopal brushes of field-collected females of O. cornuta and three closely related species to elucidate to what degree these pollen generalist bees mix pollen of different hosts in their brood cells. In striking contrast to a pure Ranunculus pollen diet, which had a lethal effect on most developing larvae of O. cornuta, larval survival, larval development time and adult body mass of both males and females remained nearly unaffected by the admixture of up to 50% of Ranunculus pollen diet to the larval food. Between 42% and 66% of all female scopal pollen loads analysed contained mixtures of pollen from two to six plant families, indicating that pollen mixing is a common behaviour in O. cornuta and the three related bee species. The present study provides the first evidence that the larvae of pollen generalist bees can benefit from the nutrient content of unfavourable pollen without being negatively affected by its unfavourable chemical properties if such pollen is mixed with favourable pollen. We conclude that the widespread pollen mixing by females of pollen generalist bees should also be considered as a possible strategy to exploit flowers with unfavourable pollen and to optimize larval food quality.

Fornecimento de substituto de pólen na redução da mortalidade de Apis mellifera L. causada pela Cria Ensacada Brasileira / Pollen substitute on the reduction of Apis mellifera L. mortality caused by Brazilian Sac Brood

A cria ensacada brasileira é uma das piores enfermidades de abelhas Apis mellifera L. africanizadas, principalmente do Sudeste do Brasil. Ela afeta o desenvolvimento das larvas, após serem alimentadas pelas abelhas nutrizes com o pólen do barbatimão (Stryphnodendron polyphyllum e Stryphnodendron adstringens), ocasionando a morte entre os estágios de pré-pupa e pupa. O objetivo deste trabalho foi verificar o efeito do substituto de pólen na redução da mortalidade da cria ensacada brasileira durante o florescimento do Stryphnodendron adstringens. Foram utilizados dois tratamentos: as colmeias do tratamento 1 receberam semanalmente substituto de pólen (mistura de farinha de soja, fubá e farinha de trigo em proporções iguais). As colmeias do tratamento 2 não receberam o substituto de pólen, tendo como único alimento o pólen que encontravam na natureza. As colmeias alimentadas apresentaram diferença significativa de 93,5 por cento de viabilidade de sobrevivência de cria em relação às não alimentadas, que apresentaram 80,4 por cento de sobrevivência. A frequência de pólen S. adstringens nos coletores de pólen, no tratamento 1, foi 0,21. Entretanto, no Tratamento 2, foi de 4,38, indicando que as colmeias que não se alimentaram com o substituto apresentaram maior frequência do pólen dessa planta, quando comparadas com as que dispunham da dieta artificial. Esse manejo sugere que a oferta do substituto de pólen, antes e durante a florada do Stryphnodendron adstringens, é importante na redução dos efeitos da cria ensacada brasileira em enxames de abelhas africanizadas.

Brazilian sac brood is one of the worst diseases of Apis mellifera L. mainly in southeast Brazil. This disease affects the larval development after having been fed with "barbatimão" pollen (Stryphnodendron polyphyllum and Stryphnodendron adstringens); the workers become intoxicated and die between the prepupae and pupae stages. This study verified the effect of pollen substitute on the reduction of Apis mellifera L. mortality, caused by Brazilian sac brood during Stryphnodendron adstringens flowering. Two treatments were used: in treatment one, colonies received pollen substitute weekly (a blend of a soy flour, maize flour and wheat flour of equal proportions). The hives of treatment 2 didn´t receive the replacement of pollen substitute, receiving only the pollen found in nature. The hives which were fed with the pollen substitute had significant difference of 93.5 percent of brood survival viability when compared to non-fed which had 80.4 percent of survival. The frequency of pollen S. adstringens in pollen traps, in treatment 1, was of 0.21. However, in treatment 2, the frequency was of 4.38, indicating that the hives that were not fed with the substitute showed a higher frequency of pollen of this plant when compared with those who had the artificial diet. The results suggest that supplying with pollen substitute, before and during Stryphnodendron adstringens flowering, is important in reducing the effects of Brazilian sac brood disease.

Tipos polínicos encontrados em colônias de abelhas africanizadas sujeitas à doença cria ensacada brasileira / Pollen types found in africanized honeybees colonies submitted to brazilian sac brood disease

O pólen de algumas espécies de Stryphnodendron (Mimosoidea), conhecidas popularmente por barbatimão, tem sido apontado como agente causador da mortandade de larvas pré-pupas observada na cria ensacada brasileira (CEB). Neste estudo, objetivou-se identificar os tipos polínicos encontrados em amostras de pólen coletadas em colméias de Apis mellifera L., no Estado do Rio de Janeiro, a fim de elucidar as fontes florais utilizadas por essa espécie de abelhas em períodos sujeitos a CEB, com especial atenção para a presença do pólen de Stryphnodendron spp. Foram coletadas amostras de pólen apícola e pão de abelhas em sete apiários, em três municípios do Estado do Rio de Janeiro. A amostragem foi tomada um mês antes e durante a ocorrência da CEB. As famílias botânicas mais representativas foram: Leguminosae, Euphorbiaceae, Asteraceae, Poaceae e Myrtaceae. O pólen de Stryphnodendron não esteve presente nas amostras analisadas. A ausência de Stryphnodendron no menu floral das colônias africanizadas acometidas pela CEB em condições naturais sugere outro fator causal para a CEB no Estado do Rio de Janeiro.

The pollen of some Stryphnodendron species (Mimosoidea), commonly known as barbatimão, has been identified as a causative agent of pre-pulps larvae mortality, observed in Brazilian sac brood disease (BSB). The goal of this study is to identify the pollen types found in samples of pollen collected by Apis mellifera L. to elucidate the floral diet used by this honeybee specie in Rio de Janeiro state during the periods where BSB usually occurs. Particular attention was given to the pollen of Stryphnodendron spp presence. Samples of pollen and bee bread were collected from seven apiaries located in Rio de Janeiro state, Brazil. Sampling procedures were done a month before BSB usual occurrence and also during its occurrence. Leguminosae, Euphorbiaceae, Asteraceae, Poaceae and Myrtaceae were most representative botanical families on pollen sampling. Pollen from Stryphnodendron was not presented at the analyzed samples. In order to Stryphnodendron absent at the floral diet of Africanized hives affected by BSB in natural conditions, it is proposed another causal factor for BSB in the state of Rio de Janeiro.