Honey bee (Hymenoptera: Apidae) foraging responses to phenolic-rich nectars

We determined honey bee (Apis mellifera L.) responses to five phenolic-rich aqueous solutions presented in an artificial flower feeder. Low concentrations of caffeic acid and genistic acid in 30% sucrose were preferred over a sucrose control. High concen trations of the acids inhibited imbibition. Phenolic-rich solutions made from aloe (Aloe littoralis) nectar, almond (Prunus dulcis) honey, or salt cedar (Tamarix pentrandra) honey deterred collection. Our results suggest that phenolics may affect honey bee imbibition behavior by functioning as a strong repellent or deterrent. Floral nectars are mixtures of natural products consisting primarily of sugars, with amino acids, proteins, lipids, salts, and organic acids in lesser amounts (Baker and Baker, 1983a). Nectars also contain secondary plant substances such as phe nolics and nitrogenous alkaloids that may deter some nectar feeders. Baker and Baker (1983b) found phenolic-rich nectar compounds in the nectars of 333 out of 850 plant species tested, suggesting that phenolic-rich nectars are widespread in nature. Phenolic substances in nectar need more critical analysis to evaluate their biological effects. Although the effects of phenolic compounds in nectar have not been determined, they could have several effects. Their fluorescent properties may be a visual cue to insects (Thorp et al., 1975). Phenolics might render car bohydrates and amino acids less or even non-metabolizable and therefore nutri tionally unavailable for non-pollinating insects. They may impart an unfavorable taste to nectar for non-pollinators (e.g., nectar "thieves") and the right taste (phagostimulants) for adapted pollinators. Phenolics might also be antimicrobial and keep microbes from degrading carbohydrates in nectar. Phenolics may be secondary plant metabolites shunted into the nectar as a form of "excretion," although this is unlikely due to the metabolic costs to the plant (Baker, 1977; Levin, 1971). Various pollinators (insects, bats, and birds) could "exhibit" different feeding responses to phenolics in floral nectars. Baker and Baker (1978) suggested that phenolics present in high concentrations in many tropical plants may deter thiev ery by ants. Apparently ants can detect phenolics and discriminate against them, but their threshold concentrations are unknown. We have observed that the nectars of several desert plant species contain high concentrations of phenolics. Almond nectar/honey contains three to four bright blue fluorescent compounds (under longwave U.V. illumination) and one reddish component. Salt cedar honey fluoresces a brilliant yellow-green and contains at least eight phenolics. Aloe littoralis nectar is exceedingly rich in phenolic acids and other phenolics. Preliminary 2-D TLC (S.L.B., unpubl. data) indicates the 1 Agricultural Research Service, U.S. Department of Agriculture, Western Cotton Research Labo ratory, 4135 E. Broadway Road, Phoenix, Arizona, U.S.A. 2 Agricultural Research Service, U.S. Department of Agriculture, Carl Hayden Research Center, 2000 East Allen Road, Tucson, Arizona, U.S.A. Accepted for publication 20 November 1992. This content downloaded from 157.55. 39.208 on Wed, 28 Sep 2016 06:30:20 UTC All use subject to http://about.jstor. org/terms 224 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY presence of at least 18 fluorescent phenolics (e.g., blue, blue-green, dark green, green, orange, red-violet, olive, purple, brilliant bright green, ochre and blue violet), under 254 nm U.V. illumination. Once these compounds are isolated, they can be tested individually and in combination against nectarivores such as honey bees, birds, and nectar-marauding ants for antifeedent properties. A small literature base is available on the phenolics present in leaf exudates from Aloe spp. (Conner et al., 1987; Beaumont et al., 1985; Reynolds, 1986; Reynolds and Nichols, 1986). These phenolics have been identified as anthra guinones, anthrones, and phenylpyrones; major leaf constituents (e.g., aloenin, nataloe-emodin) are likely to be some of the bitter phenolics in A. littoralis nectar. Hoffman (1988) studied the pollination ecology of Aloe ferox in South Africa and found that bees collected pollen but rarely imbibed nectar. No mention was made of the phenolic components of nectar in this species, nor any effects on floral visitors or nectar "robbers. " Honey bees were observed foraging during a six-year period on cultivated A. littoralis plants in Tucson, Arizona (S.L.B., unpubl. data). They routinely collected pollen from the bright orange exserted anthers, but always avoided the nectar. The copious nectar (often 200-300 ^I/flower) went uncollected by honey bees, native bees, and hummingbirds. Honey bees that contacted nectar with their mouth parts during floral manipulations invariably groomed off the nectar. The nectar was a light yellow-green at anthesis but darkened (photo-oxidized) to an amber color by the following day. Large droplets of dried nectar remained when the flowers senesced and abscised. This study tested the responses of honey bee (Apis mellifera) workers to the phenolic compounds genistic acid and caffeic acid, along with the phenolic-rich nectar or honeys, Aloe littoralis (aloe), Prunus dulcis (almond), and Tamarix pentandra (salt cedar). Results provide initial information on the palatability (acceptance) of these phenolic-rich solutions to feeding honey bees.