Digestion of maize and sunflower pollen by the spotted maize beetle Astylus atromaculatus (Melyridae)

Abstract

The relationship between beetles and flowers is often mutually beneficial. Flowers provide not only edible rewards and favourable micro-environments, but may also be sites for mating and egg-laying activities. Even though beetles consume various parts of flowers, including pollen, and can sometimes cause considerable damage, they are in fact important pollinators of many flowers. Pollen was once considered indigestible but is actually a highly nutritious food source for many animals (including insects, birds and mammals) which use a variety of methods to digest it. Six basic methods are discussed in detail: mechanical damage, piercing and sucking, external digestion, enzymatic action, osmotic shock and pseudogermination. In this study I investigated the mechanism and efficiency of pollen digestion of two different kinds of pollen, that of maize Zea mays and sunflower Helianthus annuus, by the spotted maize beetle Astylus atromaculatus (Melyridae) an economically important pest in South Africa Histological observations were made of the gut contents and faeces of spotted maize beetles that fed on maize and sunflower. A high percentage of maize pollen grains (88%) was found to be empty and ruptured in the anterior midgut of these beetles, while sunflower pollen, although the contents were removed from most of the grains (84%), remained intact. Osmotic shock was apparently involved in digestion of maize pollen while another method, such as enzymatic action, may be used for the digestion of sunflower pollen by this beetle. Digestion efficiency of pollen, which corrects for the number of initially empty grains, was determined for the spotted maize beetle (67% for sorghum, 72% for sunflower and 74% for maize) and was found to be high in comparison to values for various mammals, birds and insects consuming pollen of other plant species. Kroon et al. (1974) proposed osmotic shock as a prerequisite for pollen digestion in honeybees and this hypothesis was accepted into the literature without question. I investigated the effect of osmotic shock on maize pollen by looking at behaviour of pollen grains under varying osmotic concentrations. Given that rainwater can sometimes cause irreversible damage to pollen grains, distilled water was used to simulate rain and sucrose solutions the stigmatic exudates of flowers. A small number of studies have focused on differences between cultivars, therefore in vitro studies were carried out subjecting maize pollen of different cultivars to different sucrose and glucose:fructose concentrations, using distilled water as a control. Results of this study indicated that maize pollen might burst in distilled water and sugar solutions of various concentrations did not decrease the amount of rupturing compared to that in water. Few studies have looked at pollen of a single plant species being digested by different animals. I compared the efficiency and mechanism of maize pollen digestion by honeybees (Apis mellifera) and spotted maize beetles. Digestive efficiency was high (80%) in beetles compared to that of bees (21 %). Maize pollen bursts early in the midgut of maize beetles but remained intact in honeybees: this suggests that osmotic shock is not as important for bees as previously suggested.