Digestion of maize and sunflower pollen by the spotted maize beetle Astylus atromaculatus (Melyridae): is there a role for osmotic shock?

We investigated the mechanism and efficiency of digestion of two types of pollen, maize, Zea mays, and sunflower, Helianthus annuus, by the spotted maize beetle, Astylus atromaculatus (Melyridae). We found similar and high extraction efficiencies, but different mechanisms of digestion. Osmotic shock was apparently involved in digestion of the large and thin-walled maize pollen grains. In the anterior midgut most maize pollen grains were already ruptured, in contrast with the intact exines of sunflower pollen, which suggests another mechanism of digestion for the latter, such as enzymatic action. We investigated the effect of osmotic shock on maize pollen in vitro by looking at the behavior of pollen grains in varying osmotic concentrations. Maize pollen grains burst in both distilled water and sugar solutions of various concentrations, and the amount of rupturing decreased with an increase in sugar concentration. Digestion of maize pollen was much slower in honeybees than in spotted maize beetles. Maize pollen bursts early in the midgut of maize beetles, but remains intact in honeybees: this suggests that osmotic shock may not be as important for honeybees as previously suggested.