Can Tiny Thrips Provide Sufficient Pollination Service During a General Flowering Period in Tropical Rainforest

One of the most crucial and interesting topics in tropical rainforest research is the pollination systems adapted to mast species, which flower massively at multiyear intervals. Highly fecund flower thrips are the primary pollinators and can respond rapidly to such an abrupt increase of flowers. Because thrips are minute and rather weak flyers, however, it had remained unclear to what extent they could contribute to cross-pollination of mast species growing at low population densities. However, microsatellite DNA analyses of single pollen grains adhering to pollinators, combined with the direct observation of flower visitors with the aid of canopy observation systems, revealed the pollination efficiency of flower thrips during the masting period. In this study of Shorea acuminata, the principal flower visitors were flower thrips, accounting for more than 60% of the collected insects during the flowering time. The thrips visitation pattern was clearly synchronous with the flowering phenology of S. acuminata. However, the genetic analysis of pollen grains suggested geitonogamous self-pollination by flower thrips. Thus, flower thrips made a small contribution to the outcrossing of S. acuminata, although their high visitation frequency on S. acuminata synchronous with the flowering phenology of the trees brought about vast production of self-pollinated seeds. On the other hand, the greater genetic diversity of seeds than that of their adherent pollen load implied the existence of other pollinators that provide sufficient pollination service for S. acuminata. In this study, lygaeid and plant bugs (Hemiptera) were collected frequently (12.2%), placing them third after flower and stipule thrips. Lygaeid bugs typified by big-eyed bugs (Geocoris and Piocoris) are known to prey on thrips, and their visitations to S. acuminata trees increased with increasing numbers of flower thrips. Thus, it was considered that outbreaks of flower thrips may attract the visitation of lygaeid bugs to flowering S. acuminata trees as edible resources during the general flowering period, and such an ecological food chain would provide high levels of genetic diversity of seeds and promote sufficient pollination service for S. acuminata.