Aggregation of the black chafer Holotrichia loochooana loochooana (Sawada) (Coleoptera: Scarabaeidae): Function of female pheromone and possible adaptive significance

Abstract

Attracted by their released pheromone, anthranilic acid, the females of the black chafer, Holotrichia loochooana loochooana, form aggregations on their food-plant colonies. To investigate the benefits they derive from joining such aggregations, the process of aggregation formation was observed, and the frequency of males landing on lures with or without anthranilic acid [(+)- or (-)-lure] was studied as a function of the lure positioning. Subsequently, within 30 min after the first females settled on a plant, several females would form a small aggregation. Thereafter, the females would land, near the landing spot of the first females, and the aggregation would expand in both the leeward and windward directions. Since there are very large daily and individual variations in the amount of pheromone released by the females, we conducted our field experiments using two extreme pheromone-release models, i.e., a (+)-lure that releases anthranilic acid in a quantity corresponding to the maximum amount released by a female (major female), and a (-)-lure that releases no pheromone (minor female). While no male was observed to land on a single, solitary (-)-lure, the males would land on a (-)-lure when it was positioned within 20 cm, particularly if it was leeward of the (+)-lure. This means that it is possible for females that had released no pheromone to still get mates provided they land within 20 cm from the females that had released a large amount of the pheromone. When a (+)-lure was surrounded by four (+)- or (-)-lures at 10- and 20-cm intervals, the frequency of male landings on the lure was not significantly different from the case of the solitary (+)-lure, except that the landing frequency on the windward (+)-lure was lower than that on other (+)-lures. Thus, it would be more advantageous for minor females to join female aggregations rather than remaining in a solitary mode.