Gregarization of the migratory locust Locusta migratoria (Orthoptera: Acrididae) and prompt control on a subtropical island in Okinawa, Japan

Abstract

Upsurges in the populations of the migratory locust Locusta migratoria (L.) occurred in July 2011 at the airport and in sugarcane fields on Shimoji Island, Okinawa, Japan. In sugarcane fields and pathways, dark-coloured nymphs at early instars marched and infested sugarcane leaves. The number of locusts was estimated as approximately 1.4 million. After prompt control by spraying insecticides, the locust populations decreased remarkably in size at the airport and in sugarcane fields. The locust populations at the airport restored gradually and dark-coloured nymphs similar in appearance to typical gregarious forms were found marching on paved roads at the end of August. Insecticides were sprayed again in early September over those areas at the airport where nymphs were abundant. The locust populations rapidly declined and only a few locusts were observed thereafter on the island before winter. These observations indicate the importance of constant monitoring and prompt action for the control of locusts. The seasonal life cycle of this locust species on this island was studied by regularly sampling the locusts in the field, which revealed that this locust species produces three generations per year with different overwintering stages.