Biology and host specificity of gall-inducing Acythopeus burkhartorum (Coleoptera: Curculionidae), a biological-control agent for the invasive weed Coccinia grandis (Cucurbitaceae) in Guam and Saipan

Abstract

An introduced cucurbit Coccinia grandis (Linnaeus) Voigt has grown into problem proportions in Hawaii and the Pacific islands of Guam and Saipan. The biology of Acythopeus burkhartorum OBrien, 1998, a potential biological-control agent of C. grandis, has been described. By inducing the gall a sink for nutrients and by deriving nutrition, A. burkhartorum places C. grandis under stress. Especially during the late larval stage, the weevil displays an unusual behaviour of shredding dry gall tissues with its mandibles to prepare the pupal case and using the shredded sclerenchyma fibres to fill the open and cut ends of the pupal case. This ability to create such a pupal case is unique among weevils. Because A. burkhartorum is able to sever tender shoots of C. grandis at points where galls are induced, we consider that this weevil will be highly relevant in C. grandis management. Although non-gall-inducing species of baridine weevils have a wide host range, the known gall-inducing species are specific to their respective hosts, similar to the majority of gall-inducing insects. A. burkhartorum prefer consistently either petioles or stems, behave identically by tunnelling through soft tissues within the host organs, and induce galls. Because of the potential of A. burkhartorum in biological control of C. grandis, we tested its specificity against C. grandis and Zehneria guamensis (an endemic cucurbit of Guam and Mariana Islands), following choice and no choice test modes. No feeding hole or gall development occurred on Z. guamensis indicating a categorical response that A. burkhartorum is specific to C. grandis. This result encouraged field release of A. burkhartorum in Guam and Saipan.