A new pest borer on oil palm bunches in Indonesia (Rhabdoscelus obscurus Boisduval, Coleoptera Curculionidae)

Some large weevils have been reported for a long time on oil palm belonging to the Calandrini such as Rhynchophorus vulneratus (Pz.) and R. bilineatus (Mon.), respectively in West and East Indonesia, but even in association with other feeding pests these two species are without major incidence. Another very small Calandrini, 7-8 mm, is also very common on the fronds cut, attracted by fresh exudates, Diocalandra frumenti (F.). This insect is harmless, the larvae feeding only in the upper part of the tissues of the petiolar leaves, recently pruned. But now two bigger species, 12 and 14 mm, are found very numerous in oil palm plantation along with D. frumenti, Rhaboscelus maculata Gyll. already recorded in Malaysia and Rhabdoscelus obscurus Boisd. These two weevils have never been mentionned in Indonesia and the latter one is able to damage the bunches. R.maculata is living only as D. frumenti in the petiolar bases freshly cut, but R. obscurus can be found also in the living tissues of bunches boring inside the peduncule, the rachis or even the spiklets and the fruits. Damages in Sumatra in the first years of production are very variable according to the vegetal material and the cultural practices. Material with slow growth with often wounds during castration is more affected. The white grubs boring the peduncule before maturation of the fruits can provoke a decay of the upper part of the bunch, similar to a lack of pollination. Inside the rachis, after the feeding of the larvae, a large black longitudinal necrosis occurs reducing the development of the spiklets, and favouring the spread of bacteria in the soft tissues and fruits. The percentage of bunches affected can reached 21.4% on some crossings, but in average is only 5.1% on commercial fields, when this weevil is present. On mature palms damages are also present with the feeding cavity inside the rachis causing a frequent fracture of the bunch. The biology of this new pest has been studied, description of the adults and larvae are given. The life cycle is completed in 94 days. Until now Rhabdoscelus obscurus was known in the Pacific Islands, Papua New Guinea and Queensland. In Indonesia this pest has been only recorded in Irian Jaya and Moluccas, mostly on sugar cane, banana, coconuts and wild palms. Now this species has spread to the western part of the archipelago, and is present and well established in Sumatra oil palm plantations. Damages although still limited have to be taken into consideration and monitored. (Résumé d'auteur)