Biology of the palm weevil, rhabdoscelus maculatus in selected foliage palm nurseries in Sri Lanka

Abstract

A study on the Biology of the palm weevil Rhabdoscelus maculatus was carried out at Tropiflora Ornamental Palm Nurseries in Sri Lanka. This weevil, an important pest of ornamental palms, was detected for the first time in Sri Lanka at Tropiflora Nurseries. Although the incidence of this weevil is restricted to ornamental palms at present, there is a potential threat of the weevil becoming a pest of important cultivations such as coconut and sugarcane. The present investigation was therefore, carried out to confirm the identification of this weevil and to study its biology. The identification of the palm weevil was confirmed as R. maculatus Gyllenhal, 1838 and is the first record in Sri Lanka. Biological, morphological and behavioural aspects of the weevil were studied under laboratory, semi-natural and natural conditions. The adult is a small, reddish-brown weevil (10.38- 12.62 mm in length) with two black markings on the elytra and two on the thorax. The female is slightly larger than the male. The rostrum of the female is longer and more curved than that of the male. The ventral surface of the rostrum is roughened in the male while it is smooth in the female. Adult weevils are nocturnal in habit and hide in plant sheaths and debris during the day. They fly only short distances at a time but flight during the day was not observed. Mature female lays eggs inside cavities made in the leaf sheath. Under laboratory conditions, the period for egg incubation is 5.00 + 0.82 d. The larva is creamy white and legless with a round, highly sclerotized, reddish brown head capsule. Larvae feed on stem tissues rapidly and make tunnels. The frequency distribution analysis of head capsule width revealed six larval instars. Early weevil infestation of palms is indicated by a jelly like substance oozing out of the holes in the leaf sheaths. R. maculatus larva takes about 7.75+ 3.70 weeks to complete its de velopment and then pupate inside a cocoon made of tightly bound, fibrous material completing the pupal stage in 2.50 + 0.76 weeks. The total life cycle of this weevil is completed in 10-12 weeks. The life span of adult weevils is between 8-10 months under semi-natural conditions and four weevil generations per year occur under natural conditions.