The Sisal Weevil

Abstract

This paper amplifies a previous circular [R.A.E., A 23 67] on Scyphophorus acupunctatus, Gyll., attacking sisal (Agave sisalana) in Tanganyika Territory, and embodies the results of the author's observations to date. The most obvious symptom of attack is the presence in the leaves of holes 1 cm. in diameter. These are the result of the weevil boring into the hearts of plants under 2 years old when the young leaves are still unfolded. When the plants are healthy the injury does not develop further, but when growth is not vigourous, the reaction of the tissue around the perforations is not sufficiently active to prevent the entrance of rot-causing organisms. These infect the central shoot, which becomes red and soft, and the plant dies. Large suckers used for planting are more liable to serious injury than bulbils, which are damaged by the weevil penetrating between the bases of the outer leaves into the bulb, or small suckers with newly cut bases, which also attract it. Deep planting and injury to the leaf bases are, however, the primary causes of the death of the plants in many cases, and the weevil is then only a secondary pest. Large healthy sisal plants are sometimes attacked by the adults when the leaves are still more or less part of the heart or central shoot. When cut, the leaves are found to have areas of brown dried-out epidermis about 8 ins. from the base. This causes discoloration of the fibres, but actual damage to them is not considerable. This type of injury is exceptional in that it appears to depend on the population density of the weevils and not on the health of the plants. There is, however, evident varietal susceptibility to weevil injury; Agave amaniensis and Furcraea gigantea (Mauritius hemp) growing in small plots on sisal plantations, particularly near the coast, are severely attacked, and at Amani, 2, 500 to 3, 000 feet higher than the sisal plantations, injury to A. sisalana and A. amaniensis is negligible, damage to A. ingens is more noticeable, while Furcraea gigantea is disfigured by large holes in nearly all the leaves. When eggs are laid in the base of young bulbils or suckers, or in the hole made by the adult weevil in the central shoot of a larger plant, usually only one or two larvae develop, but numbers do so in plants of which the pole has been cut or in dead stumps. The eggs survive only if there is a certain amount of moisture, and the larvae also die if exposed to dry conditions. The egg, larval and pupal stages last 3-4, 28-55 and 19-36 days, respectively; the larvae bore through the central shoot or make irregular tunnels through the tissues till full-grown, and pupate in cocoons made from fibre and leaf debris. Larvae and pupae both develop most rapidly during the rains. The adult female appears to take a minimum period of 25 days after emerging to reach sexual maturity, so that 11 weeks are needed to complete the life-cycle and there is a possibility of 4 generations a year. The length of life of the adults is not known, nor is the reproductive capacity, but 3 females averaged 62 eggs each over a period of 3 months. The general distribution of the weevil is discussed. In Tanganyika it has now been recorded as far west as Lambeni, but the average population is greatest in the coastal belt from Moa to the Pangani River. It is least in the estates at the foot of the Usambara Mountains, where the soil is of a red lateritic type. Here termites are numerous and are active in destroying the sisal stumps. There are indications that relatively dry seasons are also periods of increased activity of the weevils, but a low average rainfall is not in itself conducive to weevil attack. The control measures recommended are similar to those already noticed [loc. cit.]; attempts to render stumps innocuous by pouring a solution of sodium arsenite into holes made in the tops were unsuccessful as the penetration was not sufficient to render the whole stump poisonous.