The Control of the Black Sage in Mauritius by Schematiza cordiae Barb. (Col., Galerucid.)

Abstract

Cordia macrostachya (Jacq.) R. & S., is a Boraginaceous shrub indigenous to the West Indies, Central America, and northern South America. It was accidentally introduced into Mauritius about 1890, probably from British Guiana, and became a weed of major importance.A comparative ecological study of Cordia in Mauritius and in Trinidad showed that its invading power in the former island rested upon a profuse production of viable fruits, whereas in Trinidad, where the plant is of no agricultural concern, fruit production was controlled by the combined action of a number of insects. It was accordingly decided to introduce suitable species of these insects into Mauritius.The first to be introduced was a leaf-eating Cassidid beetle, Physonota alutacea, but as described elsewhere, this introduction, which took place in 1947–48, was not a success.The introduction of the second species, Schematiza cordiae Barber, and its result, is described in the present paper. This Galerucid is also a leaf-feeder, but it will sometimes feed upon the inflorescences.Through the agency of the Commonwealth Bureau of Biological Control, air consignments of Schematiza were received between June and September, 1947. Breeding was started, and after feeding tests in both Mauritius and Trinidad had established its specificity to C. macrostachya, the release of laboratory reared adults was commenced in March, 1948. In February, 1949, after about 18,000 had been released, breeding was no longer necessary for large populations had developed in the field and were inflicting severe injury to Cordia scrub. Dispersion of the insect over the island followed quickly, and was assisted by the collection and distribution of about 60,000 adults. Schematiza became extremely abundant so that by the middle of 1950 most expanses of scrub in the island had either been stripped of leaves and inflorescences, or had generally been severely defoliated.As a result, much scrub is dead owing to a prolonged period without foliage, much is being subjugated by other vegetation, while partial defoliation and direct inflorescence attack have reduced the reproduction of other bushes.This rapid control of C. macrostachya is attributed to the high fecundity of the Schematiza combined with its freedom from parasites and important predators, and to its suitability to the climates of all Cordia infested regions.