The control of black sage (Cordia macrostachya) in Mauritius: the introduction, biology and bionomics of a species of Eurytoma (Hymenoptera, Chalcidoidea)

Extract

An account is given of the biology of the phytophagous Chalcidoid, Eurytoma sp. near howardi D.T., and of its bionomics in Mauritius. The insect was introduced into Mauritius from Trinidad to supplement the activity of the Galerucid beetle, Schematiza cordiae Barber, in the control of Cordia macrostachya, an introduced shrub, indigenous to Central America and the Caribbean Islands. The egg and larval stages are described and figured.

The entire development of Eurytoma takes place within the fruit of Cordia, which contains usually only one seed when mature and has a very hard and stony endocarp and a soft and fleshy mesocarp. Oviposition by Eurytoma takes place before the endocarp hardens; the egg is laid inside the seed and the larva feeds on the nucellus and endosperm and later on the developing cotyledons, until the entire seed, except for the seed coat, has been consumed. The attacked fruit continues to grow and its endocarp hardens, but final swelling of the mesocarp does not take place. Although more than one egg may be laid in a seed, only a small percentage of the many seeds dissected contained two mature larvae or two pupae. Pupation takes place within the seed, from which the adult escapes by boring a round hole through the endocarp.

Development from egg to adult occupies about 23 days in the hot season (November–April), but in the cooler months (June–September) the fully-grown larva enters a resting period, which may last several months.

Adults kept in jars, and provided with honey in water, at a mean temperature of 25°C, lived for an average of 11 days (males) or 20 days (females). A newly emerged female contains about 12 fully developed eggs. Virgin females oviposit readily, all the progeny being male.

Releases of adults from material received from Trinidad were made between October 1949 and May 1950; by the latter date it was evident that the insect had become established at the first liberation site and further colonies were established in various parts of the island from material collected there. Estimates of infestation of samples of fruit collected in October 1951 at or near 22 liberation points ranged from 16 to 81 per cent, (mean, 53·7). The latest island-wide estimates, in February, June and October 1953, showed averages of 64·7, 40·8 and 48·7 per cent, infestation, respectively.

Infestation of fruits is highest (70–80%) in the hot season, when fruit production is at its maximum, and lowest in the cooler months, when, although the rate of fruiting declines, the life-cycle of Eurytoma is prolonged.

Schematiza cordiae had achieved a high degree of control of Cordia before Eurytoma became widely established, and it is difficult to assess the contribution made by the latter, which has no known specific natural enemies in Mauritius; but the two unrelated insects acting jointly are likely to stabilise control more effectively than would one insect alone.

In an appendix, P. 0. Wiehe gives notes on the present ecological status of Cordia in Mauritius and contrasts it with that prevailing before the introduction of Schematiza and Eurytoma. He concludes that the secondary succession of vegetation has now reverted to its former course.