Contribution to the Study of some Phytophagous Acarina and their Predators in Mauritius

Extract

During investigations on the phytophagous mite fauna of Mauritius, of which up to the present five species only had been recorded, 21 species were collected, of which ten species belong to the TETRANYCHIDAE. In addition, nine species, of which the majority are believed to be predacious, were observed.

An annotated list of the species, together with their food-plants, their geographical distribution and the countries from which they might have been introduced into Mauritius, is given.

Of the species of economic importance the following are worthy of mention: — In the TETRANYCHIDAE: Eutetranychus banksi (McG.) on Citrus, Oligonychus biharensis (Hirst) and Oligonychus mangiferus (Rahm. & Sapra) on various fruit trees, including avocado, mango and litchi, Oligonychus sp. (pratensis (Banks) group) on sugar-cane, Tetranychus marianae McG. and Tetranychus sp. (ludeni Zacher group) on many vegetable crops, including bean, egg-plant, tomato, etc. In the ERIOPHYIDAE: Phyllocoptruta oleivora (Ashm.) and Tegonotus sp., which are potential enemies of Citrus and tomato, respectively. In the TENUIPALPIDAE: Brevipalpus phoenicis (Geijskes) has a very wide range of food-plants and its future importance in the cultivation of tea and Citrus are mentioned, whilst Raoiella indica Hirst is an important pest of coconut plantations. In the TARSONEMIDAE: Hemitarsonemus latus (Banks) ranks as an important pest of a large number of economic plants including bean, tea and water-cress.

Only two predatory mites of great economic importance are recorded, viz.:— Typhlodromus caudatus (Berl.) and T. ovalis Evans. The bionomics of the former species are fully described and its relation to Raoiella indica on coconut is discussed. The life-cycle of T. caudatus from egg to adult varies from 4–8 days, in summer to 13–18 days in winter.

Results are given of a survey of the predatory insects associated with the phytophagous mites. The most important predators on Tetranychus spp. are the Coccinellid, Stethorus vinsoni Kapur, and the Cecidomyiid, Feltiella sp. near tetranychi Rübs. The daily mite consumption of these predators is given. Other predators of secondary importance are Oligota pallidicornis Cam. and Scolothrips sp. near indicus Priesn.

The bionomics of T. marianae on tomato and of Raoiella indica on coconut were studied. Duration of the life-cycle of the former species varies from 4–9 days in summer to 15–22 days in winter. Twenty four to 30 generations of this species are estimated to occur in a year. Raoiella has a development period of 18–26 days from egg to adult in summer and of 30–36 days in winter.

A biological study of the predatory insects, Stethorus vinsoni and Feltiella, sp., is given. Their beneficial effect is discussed. These two predators do not exert a control in time to check the build-up of the mite population. Their abundance is directly proportional to the incidence of the mite and the peak is generally only attained when the depredations of the latter have reached a disastrous level.

The various possible factors which have contributed to the sudden increase in the mite population on tomato during the last 6–8 years are discussed. Amongst the important factors are the following: —

(a) Absence of cyclonic disturbances with their heavy rains and violent gales which generally upset considerably the development of the mite population and reduce it to a minimum each year.

(b) Dressing of tomato and other vegetable crops with nitro-phosphatic fertilisers which were not applied until 10 years ago and which, it is thought, may have favoured an increase in the mite population.

(c) The abundance of natural reservoirs of wild food-plants that harbour Tetranychus spp. from which the subsequent spread results in the building up of mite colonies in cultivated areas.

(d) The ineffectiveness of Stethorus vinsoni as a predator on infested tomato plants.