Investigations on the Control of the American and Red Bollworms of Cotton in S. Africa

Summary

Methods have been devised and applied to an intensive study of the course of illworm activity on numerous rain-grown and irrigated crops. These were developed the first instance to enable accumulation of data considered essential for evaluating eusefulness of certain egg and larval parasites when released experimentally. and they have now been extended to the point of providing continuous, quantitative data on bollworm-food-plant relations. This work has been placed on a routine basis. The information that is supplied comprises:—

1. The course of oviposition in point of duration, magnitude and time of occurrence in the life of a host crop. This serves well to indicate the true course of moth activity on different food-plants, the relative attractiveness of these and to reveal if such matters are maintained under different conditions and bear definable relations to growth stages of a food-plant.

2. The identity and activity of egg and larval parasites under natural conditions.

3. The identity and activity of insects predatory on the eggs and larvae, as occurring in the varied insect communities associated with different crops.

4. Comparative mortality of larvae in the different crops where oviposition has been recorded.

Continuous records of egg-laying by the American bollworm, H. obsoleta, F., have been taken at the Cotton Station, Barberton and on several farms in the neighbourhood. Cotton and maize were the principal crops, but the surveys included a number of other food-plants. Egg-laying was found to be very closely associated with the period of bud and flower production, and its duration on different hosts thus varies according to the flowering habit.

In the case of maize, egg-laying hi quantity commences when the tassels (stam-inate flowers) are extruding, and the peak of the oviposition is passed before silks (pistillate flowers) are abundant. This is at variance with the generally accepted view that the plant is most heavily oviposited upon during the silking period. The typical course of oviposition on maize here lasts for two to three weeks only.

Numerous instances were observed where oviposition by Heliothis occurred on maize in tassel to the practical exclusion of neighbouring cotton in freely fruiting and healthy condition, and the possibilities of exploiting this relationship are discussed.

As regards winter-irrigated vegetable crops and citrus orchards, data are given showing that the American Bollworm breeds extensively in these crops, particularly so a month or two prior to the appearance of rain-grown cotton, maize and tobacco.

The irrigated crops in the Barberton and contiguous districts are regarded as a main source from whence bollworm on cotton is derived. Citrus orchards of which there are relatively large acreages may be of particular importance in this regard.

The methods of obtaining information on the course of moth emergence are described. Data are given that explain the course of moth activity in winter crops and indicate when moths enter rain crops from winter breeding areas.

The only larval parasite of importance under natural conditions is Sturmia inconspicua, but this fly does not parasitise young larvae. There is a scarcity of insects parasitic on American bollworm larvae, and the question of importing them should be considered. Breeding and experimental releases of a larval parasite, Microbracon brevicornis, are discussed.

Two important egg parasites are a species of Phanurus and Trichogramma lutea.

An Anthocorid bug, Orius sp., destroys large numbers of bollworm eggs and probably also young larvae, and this insect and certain species of ants are the most important agents in natural control.

As regards the red bollworm, Diparopsis castanea, Hmps., it is concluded that the use of trap crops should be discouraged for general adoption as a means of controlling it owing to the danger, through inattention to the trap crops, of breeding this boll-worm in them. Its incidence in former years is briefly reviewed. Of late years the annual infestation has diminished greatly in some districts but not in others. The variations in this regard are attributed mainly to the presence or otherwise of ratooned cotton, particularly in mild winters and to seasons of very late planting.