On the Ecology of Acrididae near Lake Chad

Summary

1. The shore of Lake Chad was suspected of being a reservation of both the Migratory Locust and the Red Lcoust. In 1931 a preliminary survey of the shore was made, as a result of which it was tentatively concluded that about one-half or less of the shore of British Chad may offer conditions suitable for the production of swarms of the Migratory Locust from the solitary phase. The Red Locust was present only in the southern half of the shore. A village named Kalkala, at the south-western corner of the Lake, was selected as the most suitable locality in which to carry out ecological research.

2. The ecology of the two species of locusts and of about 60 other species of ACRIDIDAE was studied at Kalkala during the first six months of 1933. A description is given of agricultural and climatic conditions in the Kalkala area and some notes on the hydrography of Lake Chad are included. The vegetation, soils, fauna and microclimates of the habitats frequented by ACRIDIDAE are described.

3. Collections of Acrididae were made each month in all the principal habitats. It was found that the Acridinae occurred principally in the wettest habitats and that this subfamily exhibited a small degree of ecological plasticity. The Oedipodinae consisted chiefly of geophilous species which were most abundant on bare soil in farm-land. The Pyrgomorphinae were most abundant in two non-gramineous habitats, but were not uncommon in Cynodon dactylon; one species was associated with Calotropis procera. The Catantopinae showed a greater degree of ecological plasticity than did any of the other subfamilies; the majority of the species encountered were most abundant in the drier habitats.

4. Bionomical notes are given for the majority of the species of Acrididae met with in 1931 and 1933 in the Chad area. Evidence was obtained that many species of Catantopinae have a prolonged imaginal diapause. Certain species of Acridinae and Catantopinae are characterised by an embryonic diapause during the hottest months of the year, and one species of Pyrgomorphinae has a nymphal diapause during the early part of the dry season.

5. The Red Locust frequents tall grass habitats from November to the end of April, the adults then gradually move to short grass and farm-land and begin to breed in late June. The imaginal diapause lasts from early October to late July. Biometrical examinations of adults and other data indicate that the Red Locust is in phase transiens (congregans). There is no evidence that swarms have left the Chad area. It seems probable that conditions in this area are unsuitable for the production of phase gregaria.

6. The solitary phase of the Migratory Locust was present in the survey area throughout the six months; it was never numerous and decreased steadily in numbers from January to June in spite of the fact that it was breeding from late February to June. The decrease in numbers is thought to have been due, chiefly, to the attacks of the Carmine Bee-eater and to bush fires.Phase transiens (dissocians) was found until mid-March; there is no doubt that most of the locusts present were derived from the survivors of hopper bands which had been baited or trenched in the autumn of 1932. Swarms arrived in late May and early June and repopulated the area. The principal habitats of the Migratory Locust were Cynodon dactylon, Brachiaria ramosa and Chloris gayana. The scarcity of this species in the southern half of Chad indicated that this area is unlikely to be a reservation; but research over a number of years is necessary to confirm this opinion.