![Sample our Environment & Agriculture journals, sign in here to start your access, latest two full volumes FREE to you for 14 days]({"type":"image" , "src" : "/sda/5934/TOC-Subject-Sample-2021-Environment-Agriculture.jpg"})
Summary
The South-West of Madagascar, a region to the West of a line between Maintirano and Ranopiso is characterised by its light soils, aridity, peculiar vegetation (Didiéreacées), very extensive cultivation and pasture farming, high endemicity and speciation rates of its fauna.
Insect identity.
a) Phases and Size.
Compared with the solitaria phase, the transiens phase is characterised by much smaller females and slightly smaller males.
Compared with the transiens phase, the gregaria phase is characterised by very much larger males and slightly larger females.
Thus, the transiens phase does not seem to be merely part of a sequence of intermediate forms between the two extreme phases but a true phase in itself as distinct as the solitarious and gregarious.
b) Subspecies.
No differences in morphology, size or amplitude of phase variation have been found to justify the distinction hitherto made between Locusta migratoria capito (Saussure, 1884) and Locusta migratoria migratorioides (Reiche and Fairmaire, 1850). The latter, older name, must therefore be regarded as valid.
Some examples of insular nanism in the Indian Ocean islands, of brachypterism and gynandromorphism are related.
Life history and behaviour in the solitary phase.
The embryo's continuous development takes 11 to 50 days, according to the season. Its discontinuous development (with a quiescent period) takes 26 to 107 days. The warm season quiescence is caused by desiccation and the cool season quiescences by desiccation or cold.
The isolated larva behaviour seems to be partly induced by the necessity to preserve an optimum internal temperature. The larva development takes 24 to 75 days.
The percentage of larvae with six stages instead of five is 33% in females and 7% in males, except in the middle of the warm season in which it decreases respectively to 9% and less than 1%. The adult migrations are established by capture at light, recapture of marked insects, arrival of mature and laying females at the summer breeding areas in November and December and their departure in April. The arrivals are recognised by sudden increases of population density, female percentage and mature female percentage; the departures by inverse phenomena. The insects seem to go to the South-West at the beginning of the warm season and to the North-East at the end. The adult's maximum longevity observed in the field is three months for a female and ten for a male.
The female sexual maturation takes 12 to 45 days according to the season. There are, it seems, two female groups, one maturing quickly, the other, slowly. The number of eggs per pod is very high in the warm season: 92 in January, 81 in February. The number of pods laved by one female is about five in the warm season, three in the cool season. The average laying rythm is 5 days in the warm season, 10 in the cool season. Compared with large grasshoppers like Gastrimargus or Cyrtacanthacris, the Locusta reproductive potential increases greatly with the temperature.
Breeding in held cages and daily samples in the breeding areas indicate the possibility and probability of about four reproductions a year (11 reproductions in three years): R 1 in the cool season is sedentary and develops slowly; R 2 migrates from the permanent breeding areas to areas under cultivation, fallows and forest glades; R 3 reproduces in these transient breeding areas and often gregarises; R 4 often inclines to disperse. In normal development, the percentage of green insects and the hind femur length vary inversely during consecutive reproductions. When a gregarisation begins, they vary together.
Environment.
1. Egg: The mean ground temperature at a depth of 5 cm is a little lower than the maximum air temperature. It varies with exposition, humidity and shelter.
A late desiccation, after the inversion of the embryo, can be:
a) compensated: larvae hatch but are small;
b) uncompensated: the embryo completely develops but is unable to hatch.
An early desiccation, before the inversion of the embryo, can also be:
a) compensated: eggs become quiescent;
b) uncompensated: eggs die.
This clarifies the fact that rain after short dry periods in the warm season can induce simultaneous revivescences and hatchings and shows how vermiform larvae can be trapped in the lower part of the egg-pod and unable to ascend through quiescent or dead eggs to reach the surface of the ground. Some lower eggs can be destroyed because of excessive dampness but the entire contents of the egg pod are rarely lost in this way. Scelionid Hymenopterous destroy 1 to 32% of eggs laid, Sarcoptiform Mites 0 to 80%, Bombyliid Dipterous 1 to 17% according to the season.
2. Larva and adult: The abiotic factors of the environment are analysed. The study of vegetation in breeding areas shows the importance of Eragrostis-Panicum associations as in Africa, the diversity of Gramineae species on fallow land and the vegetation differences between permanent and temporary breeding areas, the last having less abundant hydrophilous species. It is suggested that other factors apart from shelter and nutritional value make some vegetation attractive to Locusts. Commensalism and interattraction with the Red Locust is not infrequent. Undergrazing favours Locusts on pasture lands. Predatism on larvae is an especially important controlling factor.
Some aspects of the Locust-Man connection are: acridophagy, control, environmental changes caused by agriculture such as rice fields of the rainy, irrigated and flood fall types and non-irrigated farming involving the introduction of hedges, clearing of land and burning of grass.
Traditional man favours the presence of locusts and in fact eats them. In case of great damage, his usual control means is fire. Modern man, although the most effective locust enemy, sometimes encourages their development as when he abandons his unprofitable crops, prevents grass fires and restricts overgrazing.
Gregarization factors.
Some factors leading to the initial concentration are:
1° A high egg-pod density caused by migrations, the attraction of laying areas and the interattraction of egg laying females.
2° An unusualy successful incubation promoted by a kind of seasonal pluviometric inversion.
3° Simultaneous hatchings caused by short dry periods in Summer.
4° Shelters such as fallow land, hedges and pasture land where it is scarce. Locusta reproduction potential increases violently in the warm season, which, with optimal pluviometric conditions and underfarming, results in enormous increases in larva population.