رسائل دكتوراة
English
2004
Ecological Field Studies On The Population Dynamics Of The Solitary Desert Locust Schistocerca Gregaria (Forskهl) (Acrididae: Orthoptera)
السيد البشير محمد, عثمان محمد عبد الله الفقي
كلية الزراعة-جامعة الخرطوم · السودان
Studies of the key factors influencing the population dynamics, seasonal distribution, choice of habitat in terms of survival and multiplication of the solitary desert locust Schistocerca gregaria, in particular those factors leading to gregarisation (cause phase transformation) were conducted. The specific objectives include studies on the population densities of desert locust in the summer and winter breeding zones, (correlation between desert locust population and habitat, plant community structure, plant densities and weather conditions), morphometrics of the natural and reared populations, the life cycle and the role of different food plants on development, maturation, fertility, fecundity and longevity of the solitary desert locust under field conditions, and the effect of food plants on the developmental period and survival of the nymphal stages under laboratory conditions. During the period of the study in the summer and winter zones it was found that locust activities were greatly influenced by the amount of the annual rainfall. During the regular survey at the Red Sea coast more numbers of adult solitary desert locust were found in Pennesitum fields and in Heliotropium plants communities, whereas in the summer the survey showed that solitary desert locusts were mainly confined to areas of dense and green annual vegetation of uniform structures, particularly Cenchrus spp., and Indigofera spp., followed by Cyperus spp, and Panicum spp., habitats. It was also observed that at the beginning of the season locusts were not encountered on the seedlings of cultivated Pennesitum in the summer areas and that was mainly attributed to the low vegetation density due the weeding practices in cropped areas, whereas in the winter breeding areas the crop (Pennesitum) was sown in close rows and it was not weeded hence the vegetation density in the cropland was high and it attracted more locusts as compared to the summer areas. This suggests that locusts are attracted to habitats of high vegetation density and compact structure, because they probably need to protect themselves against the unfavourable weather conditions and against the attack of natural enemies; in addition to their need for food. Locusts were clearly noticed in the croplands when the surrounding areas started to dry up and while the Pennesitum plants were still green. During this study it was found that the vast green area to which locust could move and find conditions suitable for breeding, influenced its distribution hence there was less tendency for concentration. In this study and during the field survey at the winter breeding zone it was observed that solitarious females preferred to oviposit in sandy soil characterized by dense and tall Pennesitum plants intercepted with Heliotropium spp., and some other plant species such as Lueanea spp., Fagonia spp., and Cyperus spp. Optimum oviposition, time when the maximum numbers of females were observed laying, ranged between 11.00-13.00 hr., the dry air temperature ranged between 28-33 oC, and the soil temperature at the depth of 15 cm below the soil surface ranged between 22-29 oC. The total number of eggs/pod laid by females of solitary desert locust under field conditions ranged between 100-173 eggs/pod. Rearing of adult solitary desert locusts females in cages under field conditions on certain naturally growing host plants that included Heliotropium undulatum, Aerva javanica, and Pennisetum typhoideum in winter and Heliotropium bacciferum, Pennesitum typhoideum, Frasetia lonquisilia, Cenchurs biflorus, Cyperus conglomeratus, and Tephrosia purpura, in the summer was successful because these plants appeared to provide the nutritional requirements for normal development of solitary locust except for Panicum and Tribulus, which, among other things were found inferior in their nutrient value (nitrogen content). On the other hand females reared on H. undulatum under field conditions showed highest fecundity, fertility and longevity compared to those fed on P. typhoideum and A. javanica. However these food plants when used for rearing the immature stages of solitary desert locust under field and laboratory conditions, in winter, were found to support normal development. During this study it was found that the food quality (high nitrogen content) positively influenced development, maturity, fecundity and longevity of the solitarious desert locust. The average maturation period of the solitary females reared in summer under natural field conditions at a mean temperature of 35 oC was 18.5 days, whereas for those females reared in winter the average maturation period was found to be 25.4 days at a mean temperature of 29 oC. It was also found that some food plants affected the morphometrics of the desert locust. Locusts reared on Aerva javanica showed morphometrics close to those of solitarious individuals, whereas those reared on Heliotropium undulatum and Pennesitum typhoideum showed morphometrics close to those of the gregarious individuals. Morphometric measurements of the samples collected from the natural population in summer (2000) indicated that the population was solitarious and populations of the desert locust found in winter (2000-01), in summer (2001), and in winter (2001-02) in Tokar delta were classified as transient. During years of good rainfall in winter and in summer, three generations could be produced; two generations in winter and one in summer.
