Some aspects of the biology of the sorghum shootfly,atherigona soccata rondani (diptera:anthomyiidae),with special reference to ovipositinal behaviour

Abstract

Development and 'morphology of the immature stage of Atherigorra eoccata Rondani were examinedby neans of dissections, linear measurements, light and scanning electron microscopy of the egg, the larval ins tars , and the pupa.rium. Cevelqment measurements were of body lengths and widths and the lengths of the cepaal.opharynqeal, skeleton. Survival of larvae at different groNth stages of the sorghum plant WI ae investigated in the Laboratory and was dependerrton the size of the host plant. Survival was usually highest whenplants about ~ ,veeks old were infested, lOtI in, very young plants, and 1000estin plants nore than 50 days old. Within group:; of' insects hatched from eggs laid en the sane day, the occurrence of the various larval instars overlapped. Study of the seasonal activity showed that adult sorghum shodt~~es were present throughout the year in varying nurrbers, and that the variations in nurrbers were lnainly related to rainfall. 'lhere was rapid increase in the numberof adults soon after the beginning of the rainy season due to increased adult energence from pupae caused by rainfall. '1here was apparentily a natural disproporti,on in the nurrber of ferrales; the sex ratio varied , " bebleen 1: 1..4 and 1: ~~'9 males to females• Study of scsre aspects of reproductive biology showed a .miximumof 346 eggs per female laid during a life span of 62 days. Lcngevity and fecundity of A. soa:ata were found to be affected mainly by adult diet. Both fecundi,bj and rate of -xxioviposition were found to be influenced by mating. Unmated females had a longer pre-oviposi tion period and irregUlar o viposi tion, both factors resulting in reduced fecundi,ty , si.noe only one egg develops at a tine in an ovariole. Both protein and sugar were irrportant in the reveloprrent of ovaries. A verj ICM percentage of females (29.4%)fed an food lacking in sugar had deval.opedovaries. 'Ihe percentage of ferral.es with developed ovaries (27.8%)was still lONerwhen flies were given water alone. vJhenfemale flies were fed on a diet containing both protein and sugar, 82.9%developed ovaries. Food containing sugar without protein also enabled a high percentage of females (75%) to have developed ovaries. Study of ovi position behavior showed that certain leaves an the sane plant were preferred for oviposition by the shoot fly. It was further shosn that preference for the lower leaf s~ace was dU3to a position effect' rather than leaf ;- surface characteristics. 'fuere was a ~rked preferenc:e for sorghum over the other alternative host plants. 'fue flies appear to restrain ovi position during the periods when sorghum is unavailable. Hdilever, this restraint'" did rot affect the efficiency of host plant selection for ovi position by the

flies. Within sorqhum plots there was preference for five leaf stage plan~ for ovi position. Larvae failed to develc.p uliy on yotmger plants due to insuficient food supply, . whereas plants over 50 days old were-rarely laid upon. '1he distribution of dead hearts within field plots indicated clustering of larvae, which was a result of the xxiiattracticn of plants already damaged to the ovipositing ferrales.