|dc.description.abstract||Stemboreres are major pests of maize and sorghum in Zimbabwe. An analysis of the research work on cereal stemborers in the country revealed several gaps for which information is required. Thus, as a way of bridging the identified information gaps, the studies reported in this thesis were conducted mainly to provide information on the distribution, abundance and seasonal occurrence of stemborers and their natural enemies, parasitoid seasonal carryover mechanisms and parasitoid biology, and to assess the need for the introduction of the exotic larval end parasitoid Cotesia flavipes Cameron (Hymenoptera: Braconidae).
The seasonal occurrence of cereal stemborers and their natural enemies was studies for three seasons (1999-2000, 2001-01) within the highveld (>1200M), middleveld (600-1200m) and lowveld (<600m) ecological zones of Zimbabwe. Three species of stemborers were recorded, namely, Busseola fusca Fuller (lepidoptera: Noctuidae), Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) and Sesamia calamistis Hampson (Lepidoptera: Noctuidae). Chilo partellus was the most abundant and widely distribute species, occurring at all sites, but predominantly in the lowveld and middledveld. Busseola fusca was the second most abundant species but predominated only in the highveld. Sesamia calamistis, on the other hand, appeared to be an uneconomic pest due to its overall low frequency of occurrence.
Stemborer natural enemies recorded included the larval parasitoids, Cotesia sesamiae (Cameron) (Hymenoptera: Braconidae) and Sturmiopsis parasitica (Curran) (Diptera: Tachninidae), the egg-larval parasitoid, Chelonus curvimaculatus Cameron (Hymenoptera: Braconidae), the pupal parasitoids, Procerochasmias nigromaculatus Cameron, Dentichasmias busseolae Heinrich (both Hymenoptera: Ichneumonidae) and Pediobius Furvus Gahan (Hymenoptera: Eulophidae), and an entomogenous nematode, Hexametric sp. (Mermithidae). While C. sesamiae had a much wider distribution and host range, S.parasitica occurred only within the highveld (Harare area) where it parasitized B. fusca. Outside the highveld, C.sesamiae was, however, not recovered at Muzarabani (lowveld) and also appeared to be scare at Bushu (Middleveld).
In the Harare area, S. parasitica parasitism predominated for about half of the season (December-February) and attained a peak (10.9-60.2%) between the end of January and mid-February. Thereafter, parasitism declined sharply and by the end of March, recoveries of S. parasitica had all but ceased. In contrast, C. sesamiae parasitism generally fluctuated between 0 and 20% for the remainder of the season. Sturmiopsis parasitica over wintered in disapausing larvae from which it emerged in October-December when host diapause was terminated. In contrast, the seasonal carryover mechanism of C. sesamiae could not be identified.
Laboratory experiments were conducted to evaluate B.fusca, S.Calamistis and C. partellus for their acceptability and suitability as hosts to a highveld and a lowveld C. sesamiae population. Both C. Sesamiae populations preferred the noctuids to C.Partellus for oviposition, possible reflecting differences in evolutionary history. However, the three hosts showed differential suitability for the development of the two C. sesamiae population. Whiles all were suitable for the development of the highveld. C.sesamiae, B.fusca was a partially suitable host to the lowveld population. Crosses between the highveld and lowveld. C. Sesamiae adults were compatible, producing mixed sex progenies. Based on the results of the host suitability and mating compatibility studies and a knowledge of the maize growth cycles within the different ecological zones, it is postulated that one probable. C. Sesamiae seasonal carryover mechanism in Zimbabwe could be a yearly net outward migration of the parasitoid from lowveld ''ecological islands'' to niches where populations die out each year due to a long dry season and the consequent diapause-induced host unavailability. On its own, however, sustenance of C. sustenance of C. Sesamiae in such ''ecological islands'' is not a satisfactory carryover mechanism thus other possible mechanisms need tobe investigated.
The development and levels of parasitism of S. parasitica on B. fusca, S.calamistis and C. partellus were studied in the laboratory. Parasitoid puparia recoveries were much higher on B.fusca 983.3% parasitism) than on C. partellus (15% parasitism). No development occurred on S. Calamistis due to maggot encapsulation. At 25±0.50C, parasitoid larval developmental period on non-diapause B. fusca larvae averaged 14.2 days and the pupal period ranged from 13.7 days (males) to 15.8 days (females). Maggots’ were first observed at 6 days after mating and their numbers peaked (537-848 per female) after a 10-16 day gestation. Inoculation of diapausing B.fusca larvae resulted in a greatly extended larval period. Thus, the observed seasonal carryover mechanism of s. Parsitica is simply a replication of host hormonal influences on the parasitoid's larval developmental duration in diapausing as opposed to non-diapausing host larvae.
The suitability of B. fusca and C. partellus for development of the exotic parasitoid, C. flavipes, was investigated in Zimbabwe. Although C. flavipes accepted both stem borer species for ovipositor, development was only completed on C. partellus. Busseola fusca was an unsuitable host due to egg encapsulation. While this confirms the suitability of C.partellus for development of the exotic parasitoid, the apparent absence or very low impact of the indigenous C. sesamiae at some of the C. partellus ''hotspots'' further highlight the need to release C.flavipes in the country||en_US