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Theses

FUNCTIONING OF A COMMUNITY OF LEPIDOPTERAN MAIZE STEMBORERS AND ASSOCIATED PARASITOIDS FOLLOWING THE FALL ARMYWORM INVASION IN KENYA

Abstract : Lepidopteran stemborers are still among the most important pests that are reported to cause maize yield losses in sub-Saharan Africa. In Kenya, there are three main stemborer species, Busseola fusca, Sesamia calamistis and Chilo partellus that seriously limit potential maize yield. The recent invasion of the fall armyworm (FAW), Spodoptera frugiperda (Lepidoptera: Noctuidae) in the maize growing region of the country has further complicated the management of stemborer pests in maize fields. For proper management of maize pests, the knowledge of the behavioural and chemical ecology, eco-environmental factors and interaction among various pests species in the field is essential. Therefore, this study aimed to identify the main reservoir source of maize stemborers and associated parasitoids that give rise to new populations in succeeding cropping season and evaluate the chemical basis of the female moth oviposition site preference. Furthermore, the communal larval interactions among the stemborer species and FAW larvae and the factors such as larval dispersal potential, environment temperature, larval density and duration of the interactions as well as the susceptibility of FAW larvae to the stemborer associated parasitoids were also studied. The occurrence of maize stemborers and associated parasitoids was investigated in maize stem residues and wild grasses during the non-cropping seasons as potential carry-over populations to subsequent early-season maize plants. Chemical mechanism of oviposition choice was conducted in a community of noctuids, B. fusca and S. calamistis and crambid C. partellus between un-infested, conspecific and interspecific larvae-infested maize plants. The impact of FAW introduction in maize stemborers and associated parasitoids communities was evaluated in maize fields. The performance studies were also carried out on the associated larval parasitoids of maize stemborers, namely Cotesia flavipes, C. sesamiae Inland, C. sesamiae Costal and C. typhiae (Hymenoptera: Braconidae) in laboratory conditions with FAW larvae as host. Larval dispersal either through ballooning or crawling was compared between FAW and individual stemborer species. The study of factors that influence the interactions of stemborers larval communities with the FAW with respect to temperatures (15°C, 20°C, 25°C and 30°C), larval densities (4 larvae, 8 larvae and 12 larvae), and the durations of the interactions (5 days, 10 days, 15 days and 20 days) was conducted using restricted artificial stems in the laboratory. The sources of infestation study showed that the carry-over of these stemborer species and their associated parasitoids during the non-cropping season was significantly (P < 0.05) ensured by the maize residues left in the fields from the previous harvests. The chemical basis of female moth oviposition site preference data highlighted that either conspecific or heterospecific larvae-infested maize plants elicited specific volatile signatures that attract female moths for oviposition. Following the introduction of FAW in maize fields, the study showed that single-species infestation significantly (P < 0.05) decreased in maize field upon communal stemborer and fall amryworm larval interactions. It was also recorded that multi-species significantly (P < 0.05) increased at field level with the introduction of FAW while the overall lepidopteran infestation incidences and larval densities significantly (P < 0.05) increased leading to the accelarated increase of yield losses in the fields. All the tested associated parasitoids inserted their ovipositor into the FAW larvae but without depositing eggs in them except C. typhiae. However, they induced significant (P < 0.05) non-reproductive mortality of FAW larvae. The FAW neonate larvae had a significant (P < 0.05) greater potential of dispersion than of the stemborer species counterparts. Temperature was an important factor that significantly (P < 0.05) influenced the intra- and interspecific interactions, both on survival and relative growth rates (RGR) between larvae of the three stemborer species and the FAW larvae using the same resource. However, in interspecific interactions, the competition was significantly (P < 0.05) less pronounced between the FAW and the stemborers compared to that among the stemborer species across the temperature tested. The results also showed that negative density-dependence survival and RGR significantly (P < 0.05) affected both survival and RGR. The time partitioning of the resource use significantly (P < 0.05) influenced the coexistence of these competing insect species. The carry-over niches findings give rise to new habitat management considerations in IPM strategies. Attractant volatiles identified generate applications in the development of a multi-species lure targeting female moths. FAW constitutes an additional production constraint of cereal crops that can co-exist with stemborer species along different temperature gradients. In addition, the nonreproductive mortality induced by common stemborer specific parasitoids can be explored further as part of FAW biological control contribution.
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https://hal.archives-ouvertes.fr/tel-03093686
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Submitted on : Wednesday, January 6, 2021 - 2:11:22 PM
Last modification on : Friday, January 8, 2021 - 9:33:26 AM
Long-term archiving on: : Wednesday, April 7, 2021 - 6:17:31 PM

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Bonoukpé Mawuko. FUNCTIONING OF A COMMUNITY OF LEPIDOPTERAN MAIZE STEMBORERS AND ASSOCIATED PARASITOIDS FOLLOWING THE FALL ARMYWORM INVASION IN KENYA. Life Sciences [q-bio]. Nairobi University, 2020. English. ⟨tel-03093686⟩

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