Title: Functioning of a community of lepidopteran stemborers and associated parasitoids in the context of the recent invasion of the fall armyworm 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 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. typhae (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 (15oC, 20oC, 25oC and 30oC), 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. typhae. 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.
Title of the PhD: Mechanisms of resistance in the African maize germplasm to the fall armyworm, Spodoptera frugiperda (J.E. Smith)
Abstract: Maize (Zea mays L.) is the third largest crop in the world after rice and wheat. In Africa, it is the most important food crop in terms of area harvested and alone provides more than 30% of the total calories of the human population in sub-Saharan Africa. The fall armyworm, Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae), a pest of maize native to the Americas was first reported in West Africa in 2016, is severely threatening food security in sub-Saharan Africa through the loss of tens of millions of tons of maize production each year according to FAO’s 2018 estimates. In the African context where the majority of maize producers are smallholder farmers with limited access to knowledge and adequate inputs to properly manage this new pest, the use of resistant varieties of the host, obtained either through conventional plant breeding methods or through silica induction (a known inducer of resistance in Grasses against pests), is therefore one of the most effective means of control, compatible with other integrated pest management strategies. The first step is to check whether an increase in the silica content of maize disrupts the development of S. frugiperda larvae. While silica induces a significant increase in stem diameter and height of potted maize plants, it has no influence on the development and mortality of S. frugiperda, ruling out the use of silica in maize resistance to this pest. Some resistant maize varieties have been bred and exist in the Americas against S. frugiperda but none are currently available as they are not adapted to the African continent. The other main objective of this thesis is therefore to develop a strategy to control S. frugiperda in Africa by using resistant varieties derived from African maize germplasm. The first results of the work on breeding and genetic improvement of (sub)tropical maize varieties against S. frugiperda, initiated by the International Maize and Wheat Improvement Center in Kenya (CIMMYT) between 2018 and 2019, indicate that five maize lines out of 1303 genotypes tested in greenhouses under artificial infestation have appreciable levels of resistance to S. frugiperda based on leaf and ear damage. After obtaining hybrids from these lines that are potentially resistant to S. frugiperda, this research is divided into three steps: 1) identify the mechanisms of S. frugiperda resistance in lines and hybrids selected for their resistance, 2) check whether these resistant genotypes are avoided by S. frugiperda females for oviposition, 3) and finally identify the chemical compounds responsible for resistance.
Titre de la thèse (en anglais): Environmental demogenetics of potato late blight in Colombia
Résumé (en anglais): The potato late blight is known for its part in the Irish potato famine during the 19th century. This disease has been extensively studied ever since. However, despite being one of the most well studied plant diseases in the world it remains one of the biggest threats to global food security. The late blight is caused by the Oomycete Phytophthora infestans. This is an hemi-biotrophic pathogen that infects the economically important crops potato (Solanum tuberosum) and tomato (Solanum lycopersicum).
In Colombia due to the prevalence of this disease and the extended use of susceptible potato cultivars, the main control strategy against this disease is the continuous application of fungicides. This constitutes a major problem because the repeated exposure of P. infestans to these fungicides results in the development of acquired resistance, and the effects on the health of the growers and the increased costs of continuously using fungicides during the growth cycle of the crop.
One way of reducing the continuous use of fungicides that has been practiced with some degree of success is the use of simulation and epidemiological models. These models project the proliferation of P. infestans based on environmental conditions such as temperature and relative humidity. This allows the growers to optimize the use of fungicides by applying them exclusively during the most favorable periods for the late blight development instead of a continuous use.
These models however have a few limitations. First, these do not consider the spatial configuration beyond each individual field. This could be an important addition due to the possibility of P. infestans dispersal between fields. Second, these disregard other possible management strategies and conditions that could be informative for the projection of late blight. And third, these are deterministic mechanistic models which have required decades of study to find the response of P. infestans to a variety of environmental conditions. Even though there have been adaptations of these models to tropical conditions, these models have a limited applicability outside the US because of the differences in environmental conditions and the responses of the different lineages present elsewhere.
Our main goal in this work is to develop an integrative model that considers several sources of information including environmental, epidemiological, genetic and spatial within a Bayesian learning framework. The idea is to start with a simple model that can be calibrated through this approach using collected field data. This would allow us to develop both a model for potato late blight in Colombia which would be continually calibrated with newly collected information, and a generic framework that can be used to develop models for lesser known plant pathogens that could be calibrated relying heavily on the collected field data and not previous mechanistic studies.