Bruno Le Rü

Dominant inheritance of field-evolved resistance to Bt corn in Busseola fusca

Transgenic crops expressing Bacillus thuringiensis (Bt) toxins have been adopted worldwide, notably in developing countries. In spite of their success in controlling target pests while allowing a substantial reduction of insecticide use, the sustainable control of these pest populations is threatened by the evolution of resistance. The implementation of the “high dose/refuge” strategy for managing insect resistance in transgenic crops aims at delaying the evolution of resistance to Bt crops in pest populations by promoting survival of susceptible insects. However, a crucial condition for the “high dose/refuge” strategy to be efficient is that the inheritance of resistance should be functionally recessive. Busseola fusca developed high levels of resistance to the Bt toxin Cry 1Ab expressed in Bt corn in South Africa. To test whether the inheritance of B . fusca resistance to the Bt toxin could be considered recessive we performed controlled crosses with this pest and evaluated its survival on Bt and non-Bt corn. Results show that resistance of B . fusca to Bt corn is dominant, which refutes the hypothesis of recessive inheritance. Survival on Bt corn was not lower than on non-Bt corn for both resistant larvae and the F1 progeny from resistant × susceptible parents. Hence, resistance management strategies of B . fusca to Bt corn must address non-recessive resistance.

Evolution of a Polydnavirus Gene in Relation to Parasitoid–Host Species Immune Resistance

CrV1, a polydisperse DNA virus (polydnavirus or PDV) gene contributes to the suppression of host immunity in Cotesia genus parasitoids. Its molecular evolution was analyzed in relation to levels of resistance in the sympatric host species. Natural selection for nonsynonymous substitutions (positive Darwinian selection) was observed at specific amino acid sites among CrV1 variants; particularly, between parasitoid strains immune suppressive and nonimmune suppressive to the main resistant stem borer host, Busseola fusca. In Cotesia sesamiae, geographic distribution of CrV1 alleles in Kenya was correlated to the relative abundance of B. fusca. These results suggest that PDV genes evolve through natural selection and are genetically linked to factors of suppression of local host resistance. We discuss the forces driving the evolution of CrV1 and its use as a marker to understand parasitoid adaptation to host resistance in biological control.

Influence of secondary compounds in the phloem sap of cassava on expression of antibiosis towards the mealybug Phenacoccus manihoti

Identification and assay of cyanogenic and phenolic compounds in phloem sap of cassava (Manihot esculenta Crantz, Euphorbiaceae) and in honeydew of the cassava mealybug Phenacoccus manihoti Matt. Ferr. (Homoptera, Pseudococcidae) were realised.

Disentangling dispersal, vicariance and adaptive radiation patterns: a case study using armyworms in the pest genus Spodoptera (Lepidoptera: Noctuidae).

Thanks to the recent development of integrative approaches that combine dated phylogenies with models of biogeographic evolution, it is becoming more feasible to assess the roles of dispersal and vicariance in creating complex patterns of geographical distribution. However, the historical biogeography of taxa with good dispersal abilities, like birds or flying insects, still remains largely unknown because of the lack of complete phylogenies accompanied by robust estimates of divergence times. In this study, we investigate the evolution and historical biogeography of the globally distributed pest genus Spodoptera (Lepidoptera: Noctuidae) using complete taxon sampling and an extensive set of analyses. Through the analysis of a combined morphological and molecular dataset, we provide the first robust phylogenetic framework for this widespread and economically important group of moths. Historical biogeography approaches indicate that dispersal events have been the driving force in the biogeographic history of the group. One of the most interesting findings of this study is the probable occurrence of two symmetric long-distance dispersal events between the Afrotropical and the Neotropical region, which appear to have occurred in the late Miocene. Even more remarkably, our dated phylogenies reveal that the diversification of the clade that includes specialist grass feeders has followed closely the expansion of grasslands in the Miocene, similar to the adaptive radiation of specialist grazing mammals during the same period.

The role of wild host plants in the abundance of lepidopteran stem borers along altitudinal gradient in Kenya

Abstract Presence of wild host plants of stem borers in cereal-growing areas has been considered as reservoirs of lepidopteran stem borers, responsible for attack of crops during the growing season. Surveys to catalogue hosts and borers as well as to assess the abundance of the hosts were carried out during the cropping and non-cropping seasons in different agro-ecological zones along varying altitude gradient in Kenya. A total of 61 stem borer species belonging to families Noctuidae (25), Crambidae (14), Pyralidae (9), Tortricidae (11) and Cossidae (2) were recovered from 42 wild plant species. Two noctuids, Busseola fusca (Fuller), Sesamia calamistis Hampson, and two crambids, Chilo partellus (Swinhoe) and Chilo orichalcociliellus (Strand) were the four main borer species found associated with maize plants. In the wild, B. fusca was recovered from a limited number of host plant species and among them were Sorghum arundinaceum (Desvaux) Stapf, Setaria megaphylla (Steudel) T. Durand & Schinz, Arundo donax L. and Pennisetum purpureum Schumacher. In contrast, the host range of C. partellus was considerably wider [13 for S. calamistis]. However, the number of larvae of these species was lower in the wild compared to cultivated fields, thus the role of natural habitat as a reservoir for cereal stem borers requires further studies. Importance of the wild host plants as well as borer diversity along the altitudinal gradient is discussed.

Intraspecific specialization of the generalist parasitoid Cotesia sesamiae revealed by polyDNAvirus polymorphism and associated with different Wolbachia infection

As a result of an intense host–parasite evolutionary arms race, parasitic wasps frequently display high levels of specialization on very few host species. For instance, in braconid wasps very few generalist species have been described. However, within this family, Cotesia sesamiae is a generalist species that is widespread in sub‐Saharan Africa and develops on several lepidopteran hosts. In this study, we tested the hypothesis that C. sesamiae may be a cryptic specialist when examined at the intraspecific level. We sequenced exon 2 of CrV1, a gene of the symbiotic polyDNAvirus that is integrated into the wasp genome and is associated with host immune suppression. We found that CrV1 genotype was more closely associated with the host in which the parasitoid developed than any abiotic environmental factor tested. We also tested a correlation between CrV1 genotype and an infection with Wolbachia bacteria, which are known for their ability to induce reproductive isolation. The Wolbachia bacteria infection polymorphism was also found as a major factor explaining the genetic structure of CrV1, and, in addition, the best model explaining CrV1 genetic structure involved an interaction between Wolbachia infection and host species. We suggest that Wolbachia could act as an agent capable of maintaining advantageous alleles for host specialization in different populations of C. sesamiae. This mechanism could be applicable to other insect models because of the high prevalence of Wolbachia in insects.

Phylogenetic Molecular Species Delimitations Unravel Potential New Species in the Pest Genus Spodoptera Guenée, 1852 (Lepidoptera, Noctuidae)

Nowadays molecular species delimitation methods promote the identification of species boundaries within complex taxonomic groups by adopting innovative species concepts and theories (e.g. branching patterns, coalescence). As some of them can efficiently deal with large single-locus datasets, they could speed up the process of species discovery compared to more time consuming molecular methods, and benefit from the existence of large public datasets; these methods can also particularly favour scientific research and actions dealing with threatened or economically important taxa. In this study we aim to investigate and clarify the status of economically important moths species belonging to the genus Spodoptera (Lepidoptera, Noctuidae), a complex group in which previous phylogenetic analyses and integrative approaches already suggested the possible occurrence of cryptic species and taxonomic ambiguities. In this work, the effectiveness of innovative (and faster) species delimitation approaches to infer putative species boundaries has been successfully tested in Spodoptera, by processing the most comprehensive dataset (in terms of number of species and specimens) ever achieved; results are congruent and reliable, irrespective of the set of parameters and phylogenetic models applied. Our analyses confirm the existence of three potential new species clusters (for S. exigua (Hübner, 1808), S. frugiperda (J.E. Smith, 1797) and S. mauritia (Boisduval, 1833)) and support the synonymy of S. marima (Schaus, 1904) with S. ornithogalli (Guenée, 1852). They also highlight the ambiguity of the status of S. cosmiodes (Walker, 1858) and S. descoinsi Lalanne-Cassou & Silvain, 1994. This case study highlights the interest of molecular species delimitation methods as valuable tools for species discovery and to emphasize taxonomic ambiguities.

Ecology of the African Maize Stalk Borer, Busseola fusca (Lepidoptera: Noctuidae) with Special Reference to Insect-Plant Interactions

Busseola fusca (Lepidoptera: Noctuidae) is an important pest of maize and sorghum in sub-Saharan Africa. One century after its first description by Fuller in 1901, inaccurate information based on earlier reports are still propagated on its distribution (e.g., absent from the lower altitudes in East Africa) and host plant range (e.g., feeding on a large range of wild grass species). This review provides updated information on the biology, distribution and genetics of B. fusca with emphasis on insect-plant interactions. Related to this, new avenues of stem borer management are proposed.

Phylogenetics, species boundaries and timing of resource tracking in a highly specialized group of seed beetles (Coleoptera: Chrysomelidae: Bruchinae).

Though for a long time it was hypothesized that the extraordinary diversity of phytophagous insects was better explained by a synchronous pattern of co-diversification with plants, the results of recent studies have led to question this theory, suggesting that the diversification of insects occurred well after that of their hosts. In this study we address this issue by investigating the timing of diversification of a highly specialized group of seed beetles, which mostly feeds on legume plants from the tribe Indigofereae. To that purpose, a total of 130 specimens were sequenced for six genes and analyzed under a Bayesian phylogenetic framework. Based on the resulting trees we performed several analyses that allowed a better definition of the group boundaries and to investigate the status of several taxa through the use of molecular species delimitation analyses in combination with morphological evidences. In addition the evolution of host plant use was reconstructed and different molecular-dating approaches were carried out in order to assess the ages of several clades of interest. The resulting framework suggests a more ancient than previously thought origin for seed beetles, and a pattern of rapid host plant colonization. These findings call for further similar studies in other highly specialized groups of phytophagous insects.

Parasitism of Lepidopterous Stem Borers in Cultivated and Natural Habitats

Abstract Plant infestation, stem borer density, parasitism, and parasitoid abundance were assessed during two years in two host plants, Zea mays (L.) (Cyperales: Poaceae) and Sorghum bicolor (L.) (Cyperales: Poaceae), in cultivated habitats. The four major host plants (Cyperus spp., Panicum spp., Pennisetum spp., and Sorghum spp.) found in natural habitats were also assessed, and both the cultivated and natural habitat species occurred in four agroecological zones in Kenya. Across habitats, plant infestation (23.2%), stem borer density (2.2 per plant), and larval parasitism (15.0%) were highest in maize in cultivated habitats. Pupal parasitism was not higher than 4.7% in both habitats, and did not vary with locality during each season or with host plant between each season. Cotesia sesamiae (Cameron) and C. flavipes Cameron (Hymenoptera: Braconidae) were the key parasitoids in cultivated habitats (both species accounted for 76.4% of parasitized stem borers in cereal crops), but not in natural habitats (the two Cotesia species accounted for 14.5% of parasitized stem borers in wild host plants). No single parasitoid species exerted high parasitism rates on stem borer populations in wild host plants. Low stem borer densities across seasons in natural habitats indicate that cereal stem borer pests do not necessarily survive the non-cropping season feeding actively in wild host plants. Although natural habitats provided refuges for some parasitoid species, stem borer parasitism was generally low in wild host plants. Overall, because parasitoids contribute little in reducing cereal stem borer pest populations in cultivated habitats, there is need to further enhance their effectiveness in the field to regulate these pests.