Laurent Penet

Diaphania hyalinata Linnaeus, 1767 (Lepidoptera, Crambidae), commonly known as the melonworm, is a major pest of cucurbits in the Neotropics. Yet, a comprehensive review of the parasitoid wasps that may act as biocontrol agents of this pest is still lacking. An extensive literature review was conducted to compile the current knowledge on the identity, distribution, and biology of parasitoid wasps associated with D. hyalinata across its range. Eggs, larvae, and pupae of D. hyalinata were collected in Guadeloupe and reared to document the local parasitoid diversity. Among the 37 parasitoids identified in our review, eight were recovered during our field survey, all of which are new records for the island. These parasitoids were photographed and DNA-barcoded to facilitate their potential use in future biological control programs targeting D. hyalinata . The species Trichogramma pretiosa Riley, 1879 (Trichogrammatidae; egg parasitoid) and Schoenlandella montserratensis Kang, 2021 (Braconidae; larval parasitoid) appear to be promising candidates for the biological control of D. hyalinata in Guadeloupe. It may be also worth evaluating the effectiveness of two additional larval parasitoids, Eiphosoma dentator Fabricius, 1804 (Ichneumonidae) and Apanteles impiger Muesebeck, 1958 (Braconidae), while assessing possible non-target effects on local fauna. However, it is important to consider that the presence of the hyperparasitoid Aphanogmus fijiensis Ferrière, 1933 (Ceraphronidae) could undermine biocontrol efforts. Overall, it would be valuable to gain a better understanding of the biological and environmental factors (such as agricultural practices and landscape features) that enhance the natural regulatory activities of this parasitoid community.

Mitigating the biodiversity crisis will require an understanding of how species cope with various human disturbances of habitats ranging from fully natural environments to areas with higher anthropization levels. In this study, we investigated how the local demographics of Clonistria guadeloupensis (Redtenbacher) relate to human disturbances such as agricultural fields, roads, and urbanization in Guadeloupe. We sampled the species in diverse environmental situations in the wild, both on and around focal food plants, and recorded the habitat characteristics. We subsequently analyzed the demography of the juvenile, subadult, and adult stages. We found minimal impact of anthropization factors, with a tendency of the species to be associated with habitats with greater anthropization levels, possibly because such habitats are generally more open. We also found variation in local aggregation levels of the developmental stages, which may reflect bias in dispersal during the adult stage in which males are highly motile. Overall, the species shows ecological preferences appropriate for coexistence within areas of human disturbance.

Crop disease often leads to field epidemics with serious threats to yield. Early symptoms are sometimes difficult to identify, so the origin of primary inoculum is a critical focal point in the study of plant diseases, as it can help design management strategies to reduce crop losses. Here, we investigated whether anthracnose of water yams (Dioscorea alata L.) caused by the species complex Colletotrichum gloeosporioides can start from infected seed tubers from the previous harvest. Over two years, we collected tubers with varying pathogen prevalence in the field directly from producers and conducted fungal isolations in the lab to sample C. gloeosporioides. We also proceeded to artificially inoculate tubers before planting and monitored disease development. Finally, we genotyped isolates from leaves in the fields and assessed fixation indices between plots based on plot ownership (plots with a common seed tuber origin from a single farmer) vs. samples in plots from unrelated producers in Guadeloupe, Martinique, and Barbados. We were unable to isolate the fungus from harvested tubers in either sampling survey nor did any plants grown from inoculated tubers develop any disease symptoms during growth. Also, the genetic structure of samples within each plot was independent of plot ownership, though this occurred with varying levels in the different islands. These results suggest that contaminated planting material from seed tubers is not the primary source of the disease, which is in contrast to the common perception of yam anthracnose prevalence in the Antilles.

Colletotrichum gloeosporioides is a species complex of agricultural importance as it causes anthracnose disease on many crop species worldwide, and strong impact regionally on Water Yam (Dioscorea alata) in the Caribbean. In this study, we conducted a genetic analysis of the fungi complex in three islands of the Lesser Antilles—Guadeloupe (Basse Terre, Grande Terre and Marie Galante), Martinique and Barbados. We specifically sampled yam fields and assessed the genetic diversity of strains with four microsatellite markers. We found a very high genetic diversity of all strains on each island, and intermediate to strong levels of genetic structure between islands. Migration rates were quite diverse either within (local dispersal) or between islands (long-distance dispersal), suggesting important roles of vegetation and climate as local barriers, and winds as an important factor in long-distance migration. Three distinct genetic clusters highlighted different species entities, though there was also evidence of frequent intermediates between two clusters, suggesting recurrent recombination between putative species. Together, these results demonstrated asymmetries in gene flow both between islands and clusters, and suggested the need for new approaches to anthracnose disease risk control at a regional level.

Crop pathogenic fungi may originate from reservoir pools including wild vegetation surrounding fields, and it is thus important to characterize any potential source of pathogens. We therefore investigated natural vegetation’s potential for hosting a widespread pathogenic group, Colletotrichum gloeosporioides species complex. We stratified sampling in different forest environments and natural vegetation strata to determine whether the fungi were found preferentially in specific niches and areas. We found that the fungi complex was fairly broadly distributed in the wild flora, with high prevalence in every study environment and stratum. Some significant variation in prevalence nevertheless occurred and was possibly associated with fungal growth conditions (more humid areas had greater prevalence levels while drier places had slightly lower presence). Results also highlighted potential differences in disease effects of strains between strata components of study flora, suggesting that while natural vegetation is a highly probable source of inoculums for local crops nearby, differences in aggressiveness between vegetation strata might also lead to differential impact on cultivated crops.

Ecological interactions are a fairly overlooked component in agriculture, despite their known potential in productivity, pest and disease regulation. In this study, we assessed whether a potential low pathogenic congeneric competitor of Colletotrichum gloeosporioides could decrease disease onset in yams. We set up an experiment with artificial drop inoculation on detached leaves, with individual controls and treatments as either co‐inoculation or inoculation with a harmless competitor 24 h prior to inoculating the pathogen. Our results demonstrate that direct admixing translated into accrued competition and disease symptoms, while prior inoculation of the competitor tended to decrease symptom development. There was variation in how varieties responded to treatments and in strain competitive ability altering the protective effect, but nevertheless these results suggest that incorporating such interactions in breeding programmes or working towards increased use of commensals in agriculture might confer improved disease control for crops.

Landscape effects might impede or increase spore dispersal and disease risk for crops, as trees and hedges buffer winds and can behave as spore traps, therefore limiting diffusion of fungi, or, on the contrary, behave as disease relay once vegetation is infected and become inoculum sources. In this study, we investigated weekly prevalence of the pathogenic fungus Colletotrichum gloeosporioides on guava tree leaves, differentiating impacts of leaf height on tree, age, and location within leaf. We first estimated differences in prevalence for each covariate, and then related infection rates to weather effects during the year. Our results highlighted a great variance of prevalence among individual trees, and a lower contamination of tree tops, as well as a tendency for greater odds of infection in tips of young leaves compared to older ones. Last, we show evidence that individual tree contaminations are associated with different disease dynamics: early and dispersal-based, late and growth-based, as well as with intermediate dynamic ranges. Pathogen infection dynamics will thus be greatly impacted by cover characteristics at local scale, and tree cover should not be perceived as homogeneously driving disease levels.

The transition toward sustainable agriculture requires rethinking cropping systems in the light of less intensive and chemically reliant practices. Weed management is one of the target practices to evolve cropping systems with decreased impact on the environment. While softened management will lead to increased weeds/crops coexistence, it is of importance to assess the relative benefits and drawbacks of new practices. Among the potential drawbacks of weeds/crops coexistence, disease risk may increase if weeds are hosting pathogens. In this study, we assessed the potential of weeds for hosting pathogenic generalist fungi known to translate into disease in crops. We first describe prevalence in fields after harvest and relate prevalence to species characteristics and communities. Then, we directly test the idea that weeds serve as inoculums sources during cropping with a natural experiment. This study highlights variation in host skill among feral weeds for Colletotrichum species, including potential congeneric sub-specialization on different weeds within communities. Last, prevalence within fields was more correlated to focal crop inoculation rates compared to local weed load, but there was a significant correlation effect with prevalence on weeds in the vicinity of fields, suggesting that weeds are mediating disease levels at the local scale, too. Results pointed to the importance of weed host skill in disease risk yet open the door to the potential control of pathogens via targeted weed management.