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Project leader: Laure KAISER-ARNAULD (EGCE)      

Partners :

• Inside BASC: GQE-Le Moulon
• Outside BASC: Research Institute on Insect Biology (IRBI, UMR UMR CNRS 7261 / Univ. F. Rabelais, Tours) 
• non academic: Arvalis

The maize sesamia is an insect that is on the increase in France due to the mild winter and there are no longer any effective means of control. The use of natural enemies is expected. Cotesia typhae (photo above) is a parasitoid insect newly characterised in East Africa where it specifically attacks this sesamia. Its introduction in infested areas is therefore an interesting avenue for biological control.

The scientific challenges are:

1. to evaluate the environmental risk of its introduction into France;
2. to understand the efficiency of the parasitoid strains by studying the components of their reproductive success;
3. to estimate the impact of temperature on male fertility, which is particularly sensitive to this factor;
4. to simulate the dynamics and impact of the parasitoid following a release in order to identify missing data and study control strategies.

Photos: Left: Maize sesamia caterpillar feeding inside a young ear of corn / Centre: Female adult of the caterpillar parasite Cotesia sesamiae / Right: When mature, the larvae of Cotesia sesamiae leave the body of the caterpillar where they have developed, to form their pupal cocoon around the corpse.

The Sesa Control project has made progress on all four scientific issues. To mention just two results:

• Concerning the components of reproductive success, we have shown that parasitoid populations that are several hundred kilometres apart show significant differentiation in their oviposition behaviour and virulence towards the target host insect, explained by adaptations to local host populations [1] ; we characterised the dynamics of egg and sperm production, as a preamble to the study of their sensitivity to temperature variations, which will be studied later.
• Concerning the simulation of the action of the parasitoid following a release, a model has been developed that couples the population dynamics of the pest and that of the parasitoid. The current version integrates a phenology model for S. nonagrioides but not for the parasitoid. Simulations were carried out showing that the season's temperature as well as the dates and composition of the release (eggs/larvae) could have an impact on the effectiveness of the biological control [2].

This project was essential for starting and completing the studies developed more widely within the framework of the ANR CoteBio project. It also made it possible to start mathematical modelling work with GQE-Le Moulon, which will be continued within the framework of the Phenofore project financed by the GNIS and coordinated by GQE-Le Moulon and involving EGCE. The withdrawal of the Bioline company was compensated for by a collaboration with the Institut du Végétal Arvalis, allowing access to long-term field data, raising farmers' awareness of biological control, and the setting up of the GNIS Phenofore project.

===> The researchers explain the project and its RESULTS in VIDEO (LabEx Scientific Day, January 2021)

References

[1] Benoist R., Paquet S., Decourcelle F., Guez J, Jeannette R., Calatayud P.-A., Le Ru B., Mougel F. Kaiser L. 2020. Role of egg-laying behavior, virulence and local adaptation in a parasitoid’s chances of reproducing in a new host. J. Insect Physiol. 120, 103987. https://doi.org/10.1016/j.jinsphys.2019.103987

[2] Rosero P. 2019. Development of a mathematical model to assess the efficacy of a biological control agent against a maize pest. Mémoire M2 Ecologie Evolutive et Fonctionnelle - Ecologie Théorique et Modélisation, U. P. Saclay.

Publications

> Benoist R., Paquet S., Decourcelle F., Guez J, Jeannette R., Calatayud P.-A., Le Ru B., Mougel F. Kaiser L. 2020. Role of egg-laying behavior, virulence and local adaptation in a parasitoid’s chances of reproducing in a new host. J. Insect Physiol. 120, 103987. https://doi.org/10.1016/j.jinsphys.2019.103987. Resume: "Understanding the ability of parasitoid insects to succeed in new host populations is a relevant question for biological control and adaptive mechanisms. Cotesia typhae is an African parasitoid specialized on the moth Sesamiae nonagrioides, also called the Mediterranean corn borer. Two Kenyan strains of C. typhae differ in their virulence against a new host population from France. We explored behavioral and physiological hypotheses about this differentiation. Cotesia genus belongs to a group of Hymenoptera in which females inject a domesticated virus in their host to overcome its resistance. Since viral particles are injected along with eggs and since the strain with the higher virulence injects more eggs, we hypothesized that virulence could be explained by the quantity of virus injected. To test this assumption, we measured the injected quantities of eggs and viral particles (estimated by viral DNA segments) of each parasitoid strain along several ovipositions, to vary these quantities. Unexpectedly, results showed that virulence against the French host was not correlated to the injected quantities of eggs or viral segments, indicating that virulence differentiation is explained by other causes. The virulence against the respective natural hosts of the two C. typhae strains was also measured, and results suggest that local adaptation to a more resistant natural host may explain the pre-adaptation of one strain to the new host population. We also identified a differentiation of oviposition strategy and subsequent offspring number between the parasitoid strains, which is important in a biocontrol perspective."

Outreach

> Kaiser L & Marques Fortuna T. 2020. Cotesia typhae, une nouvelle espèce prometteuse pour la lutte biologique. Passion Entomologie, 2 juin 2020. https://passion-entomologie.fr/cotesia-typhae-lutte-biologique2-2/. "Cotesia typhae Fernandez-Triana (Hymenoptera, Braconidae) est un Hyménoptère d’Afrique sub-saharienne, parasitoïde des chenilles de Sesamia nonagrioides Lefebvre (Lepidoptera, Noctuidae). Ces chenilles mangent l’intérieur des tiges de nombreuses espèces de plantes du groupe des Poales. Originaire d’Afrique, ce papillon ravage le maïs dans les pays du Sud de l’Europe, dont la France."