Hervé Maxime

PhD thesis started October 1^st, 2011
PhD defended october 15th, 2014
Fund: INRA (Junior Scientist Grant)
Direction: Anne-Marie Cortesero & Régine Delourme

Abstract:

Plants display multiple defense systems against phytophagous insects. Manipulating these defenses by means of selection could contribute to decrease damages caused by insect pests, by increasing natural resistance of crops. This strategy faces great constraints when applied to insects.

We first detail these constraints and then propose an alternative approach to classical methods. It consists in identifying key plant traits that determine the intensity of the interaction between the plant and the pest. If such traits are identified, selection could be conducted on the basis if these sole traits, without needing any insect.

We tested this approach in a system composed of oilseed rape (Brassica napus) and the pollen beetle (Meligethes aeneus), a major pest of this culture. This coleopteran generalist pollen feeder lays eggs only on certain brassicaceous plants. Agronomical damage arise before plant flowering and are caused by adults, which destroy flower buds to get the pollen they contain.

Four crucial steps of the interaction were studied: attraction, adult feeding, egg production and oviposition, and larval development. Six oilseed rape genotypes were compared in a series of experiments conducted in the laboratory. By linking insect preference/performance to large metabolic profiling of bud tissues, we identified candidate key traits. Main conclusions of this work are (i) that biochemical composition of the perianth, especially a few compounds among which sucrose, is determinant for feeding intensity; (ii) that feeding stimulation has an important impact on egg production by constraining oogenesis; (iii) that pollen nutritional quality, probably mostly determined by starch and some glucosinolates, interacts with both pollen beetle larvae and adults. Combination of several results also allows drawing more general hypotheses about the oilseed rape - pollen beetle interaction.  One of these is that the agronomical context in which the interaction takes place may have largely influenced, or even disturbed, the interaction that linked this insect and wild brassicaceous plants before oilseed rape cultivation.

This thesis showed that a new way might be possible to protect cultures against insect pests. It could be both efficient and sustainable, especially in systems where agronomical damage is caused at a temporary vulnerable plant stage.