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Thesis started 2014 - Defended in June 15
Funding: 50% Région Bretagne - 50% Chaire AEI
Direction: Anne Le Ralec, Anne-Marie Cortesero, Vincent Faloya

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Abstract

In the perspective of developing low chemical input farming systems, it is necessary to assess the efficiency of alternative crop protection techniques. Conservation biological control is based on the intensification of natural regulation services provided by pest natural enemies and is thought to be a promising lever for insect pest control. The main aim of this thesis is to quantify the efficiency of natural regulation, both on pest populations and crop production. As this last dimension of regulation efficiency is only measurable when pests actually damage crops, we first quantified their harmfulness, thereby assessing the maximum residual pest population acceptable. We then quantified the number and proportion of insect pests suppressed by predators and parasitoids and we aimed at identifying the most efficient natural enemy species or functional groups. Finally, as some of these natural enemies overwinter inside the fields and may therefore be susceptible to disturbances at the field scale, we assessed the effects of soil tillage in early spring on their emergence rate and on the level of pest regulation they provided. We worked on two types of pests (the cabbage root fly and aphids) attacking two types of brassicaceous vegetables (broccoli and turnip). This work was conducted both in experimental plots and in the fields of farmers affiliated with two partnering agricultural cooperatives. We show that the cabbage root fly is extremely harmful on turnip and that damage is more variable on broccoli. On the latter, plants may die when attacked early by the pest, but their growth may also be impacted by low pest densities, leading to production delays. We then demonstrate that ground dwelling predators significantly control both types of pests, leading to a significant decrease in plant mortality. We also show that predator precocity is a key trait determining the efficiency of this regulation. Parasitoids of the cabbage root fly also make a high contribution to pest regulation but eliminate the pest too late to prevent damage in the short term. Finally, we show that simplifying soil tillage in early spring is not sufficient to enhance pest natural regulation but that such practices could enhance regulation services in the longer term. This thesis opens new perspectives concerning conservation biological control practices, particularly relying on the preservation of early generalist predators.

Poster pdf