Role of landscape on the distribution and abundance of aphids and their predators carabid.
Aphids cause extensive damage to cereal crops and it is increasingly crucial to find alternatives to chemical control. Following the French "Grenelle de l‟Environnement", the necessity to develop a sustainable and environmentally friendly agriculture requires more research on biological control. In agricultural landscapes, the natural control of aphids is provided by various parasites and a diverse guild of predators which may play an important regulatory role in the early phases of the breeding season. The objective of this thesis is to compare the colonization aphids and their carabids predators distribution in wheat and maize fields by in contrasted landscape contexts. The work was conducted for two years on 24 cereal plots within an agricultural landscape of the ZA Armorique. We tested the effect of various landscape variables on the diversity and abundance of aphids and carabid beetles communities in wheat and maize fields. At the landscape scale, we have shown that the "fine-grain" landscape promotes the abundance of aphids in wheat but not in maize. The "coarse-grain" favors the abundance of carabid beetles in wheat and in maize. The increase in cultivated land leads to a loss of species of carabid beetles and replacement of large species by small ones. At the landscape context scale, there is a positive correlation between the abundance of aphids and the proportion of woodland and grassland at large scale (500 m and 800 m). There is a negative correlation between the abundance of carabids and the proportion of grassland and hedgerow around crop fields. The species richness of carabids is positively correlated with the proportion of hedgerows and grassland. The proportion of grassy strips affects negatively the abundance of aphids in the wheat fields, and positively in maize fields. Carabid abundance is positively correlated with the proportion of grassy strips at large scale in both types of cultures. At the local scale, the total numbers of aphids were negatively correlated with the distance from the field margin. The total numbers of carabids were positively correlated with the distance from the field margin. The presence of grass strips so weakly influences the abundance of aphids. In our study, the total abundance of aphids and abundance of carabid beetles were significantly positively correlated in the in "coarse-grain" landscape for both cultures, while in "fine-grain" landscape, this correlation was negative for wheat and positive for maize. Our results showed that spatial distribution and aphid/carabid relationships are complex as they vary according to the landscape structure, the landscape context and the type of culture.