Inter- and intra-guild interactions related to aphids in nettle (Urtica dioica L.) strips closed to field crops.

A field experiment designed to assess the biodiversity related to nettle strips closed to crops, and more particularly the aphid and related beneficial populations, was established in experimental farm located in Gembloux (Belgium). Margin strips of nettle (Urtica dioica) closed to wheat (Triticum aestivum), green pea (Pisum sativum) and rape (Brassicae napus) fields were investigated. The diversity, abundance of aphids and related predators were analysed according to the plant crop species and the differential pesticide application (treated plot and control). Insects were visually observed every week during all the cultivation season. Two main families of aphidophagous predators were found in all field crops and nettle, the Coccinellidae and Syrphidae. The diversity of the aphidophagous predators was shown to be higher on nettle than in field crops, particularly the Chrysopidae, the Anthocoridae and the Miridae. However, a striking difference of ladybird abundance was observed according to the aphid host plant. In one side, Coccinella septempunctata was much more abundant on Acyrthosiphon pisum infested green pea than on the other host plant species. At the opposite, higher occurrence of Harmonia axyridis was observed on the aphid infested nettle plants than on the crop plants. In particular, none of H. axyridis was found in wheat crop. Also, more than only a significant positive correlation between predator and aphid abundance, specialised relations between particular aphid species and some so-called generalist predators was determined in the fields. Finally, intraguild interactions between the aphidophagous predators was assessed and shown that only a significant negative correlation between Episyrphus balteatus and H. axyridis related to the nettle aphid, Micrlophium carnosum, was observed. The relative distribution of the ladybirds, namely C. septempunctata and H. axyridis according to the host plant, nettle strips and crop plots was discussed in relation to integrated pest management approach.