Trophic relationships among carabid beetles and their potential for biological control

Background/Question/Methods

Agricultural landscapes host an important part of common biodiversity. Recent intensification of agricultural practices allowed satisfying food demand of growing populations but was detrimental for the majority of species and ecosystem services they provide. The promotion of biological control of pests by their natural enemies is a promising alternative to chemical treatments for crop protection and identifying factors enhancing this ecosystem service is fundamental.

Many studies failed in finding evidence for a clear relationship between pest regulation and natural enemies’ biodiversity. Our hypothesis is that regulation results rather from the structure of interaction network than from the number and abundance of species. However, even the simple description of interactions networks remained until recently hardly approachable. The emergence of environmental genomics, thanks to new molecular technologies, such as barcoding and high-throughput sequencing, opens exciting opportunities for studying trophic webs at a large scale.

Ground beetles are a group of particular interest as they are suspected to play an important role in regulating pest populations in agricultural fields. However, their diet remains difficult to precise in the field, and their role as auxiliaries remains still controversial.

In order to assess the potential of carabid beetles as biological control agents, we developed high-throughput sequencing approach in order to identify preys consumed by the most common carabid species encountered in main crops of an agricultural Breton landscape.

Living individuals were trapped in six fields of cereals and oilseed rape within the Long Term Research Area “Armorique” situated to the south of the Mont-Saint-Michel Bay in France. Faeces containing prey remains were amplified with two sets of 16Smt universal primers for arthropods/mollusks and earthworms/ice worms, one set of 16Smt carabid specific primers and one set of trnL chloroplastic primers targeting plants.

Results/Conclusions

A set of 225 000 sequences obtained with 454 Roche technology for the three most common carabid species, reveals a low global diversity of preys consumed. With regard to animals, most sequences were assigned to the genus Pardosa (Lycosidae), a group of very common spiders in agricultural fields. In plants, a wider diversity was found, in particular Poecilus cupreus was showed to feed on an unsuspected diversity of plants. Moreover, a large number of sequences were assigned to the genus Juncus comprising typical plants growing in grasslands suggesting a high mobility of the species. Finally, the three species under study were showed to consume cultivated plants, especially oil-seed rape.