Can landscape protect crops against pests?

Effect of factors relative to the field plot and landscape

Such studies often focus on one species or one spatial scale. We have on the contrary studied simultaneously the effect of local factors (directly relative to the field plot) and landscape features (500m zone around the crop) on a pest and several of its natural enemies. We measured crop colonization by the pest, the regulatory impact of natural enemies and the final crop damage. Our model was the root maggot Delia radicum, a major pest of cole crops (e.g. cabbage), and its main predators and parasitoid insects.

Vast complexity of interactions

Our study reveals the vast complexity of interactions at play. Indeed, the pest and its natural enemies react both to local field characteristics (including cultivation method) and to landscape features (at least 500m around the crop), but not in the same way. Some of our results are counter-intuitive: colonization of the crop by the pest is reduced when cole crops are abundant nearby, and it is not increased after a cole crop precedent in the field. On the contrary, fields surrounded by hedges might increase colonization and therefore, damage, but this tendency will have to be confirmed over the three year of the study. Other results were expected, such as a higher presence of natural enemies when the landscape is richer in woody areas, which are known to act as reservoirs.

In the end, our study underlines the formidable challenge of using landscape ecology against agricultural pests. It suggests indeed that it will probably be difficult to obtain a pest control equivalent to classical methods by manipulating landscape variables, even if our results demonstrate that it is fortunately not necessary to modify the agricultural landscape at a very large scale to obtain measurable effects.

Josso C., Le Ralec A., Raymond L., Saulais J., Baudry J., Poinsot D. & Cortesero A.M. (2013). Effects of field and landscape variables on crop colonization and biological control of the cabbage root fly Delia radicum. Landscape Ecology, 28(9): 1697-1715. DOI