Synergistic effects of ground cover and adjacent vegetation on natural enemies of olive insect pests

The use of pesticides in conventional agriculture poses several risks to humans and to the environment, and may turn out to be inefficient in the long-term as pests can develop resistance to pesticides. Non-chemical control methods can be preferable to prevent pest damage. One way to achieve this involves the establishment of ground cover or the restoration of vegetation adjacent to the crop. Either of these methods can effectively increase the abundance of natural enemies, particularly in perennial crops, but their interaction has been typically neglected. In this study we used maximum likelihood methods to analyse the synergistic effects of ground cover and different types of adjacent vegetation (herbaceous, woody) on the abundance of the main natural enemy groups of insect pests in olive groves. A Gaussian function was used to predict their abundance as a response of time, ground cover, different types of adjacent vegetation and year (2010, 2011). We examined 40 different alternative models for each group of natural enemies: spiders, ants, predatory Heteroptera, and parasitoids. Spiders, parasitoids, and one species of predatory Heteroptera (Deraeocoris punctum), showed a greater abundance in ground cover plots. Overall, herbaceous and large woody vegetation adjacent to the crop influenced the abundance of natural enemies more than small woody vegetation. However, this effect was modulated by ground cover. When both structures were present in the crop, the abundance of some groups of natural enemies (spider and parasitoids) was positively influenced by adjacent vegetation, whereas this effect was lower or even reversed in bare soil crops. We thus encourage olive farmers to use both habitat management approaches simultaneously, since the interaction of these ecological infrastructures produce an effect that maximises the abundance of natural enemies.