Landscape heterogeneity and a competing herbivore reduce olive fruit infestation by the pest Prays oleae

Monocultural agricultural systems can favour arthropod pests through direct effects, such as promoting pest dispersion, and indirect effects, such as limiting natural pest control. Therefore, promoting the presence of non-crop habitats in agricultural landscapes has the potential to reduce pest damage. However, likely in part due to the trophic complexity surrounding arthropod pests, these effects vary strongly among different pests and crops, and they are rarely consistent on a year-to-year basis. Here, we formulated Generalized Additive Mixed Models with data of 11 years from 25 olive orchards from southern Spain to evaluate the effects of landscape composition at different spatial scales on fruit infestation by the moth Prays oleae, a key Mediterranean olive pest. Then, we evaluated whether these effects were modulated by natural enemies or heterospecific herbivores. Fruit infestation by P. oleae was affected by landscape composition at small spatial scales (100 m radius), but not at medium or large scales (500 m and 1000 m radii). Fruit infestation was negatively affected by the proportion of semi-natural habitats, while it was positively affected by the proportion of olive groves in the landscape. We did not find evidence that these effects were modulated by natural enemies. However, fruit infestation by P. oleae was negatively affected by the presence of Euphyllura olivina, which is considered a secondary olive pest. Overall, this study shows that the negative response of P. oleae to habitat heterogeneity is consistent across years, but these effects were found only at smaller spatial scales. Therefore, we recommend implementing patches of semi-natural vegetation within olive groves as it can help reduce crop damage caused by P. oleae. Specifically, our models showed that maintaining 25 % of non-crop vegetation within 100 m radius buffer zones would reduce by half the proportion of fruits infested by P. oleae. Interestingly, this study also suggests that the secondary pest E. olivina can prevent fruit infestation by P. oleae, which in turn could increase crop yield.