The influence of management and environmental factors on insect attack on cork oak canopy

Abstract

The decline of oak (Quercus spp.) forests is a current trend in Northern Hemisphere and is characterized by a loss in tree vigour and increased mortality. The canopy insects are suspected to have role in this decline, but there is poor knowledge about their incidence in evergreen-oak stands. The main aim of this study is to characterize the incidence of main insect groups affecting branches and leaves of an evergreen-oak species (the cork oak Quercus suber) and evaluate which management practices and environmental traits of agroforestry systems affect it. In the spring/summer of 2018, we measured the incidence of attacks on branches and leaves by gall-makers (Diptera, Cecidomyiidae), blade-miners (Lepidoptera and Coleoptera), midrib-miners (Lepidoptera, Heliozelidae), chewer caterpillars (Lepidoptera), chewer sawflies (Hymenoptera, Tenthredinidae), weevils (Coleoptera, Curculionidae) and branch-borers (Coleoptera, Buprestidae). We analysed the frequency of pest signs according to different indicators: management practices, tree maturity, forest diversity, forest fragmentation, and latitude and longitude. The most frequent signs of insect pests on tree leaves corresponded to blade-miners, midrib-miners, chewer caterpillars and chewer sawflies. With exception of midrib-miners and branch-borers, all insect pests were found on cork oak stands experiencing decline and benefited from management intensification. Our study suggests that a diverse-aged stand may reduce the frequency of midrib-miners and chewer caterpillars, as well the attack of branch-borers. Moreover, a high plant diversity in forests can contribute to reduce the impact of defoliators on cork oaks (e.g., chewer sawflies) and understory reduction decreased the exposure of trees to gall-makers. Moreover, we found that forest fragmentation may increase the frequency of blade-miners and chewer caterpillars. We conclude that insect pests have a high incidence in cork oak stands and thus, may have and important role in its decline. Considering that a high frequency of pests is often associated with increased management intensity, a change to a more sustainable use of these systems is urgently needed.