Indirect effects of genetically modified herbicide tolerant crops on biodiversity and ecosystem services: the biological control example

Weeds can directly affect arthropods by providing favorable microclimates, food, shelter, mating sites and oviposition substrates. Indirect effects on herbivores and higher trophic level arthropods can occur if weed species interact with each other and with crop plants modifying plant physiology, chemical and visual cues and competing for resources. Hence, weed management can interfere in many different ways with arthropods. Evidence from literature is that our understanding of the mechanisms underlying the observed effects is still very limited. Higher plant diversity does not necessarily mean increased biological control. Genetically Modified Herbicide-Tolerant (GMHT) crops offer new possibilities for weed management with potential consequences for conservation biological control. For example, farmers have more flexibility in timing herbicide applications (early vs. late overall sprays), and band spraying in row crops have been shown to modify weed abundance and composition in favor of a number of predators and parasitoids. There is some evidence that maximum weed control with overall glyphosate spray of GMHT crops results in reduced weed biomass and lower arthropod abundance, including beneficial species. Conversely, many articles show that maintaining some weed enhances arthropod densities and natural biological control. GMHT crops may increase adoption of minimum and no tillage systems with possible effects on weeds and arthropods. Weed management systems with GMHT plants have a great potential to alter plant diversity in crops in favor of conservation biological control. However, careful consideration of other management options such as crop and herbicide rotation should be evaluated to make sustainable use of the benefits of GMHT crops.