BIOLOGICAL PEST-CONTROL

For the purposes of energy forests, we argue that biological pest control should be interpreted as any method of using natural organisms or their products for the regulation of herbivores, below the economic threshold. The organisms include the energy forest crop species and natural enemies of pest herbivores. In natural vegetation, potentially harmful arthropod herbivores are normally regulated at low populations through a combination of bottom-up forces in food webs including edaphic factors and plant factors, and top-down effects from carnivores. Bottom-up effects tend to be strong, impacting both herbivores and carnivores in the food web, forcing the consideration of three-trophic-level systems as the minimal unit of study. Examples are provided of three-trophic-level interactions, including,some on willows and poplars. Bottom-up effects may be particularly strong in juvenile Salicaceae, which are used in energy forest production, because plants are growing vigorously, and many herbivores respond positively. Some major pests on willows and poplars appear to have weak top-down regulation, and they may be influenced most by bottom-up effects such as plant resistance through genetic variation, ontogenetic aging and physiological aging. Balancing the bottom-up and top-down forces in energy forests may be difficult in short-rotation crops, but harvesting may prove to be important as a control during pest outbreaks damaging to yield. However, we suggest that many aspects of energy forests are conducive to pest control, which are enumerated, and emphasize the need for further research on native three-trophic-level systems, economic thresholds, cultural methods and crop species and genotype selection.