Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/21/2013
Publication Date: 8/1/2013
Citation: Smith, L., Beck, J.J. 2013. Effect of mechanical damage on emission of volatile organic compounds from plant leaves and implications for evaluation of host plant specificity of prospective biological control agents of weeds. Biocontrol Science and Technology. 23(8):880-907. Interpretive Summary: Invasive alien weeds are an increasing problem affecting many agricultural and natural ecosystems. Classical biological control, the introduction of exotic natural enemies, is a principal strategy for controlling invasive alien weeds. However, to prevent possible risk to nontarget species, it is critical to determine the host plant specificity of prospective biological control agents. This is currently done by performing choice or no-choice behavioral experiments using plants in the laboratory or under field conditions. The results of such experiments are empirical, indicating which plants are acceptable or suitable for a prospective agent under specific experimental conditions; however, they do not answer the question why some plants are acceptable and others are not. In situations where a nontarget plant may be suitable for development of a prospective agent in laboratory experiments, but the plant is not attacked under normal field conditions, understanding why the herbivore is so selective in the field could help improve our assessment of its potential risk to the nontarget plant. Some volatile organic compounds (VOC) emitted from plants are known to act as attractants or repellants to insects. However, only recently have scientists begun to study the role of VOCs for classical biological control of weeds. These studies have typically involved undamaged plants; however, the effects of prior damage on qualitative and quantitative emission of VOCs is not known. Because both olfactory and gustatory stimuli may be important in host plant selection, studies of undamaged plants may overlook VOCs that are released only after damage. Our results showed that damaging plants increased the number of VOCs emitted and changed which compounds are important for distinguishing between three plants, thus insects are likely to respond differently to undamaged versus damaged plants.
Technical Abstract: Assessment of host plant specificity is a critical step in the evaluation of classical biological control agents of weeds, which is necessary for avoiding possible damage to nontarget plants. Volatile organic compounds (VOC) emitted by plants likely play an important role in determining which plants attract and are accepted by a prospective arthropod agent. However, current methods usually rely on empirical choice and no-choice behavioral experiments, with little knowledge about what chemical or physical attributes are stimulating the insect. We conducted experiments to measure the quantitative and qualitative effects on emission of VOCs caused by simple mechanical damage or light intensity. Experiments were conducted with three plant species: Centaurea solsitialis, the preferred host of the weevil Ceratapion basicorne, C. cyanus, which is acceptable but less preferred, and C. cineraria, which is unacceptable. In one experiment a leaf on a plant was exposed to one of four types of damage (undamaged, cut, poked or mangled). More VOCs were detected from damaged plants than from undamaged plants for all three species. Discriminant analysis was able to correctly distinguish the taxonomic identity of all plants based on their VOC profiles; however, the VOCs that discriminated species among undamaged plants were completely different than those that discriminated among damaged plants. Thus, damaged and undamaged plants present a drastically different VOC profile to insects, which should be considered when conducting host plant specificity experiments. Additional experiments indicated that injection of 1-decanol as an internal standard into the septum bag in which the solid-phase microextraction (SPME) fiber was exposed did not help to reduce variation of the data. Exposure of two species of plant to a high intensity lamp reduced the amount of some VOCs detected by GC-MS. So, experiments conducted under ambient laboratory lighting are likely to expose insects to higher amounts of VOCs than under bright light.