Submitted to: Acta Oecologica
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/1/2010
Publication Date: 2/23/2011
Citation: Reddy, A.M., Carruthers, R.I., Mills, N.J. 2011. No evidence for increased performance of a specialist psyllid on invasive French broom. Acta Oecologica. 37:79-86. Interpretive Summary: Some introduced invasive plants show greater growth (vigor) and reproduction in their new habitats (introduced region) than in their area of origin (native region). The improved performance may be a consequence of genetic changes the plants undergo in their new habitats. For example, the hypothesis “Evolution of Increased Competitive Ability hypothesis (EICA)” proposes that since introduced plants are no longer exposed to their natural insect feeders in their new habitats, resources previously used by the plant to produce costly chemical defenses to deter insect feeding will be reallocated toward increased growth/reproduction. Introduced plants that reallocate resources will have a competitive advantage over resident plants, and in time, plant genotypes that show more vigorous growth will be genetically selected for. We tested the prediction that introduced French broom (Genista monspessulana) plants have evolved to be less well defended (less resistant) to feeding by a psyllid (Arytinnis hakani) than native French broom plants. We measured and compared psyllid growth/reproduction on plants from the native (southern France) and introduced (California, U.S.) regions. The psyllid performed equally well on plants from the native and introduced regions as there were no differences in psyllid egg and nymphal development, nymphal survival, female longevity or egg production between the test plants. Egg survival was higher on native plants, but the difference was minimal. Our results provide evidence that native and introduced French broom plants are equally resistant to psyllid feeding and do not support the EICA hypothesis prediction of reduced investment in defense in introduced French broom plants. Possible explanations for the lack of differences in psyllid performance between regions include the feeding method of the insect we used to test the hypothesis and that genetic changes in plant defense in introduced French broom plants may not have occurred.
Technical Abstract: Some invasive plants perform better in their area of introduction than in their native region. This may be a consequence of rapid evolutionary change due to different selection pressures encountered in introduced regions. The Evolution of Increased Competitive Ability hypothesis (EICA) suggests that release from natural enemies results in selection of more vigorous plant genotypes as a result of plants allocating resources away from costly herbivore-resistance traits and toward increased growth. We tested the prediction that introduced plant genotypes of Genista monspessulana (Fabaceae) are less resistant to herbivory by a specialist psyllid, Arytinnis hakani (Homoptera: Psyllidae) by measuring and comparing A. hakani performance on plants from native (southern France) and introduced (California, U.S.) populations. Arytinnis hakani performed equally well on plants from the native and introduced regions; as there were no significant differences in psyllid egg and nymphal development, nymphal survival rates, female longevity or fecundity between the test plants. Egg survival rates were significantly higher on native populations, but the difference was minimal. These results provide evidence that native and introduced G. monspessulana populations are equally resistant to A. hakani and do not support the EICA hypothesis prediction of reduced investment in defense in introduced plant populations. Possible explanations for the lack of effects found in this study include the type of parameters measured and the feeding ecology of the herbivore used to test EICA, and finally, that evolutionary changes in plant defense in introduced G. monspessulana populations may not have occurred.