A field study of potential ecological costs of resistance by ‘stem ducking’ in tall goldenrod, Solidago altissima.

The maintenance of genetic variation for resistance in a plant population that experiences herbivory suggests that resistance entails costs as well as benefits. Ecological costs – which result from indirect effects that a resistance trait has on other members of the host plant’s community – constitute a potentially widespread but relatively understudied constraint on the evolution of resistance. Here, we tested the hypothesis that ecological costs may act to constrain the evolution of the recently identified ‘stem-ducking’ resistance trait in Solidago altissima L. (Asteraceae). Previous studies have shown that, although ducking is effective against some harmful apex-attacking insects, ducking genets are consistently in the minority in S. altissima populations. Potential ecological costs of ducking include increased susceptibility to non-target herbivores and reduction of attack by the third trophic level (i.e., natural enemies of herbivores). In a field study involving more than 2 100 stems, erect stems of S. altissima were galled about 50% more often than ducking stems by the rosette galler Rhopalomyia solidaginis (Loew) (Diptera: Cecidomyiidae). The two other common stem herbivores in this study – the stem-boring moths (Lepidoptera) Dichomeris inserrata (Walsingham) (Gelechiidae) and Oidaematophorus kellicottii (Fish) (Pterophoridae) – were equally abundant on ducking and erect-stemmed plants, suggesting a lack of ecological cost of ducking in terms of susceptibility to other herbivores. Rhopalomyia solidaginis eggs were parasitized by the wasp Platygaster variabilis Ashmead (Hymenoptera: Platygasteridae) at the same rate whether the eggs were laid on ducking or erect stems, suggesting a lack of ecological costs to ducking in terms of reduced attraction of the third trophic level. This study confirmed that ducking is an effective resistance against apex gallers but did not support the hypothesis that ecological costs keep ducking stems in the minority. [ABSTRACT FROM AUTHOR]