A test of sexual dimorphism in <i>Valeriana edulis</i> resistance and induced responses to herbivory

Abstract

Dioecious plants have been observed to exhibit differences in growth rates and levels of herbivory defense between the sexes, with slower growing females investing more into constitutive defenses than fast-growing males. There are no predictions or empirical data, though, regarding differences in induced or indirect defense based upon plant sex. We tested for differences in levels of constitutive and inducible direct and indirect defense for male and female plants. We first measured the performance of an unidentified caterpillar species feeding on the inflorescences of male and female Valeriana edulis that either had or had not received previous damage by the same herbivore. After caterpillar pupation, we stocked damaged and undamaged plants with a constant density of the aphid Aphis valerianae and monitored population growth, rates of ant attendance and attack by natural enemies. Caterpillar pupa weights were lower on previously damaged plants than on previously undamaged plants, and the caterpillar mortality was significantly higher on male plants than on female plants. In addition, aphid colony sizes were larger on female plants, and ant tending occurred more frequently on female plants. Aphid predators visited damaged plants more frequently than undamaged plants. However, because there were no interactive effects of damaged and undamaged plants with male and female plants, V. edulis did not display any difference between males and females for induced or indirect defense.

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