Humans have spread domesticated crop species across the globe, expanding the distributions of these plants beyond their native ranges. This has been accompanied by range expansions of the pests that infest crops and inflict great economic damage. Though much research has been devoted to understanding pest effects on crops, much less is understood about their effects on native plants in invaded ranges. Here, we investigated the interaction between the cabbage seedpod weevil, Ceutorhynchus obstrictus, and cedar gladecress, Leavenworthia stylosa. The cabbage seedpod weevil is a pest of canola and other mustard crops (Brassicaceae) and has been introduced to North America from Europe. Wild mustard species such as cedar gladecress, a species endemic to central Tennessee, also host the seedpod weevil, which parasitize fruits and consume seeds, flowers and foliage. We used field and greenhouse experiments in 2018 and 2019 to evaluate the effects of these invasive weevils on gladecress and to understand the capacity for gladecress to resist weevil damage. We used jasmonic acid (JA) to induce chemical defenses in four yellow and four white-flowered populations of gladecress and assessed differences in weevil damage and seed production and germination across defense induction treatments and flower color.
Results/Conclusions
In 2018, seedpod weevils parasitized 60.6% of fruits across all experimental gladecress plants. Parasitism rate, fruit abundance, viable seed abundance and seed mass did not differ between JA treatments and flower colors. However, JA application and flower color had an interactive effect on weevil larvae presence. In yellow-flowered populations, weevil larvae were found less frequently in fruits of JA-treated than untreated plants; larvae were found at an intermediate rate within fruits of white-flowered populations, regardless of JA treatment. Seeds produced by JA-treated and yellow-flowered plants had higher germination rates and germinated earlier than those from plants without JA application or from white-flowered populations, particularly at larger seed sizes. In 2019, we confirmed that JA treatment induced higher concentrations of branched-chain glucosinolates in plants in both yellow and white-flowered populations. We are currently measuring weevil damage and seed production for gladecress in 2019. Thus far, our results suggest that cabbage seedpod weevils can inflict great reproductive costs to native gladecress, and that yellow-flowered populations may have greater capacity to resist this damage. Further research on interactions between crop pests and alternative hosts is needed to predict damage to both crops and native vegetation.