Impact of an herbivorous biological control agent on the seed production and viability of an invasive annual vine
Understanding factors contributing to the reproductive potential of invasive plants, especially annual species, is essential for understanding and predicting their long-term persistence and spread. Many studies have demonstrated how herbivorous biological control agents can limit the reproductive potential of their target plant, primarily by reducing the number of seeds produced. Our study focused on the stem-boring weevil Rhinoncomimus latipes, which has been introduced into North America to control the invasive vine mile-a-minute weed, Persicaria perfoliata. Weevil feeding has been shown to reduce both the percent cover of mile-a-minute and the number of seeds produced per plant, but prior work has not examined the potential impact of the weevil on seed viability and as a result total reproductive potential. In one study we examined the impact of adult weevil feeding on developing seed clusters by enclosing developing reproductive structure in the field with a small mesh bag containing either 0 or 5 weevils. In a companion greenhouse experiment we studied the impacts of both adult and larval weevil feeding on the entirety of the plant. With both experiments we measured the number, weight, and viability of seeds produced, and monitored the timing of seed production in the greenhouse trial.
In the field trial that confined adult weevils to developing seed clusters, we found that weevil feeding reduced the number of seeds per cluster from 8.5 to 7.1, the weight of those seeds by 3%, and the percent viability from 99% to 75%. Overall, the number of viable seeds per cluster was reduced by 37% in the presence of adult weevils. In a greenhouse trial, weevil activity delayed both the production and maturation of seed clusters by 7 weeks. Cages containing weevils produced 33% fewer total seeds, 62% fewer seeds surrounded by mature fruits, and 36% fewer seeds per cluster. However, there was no reduction in seed weight or viability, suggesting that, when given a choice, adult weevils fed primarily on foliage rather than on seed clusters. Overall, weevils in greenhouse cages reduced the reproductive potential of P. perfoliata by 35%. Taken together, these findings demonstrate that this biological control agent can have important impacts on the reproductive potential of its target weed through mechanisms that have not previously been shown. Knowledge of these mechanisms may help in more accurate modeling of its population dynamics.