Seed viability of an invasive weed species

Jeffrey, Smith
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University of Delaware
Persicaria perfoliata, commonly known as mile-a-minute weed, is an aggressive invasive vine throughout the Mid-Atlantic and Northeastern United States. The plant has been shown to be highly problematic in both natural and agricultural areas, prompting extensive research into potential control methods. As an annual plant that reproduces primarily through seeds, specific attention has focused on ways to reduce the reproductive potential of P. perfoliata. In addition to a regimen of physical and mechanical controls, a classical biological control agent was identified and introduced in 2004. This insect, Rhinoncomimus latipes, is entirely host specific on P. perfoliata with the adults feeding primarily on leaves and the larvae boring into stems. Previous studies have shown that R. latipes reduces both the percent cover and number of seeds produced by P. perfoliata. My first study aimed to identify how seed viability is affected by the maturity status of the fruit surrounding the seed and the time of year of seed collection. In order to do this, seeds were collected from five field sites in the Mid-Atlantic and returned to the lab where their viability was assessed using a chemical assay. The second study aimed to quantify the impact of R. latipes on the reproductive potential of P. perfoliata, both by determining the reduction in number of seeds produced and the proportion of resultant seeds which are viable. One experiment in this study solely investigated the effects of adult weevils confined to developing seed clusters while the other tested the effects of feeding by larvae and adults on the entirety of a P. perfoliata plant. The first study revealed that seeds surrounded by immature fruits are viable, and the rate at which they are viable varies across the season. I found that at the onset of seed production, in mid-August, 35% of seeds with immature fruits were viable, but this rate increased to 84% in late September. As has been seen in previous studies, nearly all seeds with mature fruits (96%) were found to be viable. In the second study, I found that feeding by the entire lifecycle of weevils reduced the total number of seeds produced per plant by 32%. Further, the number of seeds per cluster was reduced with feeding strictly from adults on developing seed clusters (17% reduction) and with feeding by the entire weevil lifecycle on the whole plant (36% reduction). Feeding strictly by adults on developing clusters reduced seed viability by 24%, but I found no effect of feeding by adults and larvae across the entire plant on seed viability. Our first study indicates to land managers that if they do want to apply physical or chemical control methods, they should do so prior to the onset of any seed production and not simply before fruit maturation. If they have to apply control methods after fruit set, our findings indicate they should do so as early in the season as possible. My second study demonstrates that any feeding activity by R. latipes will reduce reproductive potential by decreasing the number of seeds produced. However, only feeding by adults directly on clusters reduced seed viability, further lowering reproductive potential. Lastly, I showed that weevil activity delays the onset of fruit production and maturation, possibly limiting the capacity of the seed to spread via wildlife that are attracted to the mature fruits. This study outlines previously unknown ways in which R. latipes can help to stop to spread of a noxious invasive weed.