Interactive Effects of Plant-Plant Competition and an Introduced Biocontrol Agent on Yellow Starthistle (Centaurea solstitialis) Seed Production

Yellow starthistle (YST) is an annual invasive weed throughout the western United States. Several efforts have been made to control YST population spread. One of the most notable of these was the introduction of a European weevil species, Eustenopis villosus, which parasitizes developing YST flower heads. Past studies have demonstrated that late-season forbs with growth patterns similar to YST can also suppress its growth in experimental plant communities. This study examined the effects of competitive interactions between tarweed species (a group of late-season forbs) and YST individuals through the period of YST flowering using log response ratios. The study also examined the interplay between these competitive interactions and the effects of E. villosus on YST individual seed production by statistically controlling for E. villosus infestation in a method similar to ANCOVA. E. villosus infestation generally interacted weakly and positively with plant competitive effects---YST individuals that grew larger and escaped plant competition were also able to escape the effects of E. villosus infestation to a degree, and were able to produce more seeds. However, stronger negative interactions were found between the effects of the strongest water competitors on YST seed production and E. villosus later in the flowering period. As the effects of plant competition for water manifested later during the flowering period, E. villosus infestation may have been effected by YST quality.