Plant-insect interactions and management options for Verbascum thapsus L. in Colorado.

摘要

In the field of invasion biology, understanding the ecology of a species goes hand and hand with understanding management options. Common mullein ( Verbascum thapsus L.) is a non-native plant introduced to North America from Europe in the 1700s. It has naturalized and spread throughout North America, with populations establishing after disturbance. Through a series of field experiments, I tested the effect of insect herbivory on the growth of V. thapsus in Colorado and also investigated management options for the reproductive stage of this biennial plant.;Verbascum thapsus, while widespread, is rarely problematic in undisturbed habitats. This may be due to interactions with new community members, such as insect herbivores, that limit population size. Starting in 2009, I followed a population of V. thapsus plants in a disturbed landscape in Loveland, Colorado. A subset of the population was protected from herbivores with insecticides, reducing overall herbivore damage during the bolting year, for the first year, and throughout the lifecycle of the plants. Reducing herbivory in the first year increased the size that V. thapsus rosettes reached by the fall. Reducing herbivory during only the bolting year of the plant increased the height of the bolting plants. Reducing herbivory over both growing seasons increased plant height, the length of the reproductive stalk, and seed set. Plants with reduced herbivory also had higher rates of survival. Thus, insect herbivory reduced performance of V. thapsus in this study and may be one of the factors that prevents this introduced plant from dominating its recipient communities.;Mechanical control of V. thapsus via digging or cutting stalks is considered the best option for management. Current practice encourages the collection of reproductive stalks from the field to limit the spread of seeds. While effective, this substantially increases the labor involved in management. I tested the viability of V. thapsus seeds collected at three sites in Colorado at various times during the growing season to see if the collection and disposal of reproductive stalks is necessary. I found that harvesting before seed capsules had turned brown reduced seed viability dramatically (mean early reproductive stage = 0.08%, 95% CI = 0.06%, 0.67%; mean mid reproductive stage = 1.52%, 95% CI = 0.49%, 3.11%). I suggest focusing mechanical control efforts during the early and mid season if possible, as at those times plants can safely be left on site. When control efforts occur later in the season after seed capsules begin to turn brown, it is important that reproductive stalks be removed from the field.