Impact of Tall Fescue Grass andCarduusThistle Weevils on the Growth and Development of Musk Thistle (Carduus nutans)

Abstract

Field plot experiments were conducted to examine the interactions between tall fescue grass (Festuca arundinaceaSchreb.), musk thistle (Carduus nutansL. =thoermeriWeinmann # CRUNU), and two thistle weevilsTrichosirocalus horridus(Panzer) andRhinocyllus conicusFroelich. Restriction of musk thistle growth was greatest when the weevils were allowed to feed on the musk thistles competing with tall fescue. Significant reductions were found in total musk thistle seeds per plant, root weight, flower buds per plant, stem dry weight, seeds per head, root crown diameter, stem height, rosette diameter, and head diameter. Seed weight and viability were not reduced. Of the three stress factors (tall fescue and the twoCarduusthistle weevils), tall fescue had the greatest impact. When musk thistle seeds were planted in 1-yr-old tall fescue, germination was low. Seeds that germinated did not grow more than four Leaves and none of the thistles developed to the reproductive stage. A 1-year-old tall fescue stand effectively prevented musk thistle reproduction. Thus, tall fescue helped suppress musk thistle growth more quickly than the use ofCarduusthistle weevils alone without competitive vegetation. Dry weight of tall fescue grass was lower in musk thistle-infested plots than in the thistle-free plots.