Masters Theses

Date of Award

5-1991

Degree Type

Thesis

Degree Name

Master of Science

Major

Entomology and Plant Pathology

Major Professor

Jerome F. Grant

Committee Members

Paris L. Lambdin, Harry Williams

Abstract

Musk thistle, Carduus thoermeri (Weinmann) (Compositae: Cynareae: Carduinae), is native to Europe and was introduced into North America in the late 1800's along the eastern seaboard of Canada and the United States and in Alabama (Batra et al. 1981). Musk thistle spread quickly from the mid- Atlantic States, where it was considered to be a weed by the early 1900's, to the Midwest, and became an economically important pest by 1950 (Batra et al. 1981).

Cultural controls, including mowing, reduced grazing, minimization of erosion, and periodic reseeding of grass, are important in maintaining low numbers of thistle seedlings (Trumble and Kok 1982). Although chemical herbicides achieve a measure of control against thistles, rough terrain and the potential of groundwater contamination limit their use. These two limitations, as well as the threat of reintroduction of thistle from untreated areas, provided the impetus for the evaluation of alternative control techniques (Trumble and Kok 1982) .

In 1968, the head weevil, Rhinocyllus conicus (Froelich) (Coleoptera: Curculionidae), was introduced into the United States for biological control of thistles, particularly musk thistle (Hodgson and Rees 1976). Rhinocyllus conicus reduces the number of seeds produced by infested thistle plants (Roberts and Kok 1979). Another biological control agent of musk thistle, the rosette weevil, Trichosirocalus horridus (Panzer) (Coleoptera: Curculionidae), was introduced into the United States in 1974 (Batra et al. 1981). Trichosirocalus horridus feeds on the crown tissue (Kok and trumble 1979).

In 1989, R. conicus and T. horridus were introduced into Tennessee to assess their potential for biological control of musk thistle (Lambdin and Grant 1989). The natural enemies of musk thistle that coexisted with and helped to suppress this plant pest in its native habitat are not present to help suppress the population of this weed in Tennessee (Lambdin and Grant 1989) . A two-year study was initiated in conjunction with this research to better understand the diversity and specialization of the arthropod fauna that is associated with musk thistle in Tennessee. The specific objectives were to determine the species composition and seasonality of insects and arachnids associated with musk thistle and to observe the impact of selected arthropods on the plant. This introduced plant provides numerous established arthropod species with food or protection during its growing season. This research should provide preliminary information on the potential impact that R. conicus and T. horridus may have on the established arthropod fauna due to the resulting increase in competition for the resources of musk thistle and the eventual decrease in musk thistle populations.

During this two-year study, approximately 103 arthropod species or groups were found on musk, thistle in Tennessee. Fifty-seven families, representing thirteen orders of insects, as well as eight families of arachnids, were collected from musk thistle. The most extensive arthropod diversity was found during the flowering stage of musk thistle. The most frequently encountered orders of insects were Coleoptera, Hemiptera, Hymenoptera, Lepidoptera, and Homoptera. The two most frequently encountered families of spiders were Salticidae and Thomisidae.

Only a few of the established arthropods in Tennessee were observed to cause significant damage to musk thistle. When present in large numbers, the nymphs and adults of two species of froghoppers (Homoptera: Cercopidae), Lepyronia quadrangularis (Say) and the meadow spittlebug, Philaenus spumarius (L.), apparently stunted the growth of the plant. Froghopper nymphs were most numerous during April and early May, while the greatest number of adults were observed from early May until early June. During the bud stage, the fourlined plant bug (Hemiptera: Miridae), Poecilocapsus lineatus (F.), caused foliar damage, but was not frequently found on the plant. Poecilocapsus lineatus was observed on musk thistle between the middle of May and late June.

The larva of the pyralid moth, Dicymolomia julianalis (Walker), was observed to feed on developing seeds within the seed head of musk thistle. Infestation levels of D. julianalis on musk thistle seed heads during the summer of 1990 at sites in middle Tennessee averaged 10 to 15%, while those in eastern Tennessee averaged 20 to 25%. Most of the adults of this pyralid moth emerged between late July and late September.

Arthropods were found on musk thistle throughout its growing season. Stem and leaf feeders (e.g., grasshoppers and froghoppers) were found in large numbers before plant flowering, which attracted the greatest variety of arthropods. A number of arthropods, such as assassin bugs, minute pirate bugs, ambush bugs, and spiders, were predaceous upon many of the insects found within the flower. Although many established arthropods utilize the resources of musk thistle, few of these arthropods cause serious damage to the reproductive capabilities of the plant.

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