Masters Theses

Date of Award

8-2004

Degree Type

Thesis

Degree Name

Master of Science

Major

Entomology and Plant Pathology

Major Professor

Jerome F. Grant

Committee Members

Paris L. Lambdin, B. Eugene Wofford

Abstract

Spotted knapweed [Centaurea stoebe L. ssp. micranthos (Gugler) Hayek] (formerly C. maculosa Lam. and C. biebersteinii DC.) (Asteracea) (referred to here as C. stoebe L. sensu lato) a non-indigenous, invasive weed, has been the focus of a biological control program using a complex of insects for more than 30 years in North America. Spotted knapweed is a prolific seed producer and produces two phytotoxic chemicals (catechin and cnicin), both enhancing the invasiveness of the weed. In Tennessee, information about this common weed of roadsides and its associated insects is not well known.

This research consists of five components: (1) Determine family composition and seasonality of insects associated with spotted knapweed, Centaurea stoebe L. ssp. micranthos (Gugler) Hayek, in eastern Tennessee. (2) Determine the distribution of Urophora quadrifasciata (Meigen) (Diptera: Tephritidae) on spotted knapweed in eastern Tennessee. (3) Assess the impact of U. quadrifasciata on the production of seeds by spotted knapweed in eastern Tennessee. (4) Determine the distribution of Megalanotus sabulicola (Thompson) (Hemiptera: Lygaeidae) with spotted knapweed in eastern Tennessee. (5) Identify and determine the effects of hymenopteran parasitoids on U. quadrifasciata. The hypothesis of this research is that biological control organisms will be present on spotted knapweed in eastern Tennessee, reducing the ability of the weed populations to spread.

Insects (n = 3,122) representing 108 families in 15 orders were collected from spotted knapweed using sweep-net, direct, and beat-sheet sampling. Hymenopteran pollinators (11 families) were prevalent in sweep-net and direct sampling. These hymenopterans (Anthophoridae, Apidae, Halictidae, Megachilidae, Sphecidae, and Vespidae) found throughout the summer months contribute to the previous limited knowledge of only honeybees acting as pollinators of spotted knapweed, enabling the plant to produce offspring.

The most numerous insect recovered from sweep-net, direct, and beat-sheet sampling was the biological control organism U. quadrifasciata (n = 605; 19.4% of all insects collected). U. quadrifasciata was intentionally released in Beltsville, Maryland, in 1983 and has since dispersed southward. This gall-forming tephritid was released as a component of a complex of 13 biological control organisms where each species targets a specific part of spotted knapweed. The gall that the larva of U. quadrifasciata forms in the capitulum replaces the available space for seed development and appropriates nutrients from other plant locations. In eastern Tennessee, U. quadrifasciata was found to reduce seed production by 24% in infested capitula. The low density in the capitula of U. quadrifasciata (mean of 0.47 ± 0.02 S. E. individuals per capitulum) was offset by the prolific seed production of spotted knapweed in infested (6.01 ± 0.19 S. E. seeds per capitulum) and non-infested (7.94 ± 0.17 S. E. seeds per capitulum) capitulum to effectively reduce the population. One or more U. quadrifasciata immature was present in 78.4% of all dissected capitula of spotted knapweed. Immature U. quadrifasciata were collected from May 2003 through January 2004; adults were collected from May through August 2003.

Three solitary parasitoids (n = 412) of immature U. quadrifasciata were reared from field-collected capitula. These included Pteromalus cardui (Erdös) (Pteromalidae), Brasema sp. (Eupelmidae), and Eurytoma sp. (Eurytomidae). P. cardui was the most numerous (n = 346) and reduced the U. quadrifasciata population by 33.5%. In combination, all three parasitoid species have the potential to dramatically reduce field populations of U. quadrifasciata.

Another potential biological control organism, M. sabulicola (Thompson), was found in only two locations in small numbers (n = 10). M. sabulicola, a naturalized forager, is present throughout the eastern United States. It feeds on dispersed seeds of Centaurea spp. and therefore, has the potential to reduce the establishment of new spotted knapweed plants by consuming the dispersed seeds prior to germination. M. sabulicola had a density too low to be considered successful; however, future research into its rearing and effectiveness should be investigated.

This research is the first official confirmed collection of U. quadrifasciata, M. sabulicola, and the three parasitoids; P. cardui, Brasema sp., and Eurytoma sp., from Tennessee and the southern United States. The data on these aforementioned insects contribute to the distribution and impact of insects in the spotted knapweed community.

This research contributes to the known range of spotted knapweed, the known associations of the insect community with the host plant, along with adding to the known distribution of both previously released biological control insects and the distribution of potential naturalized biological control insects. Research into spotted knapweed and its associated insects in eastern Tennessee should be continued since this paper provides a solid foundation from which to base future studies.

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