Masters Theses

Date of Award

5-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Entomology and Plant Pathology

Major Professor

Jerome F. Grant and Mark T. Windham

Committee Members

Frank A. Hale, Alan Windham

Abstract

In the last decade the flowering dogwood, Cornus florida Link., has become threatened by a fungal disease, dogwood anthracnose (caused by Discula destructive Redlin). Symptoms include leaf spots, twig blights, and limb/trunk cankers, which lead to the eventual death of the tree. Mortality of the flowering dogwood is extremely high in forested areas, such as the Great Smoky Mountains National Park, where dogwood density has declined in recent years.

Little information is available about environmental conditions that may predispose a tree to conidia of D. destructiva, which is believed to be dispersed by wind or rain. Researchers have suggested that insect species also may play an important role in the epidemiology of dogwood anthracnose. Unfortunately, little information is available on the insect communities associated with dogwoods in forest and nursery environments. Because of concern about the possible loss of dogwoods from the forest ecosystem and the lack of information on insects on dogwoods, a two-year research project was initiated to: 1) identify insect taxa associated with anthracnose-infected (diseased) and non-infected (healthy) dogwoods in natural and nursery/ornamental settings, 2) monitor the seasonality of selected insects, 3) compare insect densities on Anthracnose-infected and non-infected dogwoods, and 4) compare insect densities among dogwood cultivars.

The most abundant insect orders collected in the Great Smoky Mountains National Park were Coleoptera, Hymenoptera, and Diptera. Significant differences in densities of Coleoptera, Diptera, and Homoptera, as well as the total number of insects and arthropods, were found among sampling dates, sites, and tree types. Greater numbers of insects were observed in 1993, when Diptera comprised approximately 50% of all taxa.

In 1993, insect densities generally peaked in June; however, peak densities were observed during July in 1994. Seasonal abundance varied among sites. In 1993, insect densities peaked in mid to late June at Cades Cove and Rich Mountain and in mid July at Cades Cove, and July at Mt. Sterling. Environmental conditions, such as harsh weather and cooler temperatures, may have contributed to delays in peak densities of insects at Mt. Sterling, the high elevation site. In this study, insect densities were greatest early in the season when incidence of D. destructiva occurs at its highest levels.

Percent composition of insect orders was similar for healthy and diseased trees. More insects, numerically, were collected from healthy than diseased trees, but no significant differences were documented between tree types within a year.

The most common insect orders collected at the Plateau Experiment Station, Crossville, TN, were Coleoptera, Homoptera, and Diptera. More insects were collected in 1994 than in 1993, and significant differences among sampling dates and cultivars were detected. In 1993, Homoptera was the most abundant insect order, while in 1994, Coleoptera was the most abundant insect order.

Insect diversity and abundance varied among the three cultivars of dogwood, each with a different foliage coloration (green-Cherokee Princess, purple-Purple Glory, or yellow-First Lady). Overall insect densities were greatest on First Lady (cultivar Y) than on the Cherokee Princess (cultivar G) or Purple Glory (cultivar P).

In 1993, densities of Coleoptera, Diptera, and Hymenoptera, as well as the total number of insects and arthropods, were significantly greater on cultivar Y than on the other cultivars. Densities of Homoptera were not significantly different among cultivars, although more (ca. 2 to 3x) were collected from cultivar Y than G or P. In 1994, the number of Homoptera were significantly greater on cultivar Y. Insect densities also were greater (ca. 3 to 6x) on this cultivar than on G or P. No other significant differences were detected among cultivars in 1994, although densities of Coleoptera were numerically greater (ca. 4 to 6x) on cultivar Y than on any other cultivar.

Seasonal abundance of insect orders was similar for each cultivar. In 1993, insect densities on each cultivar peaked in late May and again in late July on cultivar Y; on the other hand, densities in 1994 peaked during April and again in August on cultivar Y.

Previous greenhouse, laboratory, and field studies have demonstrated that insects can transport and disseminate viable conidia of D.destructiva to healthy dogwood foliage and that the foliage can develop lesions symptomatic of dogwood anthracnose. Because insect densities are greatest when incidence of conidia of D. destructiva are present, insects may play an important role in the epidemiology of dogwood anthracnose. This research will provide a better understanding of the insect taxa associated with dogwood and may be useful in the development and implementation of management programs, especially in nursery environments.

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