Masters Theses

Date of Award

5-2001

Degree Type

Thesis

Degree Name

Master of Science

Major

Entomology and Plant Pathology

Major Professor

Roberto M. Pereira

Committee Members

Bonnie Ownley, Jerome Grant, Karen Vail

Abstract

Originally from the floodplain of the Paraguay River in South America, imported fire ants are well known pests throughout the southern United States. The black imported fire ant, Solenopsis richteri Forel, and the red imported fire ant, Solenopsis invicta Buren, are believed to have arrived in the United States through Mobile, Alabama, in ships' ballast or dunnage in 1918 and the 1930s, respectively. Current federal quarantine area for fire ants covers portions of 13 states and Puerto Rico including twenty-nine counties in Tennessee.

The top northern portion of the fire ant range (northern Mississippi and Alabama, and southern Tennessee from Shelby Co. to Giles Co.) is inhabited primarily by S. richteri. The remaining range is occupied by S. invicta with the exception of a sizable band of territory between the parent species that stretches west of the Smoky Mountains to the Mississippi River, which is dominated by their hybrid. A similar pattern of species distribution occurs in their native lands in Argentina, Paraguay, Brazil, and Uruguay.

Two experimental parameters, supercooling point and survival under extended low temperature exposure, were used to examine effects of species and individual size in S. richteri, S. richteri x invicta hybrid, and S. invicta, and the effect of Thelohania solenopsae (Knell, Allen, and Hazard) infection in S. invicta on low temperature survival. Supercooling point is the lowest temperature the insect can be brought to before freezing. Based on supercooling point results for fall-collected ants, S. richteri was more cold hardy than the hybrid as shown through a significantly lower supercooling point in large- and small-sized workers. The spring-collected groups did not show this trend, and instead, the hybrid supercooled lower in the large-sized ants and there was no significant difference in the small-sized ants. Winter-collected large and small S. invicta infected with T. solenopsae supercooled to lower temperatures than those not infected. However, spring-collected colonies gave the opposite result. Large spring-collected S. richteri workers had supercooling points not significantly different from the S. invicta, but the hybrid had a significantly lower supercooling point than the parent species. Small hybrid ants were not significantly different from S. richteri, but the S. invicta supercooled to a significantly lower temperature.

In the extended exposure tests, the chilling injury of imported fire ants was measured through monitoring ant mortality during 15 days of exposure to one of three temperature regimes: +4°C, +0.5°C, and -4°C. Under the +4°C regime, both the hybrid and S. invicta infected with T. solenopsae had significantly lower mortality rates than either the S. richteri or the uninfected S. invicta by day seven. The +0.5°C regime caused increased ant mortality when compared to the +4°C regime, and the hybrids had significantly lower mortality than uninfected S. invicta from day five through the last day of exposure. The S. richteri and infected S. invicta were not significantly different. One hundred percent mortality was reached within all groups by day seven in the negative temperature regime. The uninfected S. invicta was consistently less cold tolerant than the other groups. At all three regimes, the uninfected S. invicta had the highest mortality and the hybrid had the lowest of all groups.

The supercooling point does not seem to be an appropriate measure of cold hardiness in imported fire ants. It is too easily affected by outside phenomena, such as time in the lab and thermocouple size. The differences among the supercooling points of different ant groups were not consistent and these significant differences slight, in light of natural cold weather phenomena. Furthermore, results from the extended exposure tests displayed fire ant mortality at temperatures well above their supercooling points. These results support the hypothesis that extended cold injury causes winterkill of fire ants, and may partially explain the species distribution of fire ants.

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