Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Thomas Eugene Reagan


Interactions among Louisiana sugarcane (Saccharum hybrids), weeds, nematodes, sugarcane mosaic virus (SCMV), and pest, prey, and predatory arthropods were mediated through tropic, host plant biochemical, and intra- and interspecific competitive relationships. Weed competition reduced crop biomass (15%, P $\le$ 0.005), and sugar yield (13%, P $\le$ 0.05). Weeds, however, were associated with more predatory arthropods, including Solenopsis invicta Buren, on the soil, weeds, and sugarcane plants, and with reduced Diatraea saccharalis (F.) injury (over 25%, P $\le$ 0.05). Without chemical D. saccharalis control, sugarcane production in weedy habitats was economically superior to that in weed-free habitats. Aldicarb, a nematicide-insecticide, diminished many phytophagous nematodes during the growing season, and reduced stalk-associated predators; thus, D. saccharalis injury increased by 19% (P $\le$ 0.05). Fenvalerate, a pyrethroid insecticide, reduced prey and predatory arthropods, increased Sipha flava (Forbes) infestations (63%, P $\le$ 0.0001), but provided over 70% control of D. saccharalis. Release of the crop from weed and D. saccharalis pressures was the most profitable pest management strategy. Weeds were associated with low nematode infestations and, except for Criconemella spp., failed to reservoir phytophagous nematodes. Fifteen of 17 sugarcane free amino acids (FAAs) were lower (P $\le$ 0.05) where weed competition occurred. Nematode-induced stress was associated with the reduction of four FAAs (P $\le$ 0.005), and SCMV was related to other FAA changes. A weed-virus interaction (P $\le$ 0.001) for free cysteine was correlated (r = 0.59, P $\le$ 0.001) with Tylenchorhynchus annulatus infestations. Population trends of various nematode groups were also associated with virus, weed, and nematode stress-related FAA changes. Changes in sugarcane FAA accumulations may influence levels of other phytophagous pests, including nematodes. Ecological interactions of S. invicta in North America, and the versatility of radiotracer methods for studying insect ecology and ethology were reviewed. Using radiotracers Zn-65 and Mn-54 to label two adjacent S. invicta colonies, the observation of daily territorial changes offered a unique perspective in association with environmental changes (including flooding) and colony behavior such as nest relocation. Utilizing instrumental neutron activation analysis and the tagging of S. invicta colonies with samarium, a rare earth element, revealed a 28% (P $\le$ 0.02) decrease in the size of S. invicta foraging areas due to greater food availability in weedy as compared to weed-free habitats. S. invicta foraging activity was negatively correlated; (r = $-0.69$) with that of Paratrechina vividula, one of two other ant species encountered.