Date of Award

1985

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Entomology

Abstract

A 3-yr study on southeastern Louisiana sugarcane (Saccharum officinarum L. 'CP 65-357') revealed abundance and diversity of both ground- and foliage-associated predators and prey to be higher in weedy than in weed-free habitats. The imported fire ant (IFA), Solenopsis invicta Buren, major predator of the key insect pest, the sugarcane borer, Diatraea saccharalis (F.) , was more abundant in weedy (17 mounds/ha) than in weed-free (5 mounds/ha) habitats. Azinphosmethyl, commonly used for D. saccharalis control, decreased system stability through food web route disruption by decreasing coleopterans, phytophagous insects collectively, and spiders 51, 35, and 67%, respectively. Cane and sugar yields in broadleaf habitats and returns in dollars per hectare averaged 19% higher than in the weed-free. Studies on IFA diet composition and foraging activity showed the most frequent foraging occurring in grass weed habitats which had the greatest vegetation density. Broadleaf weeds, however, appeared to provide more prey per dry biomass (g/m('2)) than grasses or sugarcane alone. A trend for higher IFA populations was observed in clayey sugar- cane fields in South Louisiana over a 2-yr period. Additionally, early harvested fields contained the highest number of IFA mounds/ha (94.2), with the lowest in the early planted fields (21.8). Significantly (P < 0.05) higher amounts of P, Ca, Mg, Na, and percent organic matter were found in clayey than in sandy fields. An updated list of spiders in sugarcane is presented and includes Geo heptagon (Hentz) and Neoantistea agilis (Keyserling), new Louisiana records. Lycosidae and Linyphiidae comprised the majority of sugarcane ground-associated spider fauna (55 and 26%, respectively). Araneidae and Theridiidae comprised the majority of the foliage-associated fauna (41 and 37%, respectively). Weed control practices seemed to have little impact on the araneid faunal composition and abundance in sugarcane. In a 2-yr study, Johnsongrass (JG), Sorghum halepense (L.) Pers. , interference reduced cane and sugar yields (36 and 31%, respectively) in plots heavily infested with JG than in weed-free plots. Sugarcane yields were inversely influenced by JG equivalents according to the prediction equation: (')Y = 68,994 - 51 x (r = -0.826, P < 0.01). Significant relationships were not observed between damage by D. saccharalis and JG infestations.

Pages

152

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