Date of Award

1987

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Entomology

First Advisor

T. E. Reagan

Abstract

Economic impact studies revealed that the sugarcane borer, Diatraea saccharalis (F.), reduced sweet sorghum yield of total sugars up to 46% in artificially infested field trials. A significant relationship was found between D. saccharalis damage and yield loss that indicated an economic injury level of 10% bored internodes. Stalk weight, percent sucrose, and total sugars were negatively correlated to D. saccharalis damage; and increased fiber content was positively correlated to percent bored internodes. Information from damage levels, along with survival records, indicated that the economic threshold was reached when approximately 5% of the sweet sorghum plants contained small D. saccharalis larvae in their leafsheaths. Comparisons of the predator-prey relationship between sweet sorghum and sugarcane plots revealed that the arthropod predator composition was similar for both crops, but predator abundance in sugarcane was 4- and 16-fold greater than that found in sweet sorghum during 1985 and 1986, respectively. Predator habitat disruption associated with cultivation practices in sweet sorghum and sugarcane is important in this relationship. The red imported fire ant, Solenopsis invicta Buren, was the dominant arthropod predator found in pitfall traps and canopy samples of each crop. Based upon pitfall trap determinations, carabid larvae, cicindelids, and the Araneae were significantly more abundant predators in sweet sorghum compared with sugarcane. Also populations of carabid larvae, chrysopids and Orius spp. were significantly greater in sweet sorghum canopy samples. Reduced damage by D. saccharalis in plots without predator suppression resulted in 22.4 and 18.6% greater yield of total sugars in sweet sorghum and sugarcane plots, respectively. Compared with sugarcane 'CP74-383', D. saccharalis larval survival inside stalk tunnels was significantly greater in sweet sorghum 'Wray' during 1985. Predators reduced D. saccharalis moth emergence in both crops by approximately 50% in 1986. A two year study was conducted to evaluate the effects of sweet sorghum stalk barrel diameter and fiber content on D. saccharalis populations. Larval survival and moth emergence were not significantly effected by resultant increased fiber content that accompanied decreased stalk barrel diameter.

Pages

85

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