Date of Award

1984

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Entomology

Abstract

Weeds which occur in crop ecosystems may cause suppressive or stimulating effects on arthropod abundance. Yet, little is known about the weed-insect interactions in Louisiana soybean. A three-part study was conducted from 1980-1983 to measure the abundance of arthropods in soybean, Glycine max Merr., as affected by the presence of absence of various weeds. A survey of arthropod abundance was conducted in commercial soybean fields which were either weedy or weed-free in central and south central Louisiana during 1980. Weedy fields had greater abundance of spiders, Geocoris punctipes (Say), and Lebia analis Dejean. Late instar green cloverworm, Anticarsia gemmatalis Hubner, were higher in clean fields. In 1981-1982 herbicides were used in field plots to manipulate weed populations and again predators were most numerous in weedy plots. The effect that weed species had on soybean looper, Pseudoplusia includens (Walker), and velvetbean caterpillar (VBC) abundance was not always consistent, but data indicated that an interrelationship existed between weeds, VBC, and soybean looper (SBL) populations. The relationship between SBL moths to 5 different nectar producing plants was investigated in greenhouse experiments conducted in 1981. Exposure of moths to cotton resulted in greatest egg production. In field cage experiments in 1982, fecundity of both SBL and VBC demonstrated significant increases (P < 0.05) when exposed to soybean containing weeds. SBL moths had a mean of 163.3 and 86.7 eggs in ovaries when exposed to weedy and weed-free soybean, respectively. Ovaries of VBC moths contained a mean of 107.5 and 27.2 eggs when exposed to weedy and weed-free soybean, respectively. Laboratory studies substantiated the importance of sugars in the diets of adult SBL and VBC and revealed that salts were required for highest egg production. Chromatographic techniques were used to determine ratios of sucrose, fructose, glucose, and arabinose in certain natural food sources. Nectar mimics and other sugars were given to moths with most sugars providing the necesssary nutrients for high egg production. In another test, rate of oviposition, longevity, and total egg numbers were greatly affected by timing and interval of exposure to carbohydrates.

Pages

108

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