Document Type

Thesis - Open Access

Award Date

1970

Degree Name

Doctor of Philosophy (PhD)

Department

Entomology

Abstract

Forty-four species of grass, 12 species of broadleaf weeds, and 15 species of broadleaf crops were screened as possible hosts of the larvae of the western corn rootworm, Diabrotica virgifera LeConte. The larve did not survive on any of the broadleaf species, but they were able to complete their immature stages on 13 grass species. Corn was the most favorable host for larval development as indicated by the number and relative size of the beetles recovered. Viable eggs were obtained from females reared as larvae on all hosts. Clean cultivation should play a role in rootworm control since 2 of the most common field weeds; green foxtail, Setaria viridis (L.) Beauv., and yellow foxtail, S. Lutescens (Weigel) Hubb; are both hosts. Broadleaf crops as well as oats, Avena sativa L., and sorghum, Sorghum vulgare Pers., can be safely used in rotation with core since they are not hosts of corn rootworm larvae. But some narrowleaf crop species (wheat, Triticum aestivum L.; spelt, T. spelta L.; barley, Hordeum vugare L.; foxtail millet, Setaria italic [L.} Beauv.; intermediate wheatgrass, Agropyron intermedium [Host] Beauv.; pubescent wheatgrass, A. trachycaulum [Link] Malte) would be injured by rootworm larvae if they were planted in egg-laden fields. Eight of the near relatives of corn from the tribe Tripsaceae (Maydeae) were intensively screened as possible sources of resistant germplasm. Adults were recovered from all of the species tested, but there appeared to be a vast difference between members of the tribe in their ability to support rootworm larvae. Tripsacum dactyloides (L.) L. appeared to be the only species tested that had a high degree of resistance to larvae of the western corn rootworm. The resistance is due to antibiosis or nonpreference and can perhaps be transferred to corn if it is not too complexly inherited. Tests were conducted to determine why some grasses were not hosts of the larvae. Olfactometer tests of both hosts and nonhosts indicated that all grasses tested attracted corn rootworm larvae up to a distance of 50 mm and that some of the grasses were significantly more attractive than corn roots. Lack of attractancy or repellency was apparently not involved in the nonhost status. Tests with maxillectomized larvae, maxillectomized and labiumectomized larvae, and normal larvae indicated that none of these ingested an appreciable amount of the roots of oats. The maxillary palpi for western corn rootworm larvae were important in the detection of sucrose (a feeding stimulant), and appeared to inhibit feeding when sucrose was absent. However, the maxillae were not the only receptors that detected sucrose absence by receptors other than the maxillary palpi. Laser radiation was used to maxillectomize the rootworm larvae. This technique may be useful in maxillectomizing other small insects and in studying the function of other sense organs. The roots of sorghum were found to be toxic to the larvae of the western corn rootworm. Preliminary tests suggested that the toxin involved was hydrocyanic acid. Other experiments showed that free cyanide was toxic to the larvae when it was ingested at a level of 10 ppm. Cyanogenetic glucosides were not toxic to larvae when ingested at a level equivalent to 2000 ppm cyanide unless appropriate hydrolytic enzymes were present. Toxic levels of hydrocyanic acid probably result during larval feeding on sorghum roots because of the action of beta glucosidase of endogenous cyanogenetic glucosides such as dhurrin.

Library of Congress Subject Headings

Grasses -- Disease and pest resistance

Western corn rootworm

Format

application/pdf

Number of Pages

88

Publisher

South Dakota State University

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