Date of Award:

5-1-1978

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Department name when degree awarded

Life Sciences:Biology

Committee Chair(s)

Donald W. Davis

Committee

Donald W. Davis

Committee

J. LaMar Anderson

Committee

Wilford J. Hanson

Committee

Jay B. Karren

Committee

Raymond Lynn

Committee

Reed S. Roberts

Abstract

Workers usually ignore damage to alfalfa by feeding adult alfalfa weevils because: (1) weevil feeding is prolonged and relatively undramatic, (2) it is difficult to sample adults because they drop to the ground, and (3) most adults, during summer, feed at night. Adult weevils were studied on caged alfalfa in the field and in the laboratory using alfalfa leaf disks. Feeding intakes were determined after 2 days, using dry weight. Both newly developed adults and old adults immediately following overwintering, were studied under various light and temperature conditions. Five new adults per recently harvested plant reduced old stem length by 23.9% and dry weight by 46%, and prevented most new growth. There was no new growth with 20 new adults per plant. Ten new adults per plant following first crop harvest retarded the second crop regrowth by 2 to 3 weeks. Each newly developed adult alfalfa weevil ate from 21.5 to 26.1 mg total at 27.7°C. Less was eaten at 22.2°C than at the higher temperatures. Old adults following overwintering reached their feeding peak near day 12 at 27.7°C or day 22 at 22.2°C in laboratory studies. Each old adult ate a total of 9 to 17 mg dry weight of alfalfa, with an average of 13 mg. New adults reached their feeding peaks faster and ate more than old adults. With new adults, most feeding peaks, in all light regimes or temperatures between 22.2°C and 27.7°C, occurred between days 6 to 8. New adults ate 15-26 mg dry weight of alfalfa, with an average of 23 mg. Each larvae of the western biotype of the alfalfa weevil eats an average of 7.34 mg during all four instars (based on composite data from several sources). If larval feeding is compared to old adult feeding, each old adult ate one and one-fourth to two and one-half times as much as each larva. New adults ate from two and one-fourth to three and one-half times as much as each larva. Since both old and new adults are same generation, the two feeding amounts added together equal a cumulative total of 36 mg dry weight eaten by each adult weevil. This total is about five times that of each larva. The significance of this is that in the past we have thought that the adult weevil caused little damage. The adult feeding is extended over a longer period of time than that of the larvae and there are fewer adults due to larval and pupal mortality. Each old adult took 2 to 4 times as long to eat the same amount as a large larva, but each new adult at 27.7°C ate as much as a large larva in an equivalent period. It was found that storing alfalfa weevil larvae at colder temperatures for as little as a week to prolong the experimental season, invalidated data obtained from adults reared from these larvae.

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