Date of Award:

5-1988

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Committee Chair(s)

Donald W. Davis

Committee

Donald W. Davis

Committee

Raymond Sanders

Committee

Wilford Hanson

Committee

Donald V. Sisson

Committee

Ting H. Hsiao

Abstract

Economic damage to forage alfalfa by the alfalfa weevil occurs frequently enough in northern Utah to warrant applications of an insecticide in some years but not all. Currently a five to ten day period is available to recognize injurious populations then make applications.

Sticky boards, pitfall traps, Berlese funnels sweep samples, stem bouquets, climatic variation and marking techniques were evaluated for alfalfa weevil population predictions. The prevailing climate was the most important factor controlling early adult activities. Early adult feeding and sexual development was the key to forecasting later larval populations. Regional surveys were not adequate for local control recommendations.

The effects of climate (especially temperature) on both weevils and alfalfa in six fields in areas with frequent alfalfa weevil damage were selected and studied in detail. The fields were selected for comparable age, alfalfa variety, type of irrigation and harvest practices. The development of both alfalfa and weevils were monitored using accumulated degree days at a developmental threshold of 9°C for the alfalfa weevil and 5°C for the alfalfa. When there was significant early season accumulated degree days (9°C), a weevil outbreak was likely. Adult weevils fed heavily and females developed eggs during this period. Cool spring conditions did not favor early weevil activity while alfalfa plants developed due to the lower development threshold.

When the climatic history of an alfalfa field was not known, growth about 5 May was an effective indicator of accumulated degree days. When the alfalfa was less than 25 cm tall and there were 3-4 total combined oviposition and feeding punctures per ten stems an injurious outbreak of weevil larvae invariably occurred about a month later. The method was not sensitive enough to detect marginal injurious population levels.

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