Yield Responses of Alfalfa to Simulated Alfalfa Weevil Injury and Development of Economic Injury Levels

Abstract

AbstractAlthough the alfalfa weevil Hypera postica (Gyllenhal)] is an important alfalfa (Medicago sativa L.) pest, relatively little is known about how defoliation at specific plant developmental stages affects yield. This study was conducted to characterize yield responses of alfalfa to simulated alfalfa weevil injury when injury was initiated at the early bud stage of the first growth cycle and to develop economic injury levels for the alfalfa weevil. The study was conducted on a silty clay loam (fine, montmorillonitic, mesic Typic Argiudoll) in a 3.6‐ha alfalfa (cv. Haymaker) field near Walton, NE. Sixteen 1 by 1‐m plots were arranged in a randomized complete block design with four treatments in each of four replicates. Treatments consisted of three levels of simulated alfalfa weevil defoliation and an undefoliated check. Leaflets were removed over 10 d. Dry matter yield and stem density responses were measured for both the first and second growth cycles. Simulated alfalfa weevil injury did not affect stem density before the first cutting or during the entire second growth cycle. When injury occurred during the early bud stage, the most significant effect was the loss of leaf tissue. Yield differences among treatments for the first growth were highly significant, and the relationship between percentage defoliation and percentage yield reduction was linear both years. Defoliation injury imposed during the early bud stage of the first growth cycle did not affect yields or stem densities of the second growth cycle. Dry matter yield responses of the first growth cycle were used to calculate economic injury levels for third and fourth instars feeding at the early bud stage of alfalfa development.