Alfalfa Development after Simulated Alfalfa Weevil Injury

Abstract

Although the alfalfa weevil [Hypera postica (Gyllenhal)] is an important insect pest of alfalfa (Medicago sativa L.), relatively little is known about how larval defoliation affects growth processes at specific developmental stages. This study was conducted to characterize the developmental responses of alfalfa to simulated alfalfa weevil injury when injury was initiated at the early‐bud stage of the first growth cycle. The study was conducted on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic, Typic Argiudolls) in a 3.6 ha ‘Haymaker’ alfalfa field near Walton, NE. A total of 16 1‐by 1‐m plots were arranged in a randomized complete block design with four treatments in each of four replicates. Treatments consisted of an undefoliated check and three levels of simulated alfalfa weevil defoliation. Morphological effects of simulated alfalfa weevil injury were determined at five postdefoliation periods. Leaf area indices, leaf area to total dry weight ratios, and leaf dry weight to total dry weight ratios were reduced linearly as defoliation percentages increased. However, defoliation injury did not affect phenological development of the first growth cycle between the completion of injury and first harvest. Most growth rates were not different among treatments, indicating that there was no postdefoliation compensatory growth. Injury initiated at the early‐bud stage of the first growth cycle did not impact alfalfa regrowth after the first harvest. Consequently, the most important effect of defoliation initiated at the early‐bud stage of the first growth cycle was to reduce dry matter yield of the first growth.

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

“…Leaf and stem tissues were separated and dried at 65 oc for 72 h. Stem densities were determined by counting stems within a 0.25-m 2 section of each plot at each sampling period. Growth responses of first and second growth alfalfa to simulated alfalfa weevil injury from this study are reported elsewhere (Peterson et a!., 1992a).…”

“…(Table 2). Of the two components of alfalfa dry matter yield, stems were least affected by defoliation injury, as indicated by stem length and stem weight growth rates (Peterson et al, 1992a) and stem dry weight data (Table 2). Stem dry weights were significantly different from the control 8 d after defoliation in 1990.…”

Compensatory growth response of the legume, Medicago sativa, to defoliation and denodulation

“…In concurrence, our results indicated that manual defoliation and defo-251 liation from a common shoot herbivore, H. postica, had similar effects on growth of M. sativa. Peterson et al (1992) also reported that simulated herbivory of M. sativa effectively mimicked defoliation from H. postica larvae. Although manual defoliation can not duplicate exactly insect defoliation, both types show clearly that above-and below-ground processes are linked.…”

Yield Response of Pumpkin and Winter Squash to Simulated Cucumber Beetle (Coleoptera: Chrysomelidae) Feeding Injury

“…Simulating insect leaf injury allows for a relatively accurate quantiÞcation of injury and thereby helps in characterizing yield loss to insect injury. Studies using simulated leaf injury have been conducted on several crops including cucumber (Burkness and Hutchison 1998), soybean (Hunt et al 1994), potato (Wellik et al 1981), and alfalfa (Peterson et al 1992). The objective of this study was to determine the yield response of pumpkin and winter squash to simulated cucumber beetle feeding injury as a basis for improved treatment thresholds and therefore improved pest management recommendations for these important pests.…”