Date of Award
Doctor of Philosophy
The response of alfalfa regrowth to actual and simulated stubble damage by the variegated cutworm (VCW), Peridroma saucia (Hubner), was investigated. A laboratory study found that most larvae exhibited 7 larval stages. Development by these larvae took 32.8 days at 24(DEGREES)C, during which 442.2 mg of alfalfa foliage was consumed;The effect of the duration of a complete suppression of regrowth on alfalfa productivity was studied using a surrogate defoliation technique. Complete regrowth suppression for 1 and 3 days did not significantly affect alfalfa regrowth. Delays of 7 and 11 days retarded plant development and suppressed the crop growth rate (CGR) of subsequent regrowth. Most of the reduction in CGR was caused by a decline in the growth rate of support (stem) structures. Leaf-weight and leaf-area growth rates were not significantly affected. The differential response of component growth rates resulted in an increase in the leaf area ratio, leaf weight ratio, and specific leaf area of regrowth;A second study found that stubble damage by various densities of VCW larvae produced the same general effects on alfalfa growth and partitioning that simulated defoliation produced. The effects, however, were significant in all years only for densities of 6 and greater larvae/0.1 m('2). These densities were large enough to completely suppress regrowth in most years for 10 to 15 days;The results of both studies indicated that plants minimized the adverse effects of stubble injury by maintaining growth rates of leaf weight and area at the expense of support-structure growth. A hypothesis based on the depletion of stored carbohydrate reserves was proposed to explain the changes observed in alfalfa regrowth;A replacement-feed cost analysis was conducted for both studies. Depending on commodity price and control costs, stubble damage was economic for a harvest system based on cutting at first bloom when regrowth was suppressed completely for 4.4 to 7.5 days or when densities of newly-molted, last-stage larvae reached 2.8 to 4.8 larvae/0.1 m('2). Action levels were reduced by approximately one-half if cutting occurred based on calendar-date. A management program for VCW in alfalfa was developed using these results, and guidelines for the management of other stubble defoliators also were proposed.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
G. David Buntin
Buntin, G. David, "The impact of actual and surrogate variegated cutworm stubble phytophagy on the growth and yield of alfalfa " (1984). Retrospective Theses and Dissertations. 8152.