Alfalfa Weevil Effects on Root Reserves, Development Rate, and Canopy Structure of Alfalfa1

Abstract Defoliation by the alfalfa weevil (Hypera postica Gyllenhal) results in a direct reduction in yield of alfalfa (Medicago sativa L.). Indirect responses to insect feeding may partially compensate for direct losses or cause further reductions in yield and feed quality. The purpose of this study was to identify indirect responses of alfalfa to weevil defoliation. We studied the effects of the alfalfa weevil on (i) root reserves, as measured by total nonstructural carbohydrates (TNC) in the taproot, (ii) morphological rate of development, and (iii) canopy structure. Alfalfa weevil defoliation produced statistically significant effects on all three factors. When the peak of the undisturbed larval population was about three/stem, TNC levels were reduced by up to 0.14 metric ton/ha while defoliation occurred. After the larval population declined, there was recovery growth and TNC yields reached levels up to 0.09 metric ton/ha higher than where weevils were controlled. The crossover from lower to higher TNC levels was associated with delayed morphological development and a prolonged TNC accumulation period where damage occurred. When alfalfa was cut before the larval population reached a peak, TNC levels were reduced during the regrowth period. Larval populations higher than two/stem reduced canopy height and leaf percentages during defoliation. During the post‐defoliation recovery period, moderately damaged plants increased in leaf percentage and produced more branches than lightly defoliated plants. However, the moderately defoliated plants remained shorter and there were resulting large differences in canopy structure. These findings showed a wide range of indirect effects of insect defoliation on alfalfa growth, and they should be considered in evaluating insect damage to the crop.