ALFALFA WEEVIL EFFECTS ON REGROWTH OF ALFALFA

Earlier work indicated that large reductions in 2nd cutting yields of alfalfa (Medicago sativa L.) could occur when larvae of the alfalfa weevil (Hypera postica Gyllenhal) fed on the stubble following the 1st harvest. To study the problem further, larval populations of known size were transferred to alfalfa plots after the 1st harvest was removed. The larvae fed on basal buds and, at the highest population (about 10,000/m2) completely destroyed active growing points in the stubble within 2 days. Measurements taken during the 40 days following the 1st harvest showed increasing regrowth delays for each increment of weevil population. Herbage yield, morphological stage, height, leaf percentage, in vitro digestibility and crude protein content were all affected. Total nonstructural carbohydrate accumulation in taproots was also delayed by weevil damage to basal buds. The maximum delay in regrowth was 6-10 days and up to 1/3 of the yield potential of the 10-day regrowth period could be lost. The regression equation to predict the percentage of the maximum loss (Y) from the insect population in larvae/m2 (X) was as follows: Y = 0.0531X. The regression equation, which was forced through the origin, had an r2 value of 0.868.