COMPETITIVE INTERACTIONS OF ALFALFA AND ANNUAL WEEDS AS AFFECTED BY ALFALFA WEEVIL (COLEOPTERA: CURCULIONIDAE) STUBBLE DEFOLIATION

Abstract

The impact of stubble defoliation by alfalfa weevil, Hypera postica (Gyllenhal), larvae on the establishment and growth of selected annual weeds in alfalfa was examined under controlled conditions in the greenhouse. Weed species were large crabgrass, Digitaria sanguinalis (L.) Scop., yellow foxtail grass, Setaria lutescens (Weig.) Hubb., redroot pigweed, Amaranthus retroflexus L. and common lambsquarters, Chenopodium album L. Stubble defoliation increased the survival of all weed species except foxtail grass. Dry matter production of aerial portions of crabgrass, foxtail grass, pigweed and lambsquarters was 28.4, 7.2, 23.7, and 7.3 times greater, respectively, when growing with defoliated than undefoliated alfalfa. Stubble defoliation also enhanced plant height and leaf number of most weed species. Stubble defoliation reduced dry matter accumulation and delayed development of alfalfa regrowth. Alfalfa root dry weight was more adversely affected than top dry weight by stubble defoliation. The presence of weeds did not significantly (P > 0.05) affect the growth and development of defoliated and undefoliated alfalfa. This result suggests that stubble defoliation by alfalfa weevil larvae reduced the competitive ability of alfalfa which allowed weeds to grow and occupy gaps within the alfalfa canopy.