Weevil (Notaris bimaculatus) Feeding Reduces Effectiveness of Glyphosate on Quackgrass (Agropyron repens)

Abstract

The life cycle ofNotaris bimaculatusFab. and the influence of this weevil on the control of quackgrass [Agropyron repens(L.) Beauv.] with glyphosate [N-(phosphonomethyl) glycine] were studied. Observations on this insect in 1978 and 1979 showed that its life cycle occurs in close association with quackgrass. The adult weevils feed on quackgrass culms and caryopses and use the inside of the culms for ovipositing. Adult populations, measured in quackgrass infestations during the summer months, ranged from 3 to 44/25 sweeps of an insect net. Larvae emerge from the eggs after 2 weeks, feed down the inside of the culms, chew an exit hole, and move into the soil where they attack the rhizomes. Larval numbers ranged from two to six/28 dm3of soil during the summer months. Larvae feed on the rhizome surface or enter the rhizomes where they devour the vascular and cortical tissue. In dense quackgrass sods treated with glyphosate at 1.7 kg/ha, feeding damage on quackgrass rhizomes caused by the larvae reduced the control of quackgrass by disrupting the translocation of glyphosate in the rhizomes. Although soil-borne larvae of several insects were found in quackgrassinfested soil, larvae ofN. bimaculatuswere always present. Controlling these soil-borne insect larvae with a soil-applied insecticide for 2 months before applying glyphosate resulted in significantly increased quackgrass control. Shoot regrowth several months after the application of glyphosate at 1.4 kg/ha to quackgrass grown in cages infested with 400 adult weevils was 298 and 611 shoots/m2in the 1979 and 1980 experiments, respectively. Similar glyphosate applications to weevil-free quackgrass resulted in only 26 and 15 shoots/m2in the 1979 and 1980 experiments.