Delayed control of weeds in glyphosate-tolerant sugar beet and the consequences on aphid infestation and yield

An experiment was set up in 1998 to study the effect of glyphosate on the w eeds and pests in glyphosate-tolerant sugar beet, in comparison with a conv entional herbicide regime. Glyphosate at 1.08 kg ha(-1) was first applied a t the two- to four-leaf 8- to 10-leaf and 12- to 14-leaf stage of the crop, followed by a second application at the same rate two to four weeks later. Weed growth did not affect sugar beet emergence or establishment, but, in untreated controls and the two later glyphosate treatments, weeds almost co mpletely covered the ground, leading to reduction of root weight, sugar con centration and yield at harvest. The number of aphids (mostly Myzus persica e Sulzer) in the beet in June was significantly higher on plots treated wit h glyphosate at the two- to four-leaf stage than on untreated plots or plot s treated later with glyphosate. Large numbers of the leaf curling plum aph id (Brachycaudus helichrysi Kaltenbach) colonised the weeds within untreate d or later-treated plots. These were accompanied by predators and parasites which eventually caused substantial mortality in the aphid populations. Th ere was evidence that glyphosate-treated weeds, although not in competition with the crop, were still able to provide sustenance for aphids. Very low levels of virus yellows were observed in the trial, and there were no signi ficant differences between treatments. The results suggest that the latest application of the first glyphosate spray in a two-spray programme should b e before the eight-leaf stage of the sugar beet to prevent weed competition reducing yield. Further studies on late control of weeds and insect divers ity are being carried out. (C) 2000 Society of Chemical Industry.