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

Abstract

An experiment was set up in 1998 to study the effect of glyphosate on the weeds and pests in glyphosate-tolerant sugar beet, in comparison with a conventional herbicide regime. Glyphosate at 1.08 kg ha À1 was ®rst applied at 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 completely covered the ground, leading to reduction of root weight, sugar concentration and yield at harvest. The number of aphids (mostly Myzus persicae Sulzer) in the beet in June was signi®cantly higher on plots treated with glyphosate at the two-to four-leaf stage than on untreated plots or plots treated later with glyphosate. Large numbers of the leaf curling plum aphid (Brachycaudus helichrysi Kaltenbach) colonised the weeds within untreated or later-treated plots. These were accompanied by predators and parasites which eventually caused substantial mortality in the aphid populations. There 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®cant differences between treatments. The results suggest that the latest application of the ®rst glyphosate spray in a two-spray programme should be before the eight-leaf stage of the sugar beet to prevent weed competition reducing yield. Further studies on late control of weeds and insect diversity are being carried out.

Ecological compatibility of GM crops and biological control

“…Uniformity in habitat structure resulting from monoculture curtails resources available to natural enemies, even when prey are abundant. Increasing plant diversity may benefit natural enemies, and enable them to better respond to pest outbreaks (Speight and Lawton, 1976;Buckelew et al, 2000;Dewar et al, 2000;Haughton et al, 2001a;Bell et al, 2002;Dewar et al, 2003;Pitre, 2004a, 2004b). But adding vegetational complexity to farmland does not always improve biological control (Bugg et al, 1987;Russell, 1989;Andow, 1991;Gurr et al, 1998).…”

“…In general, herbicide-tolerant GM crops provide farmers flexibility in the timing of weed management within their fields. Weeds can be allowed to establish and mature until they become competitive with the crop (Clay and Aguilar, 1998;Dewar et al, 2000), and these pre-economic populations provide a number of ecological services that include promoting natural enemies (Zandstra and Motooka, 1978;Altieri and Whitcomb, 1979;Norris and Kogan, 2005). Cover crops are another field-wide source of vegetation and biodiversity whose agronomic benefits are well documented (Clark, 1998), and whose management can be attuned in part to the dynamics of specific pests using herbicide-tolerant crops.…”

On the rationale and interpretation of the Farm Scale Evaluations of genetically modified herbicide-tolerant crops

“…Three factors, in particular, needed to be considered when comparing conventional practice with the combination of one of these two herbicides and a GMHT crop (Buckmann et al 2000;Dewar et al 2000Dewar et al , 2003Firbank et al 2003). First, glyphosate and glufosinate-ammonium act mainly through contact with foliage and do not reside in a herbicidal form for as long as many other herbicides.…”

Integrated weed management systems with herbicide-tolerant crops in the European Union: lessons learnt from home and abroad

“…Weeds occurring between rows after an early over-the-row band spraying could be controlled by a later broadcast treatment. [101][102][103] Some weeds can be left for a longer period between the crop rows without causing yield loss; these weeds can support beneficial invertebrates during the early to mid-season, [102] and produce seed in the autumn as food for birds. [104] These effects can only be achieved if growers do not apply long soil residual herbicides at sowing.…”