GM varieties are more than just weed beaters beetgfgfgfgfgfgfgfgfgfgfg

2 June 2000




GM varieties are more than just weed beaters beetgfgfgfgfgfgfgfgfgfgfg

Recent research suggests genetically modified sugar beet

tolerant to broad-spectrum herbicides offers much more than

simplified weed control. Andrew Swallow reports

GENETICALLY modified herbicide tolerant sugar beet seems set to give growers far more than just weed control.

Reduced pest attack and greater in-crop biodiversity should be achieved with the new lines, say scientists at IACR-Brooms Barn.

"I never expected to be working with GM crops in pest control so soon," says senior entomologist Alan Dewar. "But the 1998 herbicide tolerance trial changed my outlook on the crop and has started me thinking laterally."

Recently published results show that glyphosate-tolerant varieties would allow growers to leave weed control to at least the four-leaf stage of the crop without reducing yield (see table).

Until then a carpet of weeds grows which can confuse invading aphids, it seems. "An aphid flying in cant find the beet plants so easily," Dr Dewar explains.

But in the 1998 the number of aphids colonising the beet plants was reduced only when the first glyphosate treatment was delayed to the 8-10 leaf stage of the crop, and that incurred a yield penalty.

"Clearly, the timing of applications is going to be critical if growers are aiming for maximum biodiversity," says senior liaison officer Mike May.

Combining herbicide tolerance with band-spraying could overcome the problem, says Dr Dewar.

"In other studies band-spraying of weeds within rows, and allowing the survivors to remain between them significantly reduced aphid colonisation and virus infection."

In a conventional crop, rotovating is required to remove large weeds between the rows. That is difficult and unreliable in wet conditions, so few growers have tried it. But in a herbicide-tolerant crop a simple second application of herbicide would do the job.

Besides making the crop hard to find for beet-seeking aphids, the surrounding weeds could attract other non-damaging aphid species as a food source for beneficial predators, he says.

In 1998 speedwells drew large numbers of leaf-curling plum aphids. In turn, these attracted predators and parasites which eventually caused substantial mortality in all aphid populations. A similar luring of aphids onto volunteer potatoes has also been seen.

When applying glyphosate, growers could use coarser sprays than with conventional beet herbicides, so reducing drift risk, he adds.

Such potential environmental benefits from GM crops, if proven, could help persuade groups such as English Nature and the RSPB that UK growers should have access to the technology.

But even without the environmental advantages, growers are likely to be keen to take it up for ease of management and economic reasons, Mr May believes.

On the fen peat on which the 1998 work was done, the conventional herbicide programme cost over £200/ha (£80/acre) compared with about £30/ha (£12/acre) for the two-spray glyphosate approach.

"The benefits of herbicide tolerance in beet are so much more than in other arable crops because beet is such a poor competitor and the cost of applications are so high," says Dr Dewar.

But the herbicide savings will have to be offset against higher seed costs and possible technology fees, he says.

Both men agree that, for sugar beet growers at least, GM technology, including crops tolerant to broad-spectrum herbicides, could be a powerful tool.

But many of the potential management benefits are impossible to develop before the crop is allowed to be grown commercially, and that remains in the hands of the government and British Sugar. &#42

GM BEET ECO-BENEFITS

&#8226 Weeds hide beet from aphids.

&#8226 Non-beet aphids support beneficials.

&#8226 Possible reduced insecticide use.

&#8226 Greater biodiversity?

&#8226 Weather delays no problem.

Glyphosate-tolerant trial


Treatment Relative sugar yield Aphids/plant June 19 Aphids/plant June 29

Untreated 29 0 0.1

5-spray conventional 100 0.3 0.5

2 x glyph early 105 1.0 0.9

2 x glyph medium 76 0.2 0.3

2 x glyph late 68 0 0

All plots sown with glyphosate-tolerant beet.

Timings: Early = 2-4 & 12-14 leaves; medium = 8-10 & 12-14 leaves; late = 12-14 & 16-20 leaves.

An under-storey of weeds (left) in GM beet can provide a valuable habitat for insects and food for birds without compromising yields, provided the broad-spectrum herbicide goes on before the four-leaf stage (right), says Brooms Barn entomologist Alan Dewar.


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