Archive Article: 1997/09/20

20 September 1997

Spraying is not the only way of tackling wheat bulb fly. Gilly Johnson visits a Norfolk farm where predatory beetles are being encouraged instead.

WHEAT bulb fly larvae made a meal of cereal profits in much of East Anglia last year.

Like many of his neighbours, Norfolk grower Nigel Carey has resorted to conventional defence tactics in the past. This has meant spraying as soon as grubs are seen in the plant stems.

But that was then. Now he is experimenting with a different approach – harnessing natural predators and cultural techniques to clean up the fields. He is being helped by agronomist John Purslow and Louise Catling, who is monitoring the predatory insects.

It is not an easy task, because the risk is relatively high. The 390ha (963 acres) of sandy loam at Mill Farm, Great Witchingham, supports a rotation which includes sugar beet, malting barley, wheat and vining peas. So there are ample opportunities for the wheat bulb fly to find the bare soil it prefers for summer egg laying. And the fine, sandy soil allows the larvae easy travelling from tiller to tiller, at egg hatch in the spring.

In previous years Mr Carey has used "deadheart" sprays of dimethoate: "I feel this is a pernicious product environmentally because of its blanket kill," he comments. "We would prefer to do without, if we could."

Last spring, wheat bulb fly attack became only too painfully clear. On one field, about 40 shoots/sq m were damaged. Mr Carey was preparing to spray, but Mr Purslow advised against it.

So Mr Carey kept the sprayer out – saving money, and reducing the risk of environmental damage. "On close inspection we discovered that a third of the problem was due to opomyza, not wheat bulb fly larvae," explains Mr Purslow. This pest is not as damaging, as it does not migrate between tillers like wheat bulb fly. Instead only one shoot is affected.

"An organophosphate spray would have only been an act of revenge – it wouldnt have been cost effective," he argues. "And it would also have knocked out many insect predators such as Carabid beetles. Potential ear numbers were high at over 1,000/sq m, and so the crop was able to withstand attack."

Respectable yields this harvest show that it was the right decision, says Mr Carey. "The crop ended up with 700 ears/sq m, which on this soil was about right."

So this year hes changing course on wheat bulb fly control. First avenue of attack is cultural. Where the rotation allows, early drilling has been proved to be effective – well established crops can survive wheat bulb fly far better than late-sown cereals.

Taking this argument further; if the crop starts off with more than enough tillers, then some can be safely sacrificed to the larvae in the spring. So Mr Purslow is advising early drilling at higher seed rates than would normally be used with this light soil.

His recommendation for wheat on one field following vining peas is for 175 seeds/sq m drilled in the first week of September, going up to 195 seeds/sq m if operations are delayed into the second week. The seed rate during the second half of September would need to be higher still – 225 seeds/sq m.

Consort – "one of the few varieties suited to early drilling" – is the wheat Mr Purslow has chosen.

"This should leave us with 1,000 tillers/sq m in the early spring, enabling the crop to survive moderate attack without economic loss. Its also the case that attack is increased when prostrate tillering occurs – and this is often seen when plant numbers are low."

The cost of the extra seed must be taken into account, says Mr Purslow. But this should be weighed against savings possible from avoiding "deadheart" dimethoate sprays – at upwards of £5.75/ha (£2.30/acre) for each application. Pre-egg hatch sprays are another widely-used alternative – these cost about £17.75/ha (£7.20/acre). For comparison, using more seed would add about £5.60/ha (£2.20/acre) to the seed bill.

For the early drillings, a consistent, shallow depth is best, because a shorter shoot is less of a target. Mr Carey uses a Vaderstad drill which he believes places seed accurately, in spite of varying soil type.

"Conventional drills, which push the coulter into the soil, are more at the mercy of soil resistance. This drill holds the coulter out at the required depth and is more consistent over our varying soils within each field."

The unit also leaves the soil in a firm condition, which helps with control of wheat bulb fly and other soil pests. "Since using the Vaderstad we havent had to use slug pellets," says Mr Carey.

For later drillings following sugar beet, other factors have to be taken into account, such as soil moisture. But in the main, seed should be drilled as shallow as is feasible – because the deeper the shoot, the greater the chance of larval attack, says Mr Purslow.

Such cultural practices are no secret. More revolutionary are Mr Purslows attempts to discover how beetle predators might be encouraged to polish off wheat bulb fly eggs and larvae.

"As yet, no-one has quantified what impact ground beetles might have on this pest," he comments. So with the co- operation of Mr Carey, the field following vining peas now contains a farm trial.

Farm practice

The major area is being ploughed, then drilled – as would be standard farm practice. But one section is to be tine cultivated instead. Traps will provide the evidence as to whether the different cultivation techniques have an effect on beetle populations throughout the season.

Mr Purslow suspects that ploughing could be bad news for the ground living beetles, because of the chaos of soil inversion on the beetles habitat. And this may have a knock-on effect on wheat bulb fly larval populations in the spring.

"But theres also the question as to what happens to the wheat bulb fly eggs, when they are ploughed in – will some be killed by ploughing? Or does ploughing worsen matters, by extending the egg hatch period, because it takes longer for the warmth to reach them in the spring?"

He suspects that early egg hatch is likely to be safer for the crop, because late frosts can kill a large number of larvae.

Mr Purslow and Ms Catling will be attempting to find some answers this season. As an environmental enthusiast, Mr Carey supports their efforts – though he recognises that non- spraying strategies could involve more management time and effort.

"Part of the problem is that insecticides such as dimethoate are too cheap – its only too easy to spray just in case," he comments. "Its much more difficult working out whether or not its really justified – but thats what we must do to help nature work with us."

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