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Beet crop concerns as reports of leaf miner activity increase

Levels of leaf miner (mangold fly) and associated damage are increasing in a number of sugar beet crops and are once again causing concern for many growers, according to the British Beet Research Organisation.


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Leaf miner threat to sugar beet crop#sugarbeet

In 2015 the pest caused serious problems in beet crops in the area around the Wash in particular. In extreme situations up to 400 eggs per plant (6-8 true leaves) were found in crops in late May and there was evidence to suggest seed treatments were running out of steam at 10-11 weeks post-sowing. Yield losses as a result of attack by the pest were put at up to 15 per cent.

 

After gaining a one-off emergency authorisation for use of thiacloprid insecticide Biscaya last season, the BBRO is once again working with industry and regulatory authorities to obtain an Off-label Extension of Use for the insecticide in sugar beet crops this season.

 

Growers are reminded the previous emergency off-label approval from last season was time limited and did not extend to the current season.

 

BBRO says it has received numerous reports of leaf miner activity, with hotspots including the Wash area, parts of Norfolk, Suffolk and Cambridgeshire, where some crops are under severe attack says BBRO lead scientist Mark Stevens.

 

“The signs of leaf mining are a signal that larvae are active and they should be clearly visible to the naked eye. If unsure, hold the leaf towards the sun to check for presence of larvae.

 

“We are seeing exactly the same thing as we saw 12 months ago, almost to the day,” he says.

 

Leaf miner problems have been building over the past five seasons, probably as a result of mild winters, adds Dr Stevens.

 

Currently the only product with specific approval for foliar application for leaf miner is Hallmark Zeon (lambda-cyhalothrin) and while the level of control was not as high as with some other insecticides in BBRO trials in 2015 it still gave protection against yield loss.

 

At recommended rates a maximum of two applications can be made to sugar beet for leaf miner control, growers are advised.

 

“As with all pyrethroids we need to target applications carefully to minimise the number of applications, protect beneficial organisms and any potential for resistance build up," says Dr Stevens.

 


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Controlling leaf miner

BBRO trials work in 2015 showed potential yield reductions where populations remained untreated once the initial protection by seed treatments had elapsed.

  • The threshold for treatment is when the number of eggs and larvae exceeds the square of the number of true leaves. For example, a plant with four true leaves would need a population of 16 or more eggs and larvae to warrant treatment and a crop with six true leaves would need a population of 36 or more eggs and larvae. Control by insecticides was most effective at egg hatch/early larval development so timing of applications is important. Monitor crops closely to establish the optimum time for application
  • Target first applications when numbers of eggs and larvae are above threshold to control the first generation as soon as eggs hatch and then check the larval activity for the need of any follow up treatment. It may be prudent to retain the second application to control the next generation of egg hatch
  • As the season progresses different generations may potentially overlap. With crops planted at different dates there are likely to be populations at different stages. Check first planted crops as a priority. By regularly monitoring and recording crops for eggs and larvae number it should be possible to plot where the population is in the development cycle
  • Keep a record of numbers counted on average per plant and of the growth stage (number of leaves) to establish the treatment threshold, as well as the effectiveness of treatment
  • Check at least 10 plants in different representative areas of the field to establish a more accurate estimate of levels

Source: BBRO

 

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