Next fortnight critical for pea and bean weevil
©PGRO The next two weeks is a critical time for emerging spring pulse crops, as warmer weather could raise the pea and bean weevil threat with yield losses of up to 25%.
Spring pea and bean crops have just started to emerge in eastern areas and this is an important time, says Beck Ward, PGRO’s principal technical officer. Weevils will then start to disperse and become more widespread.
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Farmers need to be alert to signs of the pest when walking crops, which will be seen in headlands first. Early symptoms are the characteristic leaf notching.
‘Lure and kill’ approach may stalls sprays
PGRO has started a three-year project looking at biological control of the pest in collaboration with four other partners.
PGRO principal technical officer Becky Ward explains the trial is looking at a “lure and kill” approach, as an alternative to spray applications.
“Weevils are lured by the pheromone attractant into an inoculation station and become infected with a pathogenic fungi.”
When beetles leave the device they will spread the disease to other beetles, and it is hoped that this will cut insect numbers and crop yield losses, she says.
The project is partially funded by Innovate UK and the Biotechnology and Biological Sciences Research Council, with additional support from industry partners PGRO, BASF, Oecos and Exosect with Rothamsted Research.
But leaf notching is not the most damaging effect, she explains. Once emerged, the adults lay eggs which are washed into the soil. The larvae hatch and feed on the root nodules – this is what does most damage.
And failing to control the pest can lead to yield losses of 25-30%, she warns.
Monitoring
Ms Ward advises monitoring emergence with traps. And those who have drilled spring crops should place traps as soon as possible.
“Place the five traps on the headland of a field where the crop has been previously grown. This will indicate when they come out of their overwintering area [hedges and tussocky grass margins].”
She says that the traps contain a pheromone attractant.
“The traps are a good way of forecasting risk and are very efficient.
“We’ve had traps out in trial areas for one month now and have seen low numbers so far. This indicates they have started to emerge, but the cool weather has slowed down emergence.
“Weevils prefer temperatures of 12C or above, so if it warms up, we could see higher numbers.”
When to spray
Farmers who have high-risk fields and know they have a history of attack should spray at first signs of leaf notching. With traps, the threshold for spraying is an average of 30 across the five traps. These should be checked three times a week.
“We are currently seeing about five a trap, but this could increase as temperatures rise.”
Early control is key, she says, because most damage to root nodules occurs up to six weeks after egg laying.
What to spray
“Currently, we only have pyrethroids and some heavy infestations will require a second spray two weeks later.”
Ms Ward explains that the aim of the treatments is to prevent the adults laying eggs, so there is no larvae to feed on the roots.
Hutchinsons agronomist David Morris, who covers Worcestershire and Gloucestershire, advises using a good-quality cypermethrin at the first signs of notching.
“A typical early emergence spray is cypermethrin plus manganese. However, cold soils mean drilling is further behind.”
Other risk factors that lead to greater damage include stress, such as a dry spell after drilling.
“This puts plants under pressure and they are more affected by larvae feeding on roots. Stress from wet spells also increase risk.”
For more information or to source the traps, visit the PGRO website, which includes a technical guide to controlling pests.
