Seed weevils are attacking OSR crops across the country, NIAB TAG has warned, pointing out that with many reports of infestations well above the ’take action’ threshold, crops are being left vulnerable to the far more yield-penalising bladder pod midge.
’High numbers have been reported particularly in central and eastern regions,’ says Ben Freer, agronomist at NIAB TAG. ’Seed weevils haven’t been a problem for a number of years but the warm weather has brought high insect activity.’
One seed weevil in every plant will damage 20-30% of pods and reduce yields by up to 5%. But Mr Freer is keen to draw attention to the bladder pod midge, which takes advantage of the holes born by the seed weevil. ’Both cause extensive damage to pods, but the bladder pod midge can do more,’ he says.
’Where the seed weevil will bore a hole and lay a few eggs that hatch and consume a few seeds, the weaker, more feeble bladder pod midge will use the same hole to lay far more eggs. The pod midge larvae will cause complete damage to the pod’s contents.
’Pyrethroid based sprays are the chemical solution to the seed weevil problem but growers face the risk of harming beneficial insects such as bees and the parasitic wasp which targets the seed weevil.
’Don’t undertake indiscriminate spraying. Target fields which exceed the threshold of one seed weevil per plant - you have to get deep into the crop to assess the true levels of infestation.
’Field walking through a thick oilseed rape crop is hard work,’ acknowledges Mr Freer, ’but get away from the headlands to avoid a false impression.
’Pod shatter from seed weevils can also be confused with bird damage so there is no substitute for tapping the flowers over your hand or a tray to knock out the little grey weevils as they ’play dead’,’ he says.
’In the dry conditions some OSR plants try to compensate and continue to flower. While this will address the damage caused by the insects, we have also seen instances of pod abortion, the crop investing the limited water in only a few pods.’
