Patchy oilseed rape crops will need closer monitoring for pollen beetle populations to avoid damage this season, warns eastern counties independent agronomist, Andrew Blazey of Prime Agriculture. Although new recommendations indicate crops with a lower plant population are better able to compensate for pollen beetle attack by producing more pods, he believes there are many other factors that still impact on final pod numbers and, ultimately, crop yield.
Speaking to growers at a Syngenta Driving Up Oilseed Rape Yields meeting, he said: There are some good crops with strong plants at relatively low plant populations, where the new thresholds could be implemented.
However, there are far more crops that look extremely sparse as a result of the adverse conditions this season. We will have to do everything possible to manage those crops through to harvest, including preventing further losses to pollen beetle, he advised. He believed losses from pollen beetle damage to flower buds could be exacerbated in backward crops that are proving slow to recover in the cold early-spring weather – with the potential for a protracted green-bud growth stage.
Conscious of the UKs growing risk of pyrethroid resistant pollen beetle population, Mr Blazey advocated growers closely monitor beetle activity after an initial treatment. If we suspect that control has been limited following a pyrethroid application, it would be good practice to switch to a resistance breaking option, such as plenum, with any subsequent treatments on the farm.
He also believed that early intervention to prevent Pollen Beetle number building-up was a sensible approach to limit risk of resistance populations developing, and gave time to resolve issues before it was too late for crops to respond. The use of a renowned pyrethroid, such as Hallmark Zeon, could also be relied on to give a better result.
Syngenta technical manager, Chris Charnock, reminded growers that the green to yellow-bud stage is the critical timing for controlling pollen beetle and is the correct stage for Plenum treatments. The effects from the direct feeding damage of the beetles, and the subsequent hatch of larvae from eggs laid in the buds, could have been largely underestimated in backward crops that have limited time or opportunity to compensate.
Once the crop is in flower, however, the yellow petals are an attractant for beetles, where the pollen is readily accessible. Research has shown crops should be relatively safe, and no further treatments should be required.
Mr Charnock advised that a further advantage of using Plenum early at the green to yellow-bud stage is to reserve permitted Hallmark Zeon treatments to tackle the threat of seed weevil and brassica pod midge. An adult Seed Weevil can lay eggs in up to 50 pods, with the developing larvae typically consuming a quarter of the pods seed. However, the holes left also allow pod midge to lay their eggs, which can completely remove the pod.
Seed weevils must be controlled whilst active in the crop, before they lay their eggs in the pods; there is usually a window of two to three weeks from initial infestation. Good control of weevils can successfully minimise the impact of pod midge, and will help to ensure maximum survival of pods through to harvest, he added.