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Crops at risk: What to look out for this season.

30 December 2019

Wet conditions and late drilling have exacerbated some pest problems this season but some sharp frosts in November have reduced the aphid and BYDV risk by putting a stop on aphid flights into crops for the next few months, although any return of milder weather may well allow some aphids to multiply and potentially spread BYDV.

Leatherjackets are a threat in autumn-sown grass reseeds and winter cereals and checking out the cause of any poor cereal emergence or patches of poor grass establishment is worthwhile.  A DIY method is to search the soil by hand by row scratching.  Ten 30 cm lengths of crop rows from areas exhibiting suspected damage should be removed by trowel to beneath root depth.  The soil can then be hand sorted for leatherjackets, over a tray or empty fertiliser bag.  With the loss of chlorpyrifos, options, where leatherjacket damage is confirmed, are limited to rolling to try and crush the grubs in the soil.

It is a mixed picture for wheat bulb fly damage this season and late sown crops could be at risk.  Egg populations in the SRUC survey are generally below damage thresholds this season – the survey shows an average of 0.5 million wheat bulb fly eggs/ha, which is

Pic: Adas

below the damage threshold for early sown wheat (2.5 million eggs/ha) and later sown-wheat (1 million eggs/ha).  However, around 20% of the fields tested exceeded the 1 million eggs/ha threshold for risk to later sown crops (mid-October onwards), and with many wheat crops falling into the later-sown category this season and unlikely to be tillered by the time of egg hatch there is a risk of wheat bulb damage when eggs hatch in late January/February next year.  Crops that have received an Austral Plus or Signal 300 CS seed treatment should be protected.  Nothing can be done for other crops and no insecticides are now available for use at egg hatch or when deadhearts appear in February 2020.

Bear in mind that spring barley and spring wheat can get badly affected by wheat bulb fly too, particularly if sown before mid-March.  Crops at risk are those sown after potatoes, field vegetables, and fields where there was bare soil during the summer months, especially in areas with a history of the problem.  If there is a perceived risk, consider treating seed with Austral Plus or Signal 300 ES.

Slugs have enjoyed the wet autumn and wheat crops are particularly vulnerable to slugs up to GS12; beyond this stage, they can usually grow away from any further damage, although growth may have stopped now for the winter.  Slugs will be looking to attack spring wheat seed and shoots as they emerge.  Use slug traps to gauge the threat and apply pellets if there are signs of slug activity in emerging crops.  Crops, where there are gaps caused by slug damage, pests or patchy emergence, are at risk from pigeons as they like a space to land in and nibble away at the crop.  Keep an eye on crops and take measures to keep pigeons off where necessary.

Rape winter stem weevil grubs are now feeding away in several of the SRUC monitored crops.  An insecticide application now (even if travel were possible) won’t be able to control grubs that have already burrowed in the leaf petioles and stems of plants, so the gate is shut on weevil control for this season.  Watch for a lack of stem extension in the spring and manage then. Reports of cabbage stem flea beetle damage are still relatively low in Scotland which gives our crops a huge advantage relative to the south.

Andy Evans and Fiona Burnett SRUC, for the Farm Advisory Service.

Leatherjackets on soil

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