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Pests & Weed Update for All Cereals – Late June

25 June 2020


The return to cooler and damp weather has curtailed aphid flights into crops to a great extent.  However, aphids will still be building up numbers on crops. Look for aphids on leaves on warm sunny days and consider an aphicide if aphid colonies can be found on 50% of plants, or if two-thirds of the heads of flowering crops are infested with aphids. If you are seeing ladybirds or hoverflies in crops then hold off on any aphicides, as these specialist aphid predators will do a good job of mopping up aphids. Check crops again a few days later and if aphids are still above the thresholds outlined above then consider the aphicide option.

Cereal leaf beetle damage has been reported from several crops. The grub stage eats strips out of leaves. The grubs are slimy and resemble small slugs and will leave a nice stain on your trousers as you walk through the crop. Most crops are usually able to cope with the damage, but if the flag leaf is being badly grazed then yield can be affected. An insecticide applied for aphids will have an effect on cereal leaf beetle grubs. The grubs should disappear by mid-late June to be replaced by the adult beetles from early July onwards, by which time crops should be safe from any significant damage.


Now is a good time to keep an eye out for black-grass or other grass weeds visible above the crop.  Act fast if you see black-grass because it can shed viable seeds well before harvest.  Due to their low dormancy and if they are not ploughed down, most of the seeds will grow in the following crop.

It is not always clear where an infestation comes from – contaminated machinery is an obvious potential culprit, but the finger is also pointed at seed, straw and sometimes manure.  Field hygiene and prevention of spread is increasingly important as grass weeds become more difficult to control with herbicides.

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