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Warm Weather Leads To Spike In Aphid And Midge Numbers

30 June 2018

Damage to leaves (and trousers) from beetle grubs is common this year – close up of the grub

The warm spell of weather has led to aphids flying into cereal crops and numbers are starting to build up on leaves and ears. Look for aphids on the leaves of winter and spring cereals on warm sunny days but stick to thresholds for treatment. An aphicide is only needed if aphid colonies can be found on 50% of plants on the flag leaf or if two-thirds of the heads of flowering cereal crops are infested with aphids. Beneficial insects such as hoverflies and parasitised aphids in many crops will help to keep aphid numbers below damaging levels. The milky ripe  stage of grain ripening (GS73) is the cut off point for when it is uneconomic to treat for aphids.

Aphicides are available from a number of chemical groups including flonicamid, pyrethroids,  alpha-cypermethrin, esfenvalerate,  lambda-cyhalothrin, tau-fluvalinate, zeta-cypermethrin and the organophosphate product dimethoate. Check individual products for specific approvals and restrictions and note that there is evidence of resistance in the grain aphid to the pyrethroid insecticides in Scotland, so use aphicides with caution and only treat crops if thresholds have been exceeded.

Laundry bills have shot up this month as cereal leaf beetle grubs are common in crops of wheat and barley. The grubs are slimy and resemble small slugs. Most crops are usually able to cope with the damage, but if the flag leaf is being badly grazed then an insecticide treatment may be necessary. The grubs should disappear by the end of June to be replaced by the adult beetles from early-July onwards, by which time most crops should be safe from any significant damage.

Saddle gall midge is potentially a serious pest of continuous cereals such as wheat, barley and rye (but not oats), and have been seen in Scottish crops occasionally over the last few seasons. Adult saddle gall midge should have appeared by now, and lay red eggs on the leaf surface in a line or grid following the leaf veins. The red larvae burrow underneath the leaf sheaths feeding on the stem. The effect of feeding results in galls which can be seen as saddle shaped depressions on the stem. The ‘saddle’ galls weaken the stems and restrict the flow of nutrients to the ear which can develop a `bleached` appearance. Where numerous galls have formed the stem can be damaged, to the extent that it breaks with the ear falling to the ground. Growers are advised to keep an eye out for the red/orange coloured midges in second and continuous cereal crops and look at leaves for the presence of eggs.

A heavily infested crop of spring barley

Orange wheat blossom midge usually appears around about now in Scottish crops. Midges need a warm, dry spell of weather after rain to emerge, so areas that did get rain last week may now get a flush of midges. The midges only lay eggs in the ears from early ear emergence (GS51) up to GS59, so the best time to look for midges is on warm, still evenings by standing in the crop and looking for the midges resting or flying around the ears of the crop. Orange wheat blossom midges are around 3mm in size. For feed wheat crops, 1 or more midges per 3 ears are needed to justify an insecticide, and for milling wheat crops, 1 or more midges per 6 ears are needed to trigger insecticide treatment.

Pheromone traps can also monitor the appearance and numbers of midges in crops, and these need to be checked regularly during the susceptible growth stage of the crop. Thresholds for pheromone trap catches recommend that sprays are only necessary when trap catches exceed 120 midges a trap a day. Monitor fields daily when trap catches exceed 30 midges a day.

Insecticide treatments need to be applied as soon as possible once the threshold levels are reached, as they target the midges to prevent egg laying rather than the eggs and larvae of the midge. Remember that the window where damage occurs is between GS51-59, so where the crop is beyond GS59 then there is little chance of any midge larvae causing damage. Several varieties have resistance to orange wheat blossom midge so will not require any insecticide treatments (e.g. Skyfall, KWS Barrel, KWS Basset, Zulu, KWS Jackal, Elation, LG Sundance, LG Motown, Leeds, Myriad, Viscount, RGT Gravity, Gleam, KWS Kerrin, KWS Santiago, Reflection, KWS Crispin).

Fiona Burnett and Andy Evans

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