Threats to crops are above and below ground

THE mild winter and cool spring will have an impact on pest issues in both winter and spring cereal crops this season.
In the SRUC Crop Clinic we have seen several cases of winter wheat ‘standing still’ after the application of spring nitrogen. The reason the crops are not responding to nitrogen is because their root systems have been affected by the feeding of free-living nematodes over the winter months.
This leads to patches of stunted plants, occasionally with some purpling to the leaves, particularly in lighter soils. The nematodes cause the roots to become thickened and excessively branched which restricts their ability to take up nitrogen and other nutrients from the soil.
The cold soil temperatures prolong this problem, which can also affect spring cereals.
Rolling the crop before GS31 can help as the nematodes will be crushed between soil particles around the root zone giving the roots some respite.
Bear in mind that purpling of leaves could also be due to nutrient deficiencies, so if symptoms are seen in winter and spring cereals, confirm what the problem is by sending plants and soil to the SRUC’s Crop Clinic.
Above ground, once temperatures warm up, aphids will begin to fly into winter and spring cereals from mid-May onwards and any aphids that have overwintered on the winter crop will start to multiply.
On warm sunny days look for aphids on leaves and consider an aphicide if aphid colonies can be found on 50% of plants. Later in the season, if two-thirds of the heads of flowering crops are infested with aphids, then an aphicide treatment may be worthwhile before milky ripe (GS73). After that any aphicide treatment will no longer be economic.
If you are seeing ladybirds or hoverflies in crops then hold off on any aphicides, as these specialist aphid predators will do a grand job of mopping up aphids – Integrated Pest Management (IPM) in action!
Check crops again a few days later and if aphids are still on 50% of plants then consider the aphicide option. But, it is important to remember that there are changes to the authorisations and labels for the aphicide pirimicarb.
Pirimicarb products with the ‘old label’ have to be used up by the end of July, so check stores and use up stocks when necessary. ‘New label’ pirimicarb products are only approved for use on peas and beans for aphids.
Alternative aphicides include flonicamid (after GS53 only) and the pyrethroid group, such as deltamethrin, alpha-cypermethrin, esfenvalerate, lambda-cyhalothrin, tau-fluvalinate, zeta-cypermethrin, and the organophosphate dimethoate.
Beware that the pyrethroid and organophosphate products are also ‘hot’ against the predators of aphids, so should only be used when threshold numbers of aphids have been reached.
Another reason to stick to the thresholds is that there is resistance in the grain aphid to the pyrethroid insecticides in Scotland, so use with caution and only treat crops if thresholds have been exceeded to reduce the risk of full resistance.