Cereal crops could be at greater risk this autumn following the loss of seed treatment protection against aphids and heightened slug pressure.
Growers need to be vigilant over coming weeks and integrate both cultural and chemical controls. This is according to Hutchinsons technical director, Dr David Ellerton, who says: “Wet weather across many parts of the UK this summer means slug pressure in early-sown cereals could well be higher than normal where soils are moist enough."
The situation is compounded by the loss of the clothianidin-based seed treatment, Deter.
Dr Ellerton adds: “Many growers, especially those sowing early, became used to the protection afforded by Deter, so this season they could be facing issues not encountered for several years.”
In terms of the slug threat, Dr Ellerton says Deter used to effectively prevent slugs grazing on developing embryos and hollowing grains, so without it, more cultural control is needed before and after drilling, combined with targeted pellet applications where necessary.
Where treatment is required, growers still have access to metaldehyde after the ban was overturned earlier this summer, as well as ferric phosphate-based pellets.
“They both offer similar efficacy, although metaldehyde is slightly less effective in colder conditions, so these types of pellets are generally better used earlier in the season, whereas ferric phosphate may be preferable once the weather turns colder,” Dr Ellerton says.
“Always follow stewardship guidelines and, if there is any risk to watercourses, avoid using metaldehyde altogether.”
The loss of seed treatment protection against aphids potentially represents another big challenge for growers.
While much depends on the weather and aphid flight over coming months, predictions based on worst-case scenarios of early drilling, early infection, a mild autumn and prolonged high aphid activity, suggest crops could need treating with a pyrethroid multiple times before Christmas.
Hutchinsons technical manager Neil Watson says: "We do not know what the season will bring, but growers have to be aware of the risks, especially when drilling cereals early. It is something farmers in the South West have always been familiar with, even with Deter, but aphids may have been less of a concern elsewhere."
Aphids can cause crop damage through direct feeding when present in sufficient numbers in their own right, but of greater concern to many is the transmission of Barley Yellow Dwarf Virus (BYDV).
Mr Watson says a pilot study carried out by Rothamsted Reasearch in the autumn of 2018 looking at the percentage of bird cherry aphid alone carrying virus, suggests substantially more than the 3-5 per cent initially thought could be carrying virus, so the risk must not be underestimated. Testing will be scaled up this autumn.
He adds: "There is not much we can do to stop individual plants being infected when aphids first fly into the crop, so the focus is on monitoring the threat and preventing further spread from these primary infection points.
"Remember, if the crop emerges over a prolonged period of time due to dry seedbeds, and aphids colonise the crop early, the timing of treatment must be related to the earliest emerged sections. Do not wait for full crop emergence otherwise it could be too late."
The T-Sum calculation (available here) is a useful way of predicting when the third generation of aphids will emerge and spread the infection further, but it should not be relied on entirely, he says.
Do not wait
“If there are very high numbers of aphids feeding in a crop, you have to deal with them there and then. Do not just wait for the 170-degree days threshold to be reached."
Mr Watson says BYDV can be spread by different species of aphid. In southern England, the bird-cherry-oat aphid is the main vector, while further north, the grain aphid is generally more prevalent. Pyrethroid resistance is a greater concern in the latter species, but with no other chemical controls currently available, all growers must consider resistance risks and employ integrated controls.
Key cultural slug controls
- Use stubble rakes or light cultivations (for example, light harrow) to disturb slug habitat in the surface trash layer and help destroy eggs
- Prepare a fine, firm seedbed and consolidate it well. This reduces slug movement within the soil and improves seed-to-soil contact, helping crops establish quicker and grow away from potential damage. Avoid drilling into cloddy seedbeds where possible
- Drill seed deeper (4-5cm) to place germinating seed away from surface-dwelling slugs. Once through the surface, crops will be prone to grazing so vigilance is needed
- Monitor risk carefully before and after drilling to prioritise control measures. Aim for 10-13 slug traps per field, focusing on the highest risk areas (for example, wetter patches, heavier soil)
Top tips for managing aphids
- Remove the 'green bridge' between crops to prevent direct transfer of wingless aphids from volunteers or grasses to the new crop. Ensure volunteers are effectively destroyed and well before drilling. Aphids can survive for up to six weeks below ground if they are feeding on green material, and can directly transmit virus through the roots
- Drill later. The risk of indirect transfer from winged aphids flying into a crop is significantly greater when drilling early (September), but usually declines as temperatures fall later in autumn (for example, by mid-October to mid-November, depending on the season)
- Monitor risk closely. Walk crops regularly and use industry monitoring tools to assess in-season risk. Consider placing sticky traps around field boundaries, especially downwind of potential risk sources, such as grass crops, margins or woodland
- Consider variety choice. We now have the first generation of winter barley varieties with a degree of tolerance to BYDV (this does not alleviate the need to spray)
- Apply pyrethroids where and when necessary. Avoid compromising spray timing by combining herbicide and insecticide sprays to save on passes where timings do not coincide