- The Vegetable Farmer
It could be a challenging year for Brassica growers warns Allium & Brassica Agronomy's senior consultant Andy Richardson. In the first of three articles looking into the implications of the loss of neonicotinoid seed treatment Cruiser (thiamethoxam), The Vegetable Farmer asked him how brassica growers will have to adapt their crop protection this year.
With a forecast of aphids flying two weeks earlier and the loss of the seed treatment's two months of protection, the focus of pest control this year has to be aphids. In long season crops - anything that grows through both peaks of aphid flights - we could be needing to control aphids for five months with, in worst case situations, such limited options that we'll struggle beyond three months.
Published in AHDB aphid news on 8th March, the Rothamsted Insect Survey brassica and potato aphid forecasts for first capture at Kirton, Lincolnshire were; peach-potato aphid 10 April-15 May and; cabbage aphid 3 April-9 June.
The publication explained that in a reversal of last year's cold weather event, unseasonably warm weather at the end of February this year has pushed winter temperatures up by around 1oC above the 30-year average throughout most of Britain, suggesting that aphid flight may be about two weeks early.
Expect May flights
This was no surprise. During the warm spell at the end of February we were spotting aphids on the move and that's something we've never seen before. We're therefore expecting May flights so long season crops like Brussels sprouts and Dutch white cabbage could have aphids coming into them very soon after planting. And when aphids arrive early they can build up to larger numbers.
Although the number of insecticide product withdrawals has equalled authorisations over the lasttwo years, their uses have been markedly different. We have lost three that were important for aphid control – Cruiser, pirimicarb and Plenum (pymetrozine) - and gained two cyantraniliprole products for cabbage root fly and caterpillar control; Verimark and Minecto One.
In time the Teppeki (flonicamid) EAMU, gained last December for two applications to control aphids in cabbage, will help restore the balance and we hope sprout growers will have it too by 2020. But this year we fear it will be very difficult to buy, due to the dramatic increase in the area of crops needing substitutes for Cruiser, including 100,000ha of sugar beet for which Teppeki is the only authorised foliar insecticide for aphid control.
Limited foliar options
This year most brassica growers will have very limited options. In most crops aphid control will have to be achieved with two of the remaining foliar insecticides: Biscaya (thiacloprid) and Movento (spirotetramat). Sprout growers also have an EAMU for InSyst permitting one application per crop, but growers must use no more than two foliar applications of any neonicotinoid insecticide per crop.
With the maximum number of treatments per crop being two Biscaya and two Movento, based on a fortnight's activity from each we have a backbone of two months protection. Starting with a Verimark drench should add a further three to four weeks giving us in round terms three months protection. Only those lucky enough to still have some Plenum (pymetrozine) in the chemical store, or cabbage growers who can access some Teppeki, will be able to plan a programme to see themselves comfortably through both peaks of aphid fight.
But we're not just thinking about controlling aphids and their contamination now. We're also concerned about the viruses they can spread, in particular turnip yellows virus (TuYV). We've largely forgotten about it, probably because the seed treatment significantly suppressed its spread. It was effective because aphids picked it up soon after entering the crop and it controlled them before they spread much virus. Foliar insecticides are nowhere near as effective because you have to find aphids before treating so they have much more chance to spread virus before being controlled.
TuYV is symptomless in growing plants but leads to head or sprout weight reduction at harvest, followed by development of internal tip burn causing breakdown of cabbage heads in store.
Research undertaken by Warwick University and the Allium & Brassica Centre has shown the worst-case scenario – a susceptible variety infected immediately after planting – to reduce marketable yield by 60 per cent. The seed treatment virtually eliminated the issues of both yield loss and internal tip burn
Verimark may be a partial solution. Although it only has cabbage root fly on the label we know it controls caterpillars, flea beetle, leaf miner and whitefly and gives some aphid control for three to four weeks post planting. We've seen it give yield increases too, which we suspect is attributable to the wide range of foliar and soil pests controlled. But it is very highly systemic so the plant has to be growing for it to work.
Growers should however note that opting for a Verimark drench precludes subsequent foliar use of Minecto One due to the maximum total dose restriction on cyantraniliprole. You have to choose to use it as a drench or a foliar spray. Our big worry is early aphids leading to early virus infection so we're going with the drench option this year on long season crops.
Module drenches of Verimark or Tracer (spinosad) will stop early build-up of caterpillars reducing a problem we used to see carried through all the way to the end of October. But if you want to keep the option of Minecto One open for further control of caterpillars and cabbage root fly, you have to specify Tracer.
When it comes to foliar sprays we're going to have to think hard about what to use and when and that requires understanding of how they work. Movento's two-way systemicity means it should be applied when plants are actively growing and pests are actively feeding and that it shouldn't be applied when they are not i.e. during drought or cold periods.
Reaching hidden pests
The unique benefit we've found from Movento's two-way systemicity is that it works down the plant rather than up it so when you spray the older top leaves of dense sprout crops it's re-distributed down to pests hidden below the canopy.
Biscaya is mobile up the plant and within the leaf, so it controls aphids feeding on the underside of leaves. It is therefore best used earlier for knockdown when the crop structure is open or later when it's turned cooler and conditions are no longer conducive to Movento.
For whitefly, Movento is the product for population control. It's certainly not a knockdown product and takes a few days to get to work but does a very thorough job. The trick is to get it on early; don't leave it until the population has built up. And when it's hot and dry like last summer, add an adjuvant to disrupt the wax layer on plant leaves. Biscaya and Plenum will also help with whitefly by suppressing them but won't control them in the way Movento does.
Last July and August also provided ideal conditions for thrips on cabbage and they were getting though a life cycle in 10 days. Minecto One can be pretty useful on them but if you've used a Verimark drench your only knockdown option is foliar Tracer. Biscaya and Movento offer some control and will help suppress them.
More haste, less speed
Seed treatment was wonderful. You could plant a crop and forget about it for two months. That's no longer the case and we'll need to be on the ball from the kick-off. Keep a close watch on the pest intelligence services; ABC's Brassica alert; AHDB Aphid News and; AHDB Pest Bulletin. These will help you know what to be looking out for so you can get on top of them quickly. Early control is always best.
With every type of crop, short and long season, there's going to be more spraying to do so it's going to be important to resist the pressure for speed at the cost of effectiveness. Pause to think about application. As the crop develops, water volumes, nozzles and droplet size warrant consideration to ensure you get a hundred percent out of every application. With such limited options we can't afford to achieve anything less.