The hot, dry summer of 2018 was ideal for onion thrips and following the mild winter and warm, dry spring, the Allium & Brassica Agronomy team see the potential for another bad year. AB agronomist Carl Sharp says timing will be the key to effective control.
As for many pests, onion thrip control has become more challenging since the loss of chlorpyrifos four years ago and widespread resistance to pyrethroids. Growers now have two main control options; Tracer (spinosad) and Movento (spirotetramat).
Mr Sharp says that growers really need to understand how they work to use them to best effect. An important difference he notes between the two is that Movento is systemic whereas Tracer is a knockdown product.
Dutch start earlier
"Thrips fly out of cereal crops as soon as harvest begins and previously we would wait until numbers had built up to five or more per plant before applying the first insecticide. When onions were added to the Movento UK label last year the manufacturer Bayer pointed to Dutch growers' experiences of how to get the best from it. Their standard practice is to start earlier, as soon as a thrip is seen in the crop and apply with an adjuvant oil.”
Last year Bayer commissioned AB Agronomy to do a trial to test this approach in the UK. Mr Sharp ran it in Bedfordshire on red, drilled onions in a rotation with cereals where he says it's almost impossible to avoid onion thrips.
The trial compared four-spray programmes of Tracer, Movento, Movento +Actirob adjuvant oil, two Movento sprays followed by two Tracer sprays and vice-versa. The four spray programmes were included to understand what they would do rather than being suggestive of how growers should use the products in practice. The trial began on 25th June with the first insecticide application and applications followed every 14 days until the fourth and final application on 6th August.
Results were assessed in numbers of thrips per five plants on the four application dates.
Four spray programmes
Thrip numbers in the untreated control peaked at 35 per five plants on the third application date of 23rd July. The most effective programme was four sprays of Movento + Actirob, which limited build up to just 4.4 thrips per five plants at that time. This was closely followed by the programme of two sprays of Movento + Actirob followed by two of Tracer. Reversing this sequence was less effective allowing numbers to rise to 20 per five plants. Mr Sharp says the results demonstrate the importance of using Movento first and starting earlier.
"In commercial crops we need to alternate the use of insecticides for resistance management rather than using blocks of two sprays, but it is clear that beginning with Movento will stop the population exploding. It's most active on nymphs so the first spray should go on when you see the first thrips coming into the crop. As in last year's trial this is likely to be a fortnight earlier than growers previously applied their first insecticide.”
Last year there were situations where growers had to make five applications to keep on top of thrips and the situation was sufficiently serious to warrant a 120-day emergency authorisation for the use of Benevia (cyantraniliprole) for their control.
Mr Sharp says the industry has already applied for another emergency authorisation for this year and hope to have it in place for mid to late July should the anticipated large numbers materialise. "If we have Benevia it'll complement Movento well, but availability will be the issue with so many growers needing additional foliar insecticide sprays to substitute for Cruiser (thiamethoxam) seed treatment in brassicas, carrots and lettuce."
Treat; wait; check
An important difference between the approach to pest and disease control in onions he emphasises is that: "For downy mildew you have to plan a programme of sufficient fungicide applications to see you through the summer as the risk never goes away. With thrips it's much more of; treat; wait; check. If you start to see thrips again two weeks after the previous spray, treat again, but if you don't, continue watching. You may not need to run into a programme.”
Rainfall and irrigation are great allies in the battle against onion thrips Mr Sharp says. "Some are drowned by water accumulating in leaf axils and others are washed to the floor where they picked off by predators. Wet conditions also make them vulnerable to fungal infection.”
Mr Sharp advises application of Movento after irrigation.
"Particularly during a hot, dry period, the irrigation will have got the plant growing actively again which is essential for it to distribute the active substance to the growing points where nymphs are feeding."