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Crop Advice & Expertise

How to control onion thrips in 2019

In recent seasons, periods of settled warm and dry weather in late spring and early summer have led to high insect pressure. These conditions are particularly conducive for onion thrips since they get washed away and/or drown in leaf axils in wet weather.

So far in 2019 we have experienced some similar conditions with decent spells of dry and warm weather. However, these have been interspersed with periods of wetter, cooler weather. In particular, the recent heavy rains may have reduced onion thrip pressure in the short-term – but we could see temperatures rise again into the summer months and provide the same prime conditions for onion thrip development as last year.

“Thrips in bulb onions - and these are a range of different thrip species – always used to be an occasional, sporadic, localised pest issue, says Matt Garnett, commercial technical manager for Bayer in Lincolnshire. "But over the past five years or so have become a regular and much more widespread problem. A similar pattern has been seen in Europe, with onion thrips now a major pest issue for Dutch growers.”

With the loss of chlorpyrifos three years ago, and a widespread resistance to pyrethroids, Tracer (spinosad) and Movento (spirotetramat) are currently the only chemical options for growers:

“In the current climate, controlling diseases and pests is becoming increasingly difficult across a wide range of crops so we need to get the most out of what we’ve got,” Mr Garnett says.

To build an effective programme it is vital to understand the products and how they work, he explains. “Tracer is more of a knock-down product, while Movento is more of a systemic population management product, reducing populations and preventing further generations from developing, but it isn’t a rapid knock-down material.”

In 2018, onion growers managed to get approval of Benevia (cyantraniliprole) for emergency use on onion thrips. “In 2018 we did see some lack of sensitivity towards Tracer and this is where Benevia fits in very well. So, hopefully we can get a cyantraniliprole product approved again and have the option of two knock-down products in conjunction with the more systemic Movento.

“Movento will interrupt the life cycle for a thrip but when more adult thrips come in, that’s when we need the knock-down approach. Having these knock-down products would be very beneficial in helping control the pests in this crop.”

To work, Movento needs to be taken up by the plant, where it is then highly systemic, moving both up and down to get to the new tissue that pests attack.

Importantly, you won’t see direct impact on adult thrips with Movento, Mr Garnett adds. “Movento reduces egg laying, and fertility of eggs which in turn reduces the number of juveniles. It also reduces the ability for juveniles to grow and move through growth stages which suppresses populations very effectively. So, you have to use it early in the pest infestation to prevent population build-up – it won’t work anywhere near as well if used as a fire-engine style treatment.”

To get best uptake in waxy onion plants, Dutch insight is helpful as they have had access to Movento for more years than in the UK. Their experience backed up by 2018 trials and commercial experience in the UK indicates four key principles.

 

  • Go early – a lot earlier than one would normally think about onion thrip sprays – around mid-June, as nearby cereal crops which tend to host the thrips start to ripen. That way the active ingredient gets into the onion crops ready for the first infestations to pick it up – and then it limits their population spread.
  • Add an oil-based adjuvant – Dutch experience indicates this really helps to get the active into the onion plant and therefore massively helps efficacy.
  • Go once, or maybe twice with Movento, then follow with a more knock-down type material such as Tracer. 
  • Time knock-down sprays to target large migrations of thrips into the crop and thin numbers out before they can multiply.