To spray or not to spray insecticides? 

With UK farmers currently being offered £45/ha each year to comply with IPM4: ‘No use of insecticide on arable crops and permanent crops’ under the Sustainable Farming Incentive (SFI) scheme, many may be tempted to reconsider their use of pyrethroids.  

However, with a ‘perfect storm’ scenario of early drilling, high levels of volunteers with aphids already populating them, milder autumn temperatures and aphids looking for new homes once the new crop emerges, farmers would be taking a massive gamble in removing pyrethroids from their armoury in the wake of what could be a huge risk of infection from barley yellow dwarf virus (BYDV) this autumn advises Chris Martin, Agrovista’s technical manager for the North. 

The agronomist added: “I’ve been with Agrovista covering a large territory that extends from North Lincolnshire to the borders for 28 years now, and there’s little doubt that we’re seeing pests in the North that we’ve never seen before due to successive warmer autumn temperatures.  

“It’s a real conundrum for growers further North. On the back of some incredibly wet autumns in two of the last three years they understandably want to drill early in order to ensure they actually get a crop in. However, the flip side is that delayed drilling is still one of the best ways to avoid the worst effects of BYDV.” 

Mr Martin said that glyphosate is very effective in destroying the green bridge from last year’s stubble, however, while farmers are busy trying to achieve stale seedbeds as quickly as they can following the combine, the knock-on effect from this year’s variable grain quality on winter barley will be a lot of small grain seed being returned to the field through the combine.  

No silver bullets

He added: “No matter what many farmers do, there will be a green bridge infected with BYDV by wingless aphids that survive on this material post-drilling.  

“Some growers will turn to winter wheat varieties with a lower susceptibility to BYDV, but they are not a silver bullet. In my experience, they tend to perform much better against BYDV when partnered with an insecticide, rather than when replacing an insecticide.  

“So, whilst all these options have their place in any BYDV strategy, I still believe that in high-pressure aphid situations, farmers must still look at including pyrethroids as part of their defence plan. Quite simply, pyrethroids still work successfully on susceptible pests and are one of the few insecticides that can still be used on cereal crops.” 

The expert said that although all pyrethroids are susceptible to some level of resistance, they are also all different in their chemical make-up, so the best strategy is to select different pyrethroids for different crops.  

For instance, the pyrethroid esfenvalerate is not used on oilseed rape, so it is potentially a better candidate for use on cereal crops to help with potential resistance issues.  

Mr Martin continued: “Having worked with most of the market leading pyrethroids over many years, I believe esfenvalerate to be the most persistent pyrethroid, with data produced by Rothamsted Research using the Stevenson coefficient showing it’s both less aggressive and poses a lower toxic risk to bees.  

“It’s also more UV stable than most other pyrethroids and, if the weather does turn, then it’s still the most rainfast. That last benefit is particularly critical in the North, where we normally only get one chance to spray for aphid control in autumn cereals.” 

Moderate pyrethroid resistance

Supporting Mr Martin’s advice on continued use of pyrethroids, Simon Leak, business manager crop solutions for Sumitomo Chemical (UK), added: “The main vectors for BYDV are bird cherry-oat aphid and grain aphid.  

“However, whilst recent work by Rothamsted Research indicates moderate pyrethroid resistance in Sitobion avenae (grain aphid) there are no current reports of resistance or control failures in the principle BYDV vector Rhopalosiphum padi (bird cherry-oat aphid) which is excellent news for continued pyrethroid use in BYDV strategies.  

“As the sole manufacturer of esfenvalerate, Sumitomo Chemical also take the complex issues around product safety to beneficials very seriously.

“Beyond esfenvalerate’s proven lower toxicity levels to bumblebees demonstrated in work conducted by Rothamsted Research, there is also a more recent study instigated by the agronomy company Environmental Crop Management (ECM) supported by Manchester Metropolitan University that shows that the impact of esfenvalerate on web-spinning spiders – the main predator of aphids – is also minimal, with the active only proven to be initially disruptive to the spiders with no long-term harmful effect to these important beneficials.” 

Mr Leak said that when compared to other pyrethroids, esfenvalerate has the lowest grammes active per hectare at its recommended BYDV dose rate – 4gai/ha.  

“It’s also the only pyrethroid with a unique phenyl-acetate radical, giving it longer photo-stability, longer stability at higher water PH’s, and offering excellent repellency. All useful properties in an insecticide,” he concluded. 

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