With no effective insecticides to control resistant cabbage stem flea beetles in oilseed rape in the short term, affected growers will have to switch to radical non-chemical means to get on top of the pest.
While the worst pyrethroid-resistant flea beetles are still confined to hotspot areas in the eastern counties, increased crop losses in 2015 were reported in in the East Midlands, Yorkshire and Humberside and the South West.
The increasing problem is described as a “major crisis” by independent entomologist Alan Dewar – a crisis he believes the industry has brought upon itself.
He blames a programmed approach to pest control in the rapeseed crop, with many advisers not using monitoring and thresholds to make insecticide treatment decisions.
This has led to insecticide sprays applied to the crop whether needed or not and if it continues, Dr Dewar said resistance will spread until the whole UK oilseed rape area is infested with tough-to-control beetles.
The problem has also been exacerbated by the neonicotinoid seed treatment ban, which looks set to stay outside of the four counties handed derogation for use last autumn.
“If growers keep spraying, there will be more resistance, more cost and more loss leading to fewer hectares, which is actually worse for the bees than the controversial neonicotinoid seed treatments, as you are taking away their food source.
“In the short term there are no control methods. Neonics [sprays], organophosphates, other insecticide classes – none are touching these [resistant] beetles,” explained Dr Dewar.
He backed this up with data from 25 trials testing available and some commercially unavailable foliar insecticides; none adequately controlled adults or their larvae later on.
This included products such as the neonic sprays Biscaya (thiacloprid) and InSyst (acetamiprid) that were granted approval for use against flea beetles in the autumn.
While adults and their typical “shot holing” damage is the focus in early in crop development, as they have become more difficult to control, larvae hatching at the base of plants and eating into leaf petioles has increased significantly.
The graph taken from Fera’s Crop Monitor shows the worrying trend not just in the worst flea beetle areas in eastern England, but right across the UK.
“Before neonics were banned the number of larvae per plant in the autumn was generally below the economic threshold.
“However, in 2014-15, the numbers were above threshold in the east and this season it’s likely to be off the scale – I have seen plants with more than 50 larvae,” said Dr Dewar.
To put into context the potential impact of increased larval numbers, in a crop of oilseed rape with 40 plants/sq m and five larvae/plant, there will be about two million adults/ha emerging in the summer to continue its lifecycle.
Dr Dewar added that if this cycle is not broken, these larvae will continue to be the following year’s beetles, pressure will increase and the problem become more widespread.
This year has taught many in the worst affected areas some lessons on how they might cope better with the pest in the future without seed treatments and minimal sprays.
Dr Dewar said drilling early would help, with many early to mid-August sown crops growing away before the main beetle migration in late September to early October.
He also suggested higher seed rates to dilute the impact the population will have on a crop, and adequately controlling volunteers will deplete a food source for beetles waiting to attack newly emerged seedlings.
Using trap cropping around the edge of fields could also help reduce numbers, but perhaps more controversially, he suggested moving oilseed rape production away from the worst affected areas in the East to the West and North, where beetles aren’t yet resistant.
“We need to lengthen rotations and separate blocks of oilseed rape, rather than having them scattered all over the place too,” said Dr Dewar.
Finally, he gave an example of an Essex grower who grazed his sheep on a small block of oilseed rape plants infested with larvae, right down to the ground.
“He got 3.5t/ha of that crop, while the rest around it got hammered by flea beetle larvae,” he added.
Alan Dewar was speaking at the recent Association of Independent Crop Consultants (AICC) annual technical conference at Whittlebury Hall, near Towcester, Northamptonshire.