Since the withdrawal of neonicotinoid seed dressings, rapeseed producers have been solely reliant on pyrethroid insecticide applications to control the pest.
However, resistance to pyrethroids is making chemical control unreliable, so growers are manipulating drill date to avoid peak adult beetle migration and using higher seed rates to dilute feeding damage and ensure adequate plants survive into the winter.
ADAS crop physiologist Pete Berry isn’t critical of the tactics, because at present there are no obvious alternatives, but says growers must accept moving away from optimum seed rate as this will reduce crop potential and drilling too early could also be damaging.
“When you are sowing in the first half of August, you are more likely to get over-large canopies and a greater risk of lodging. Large canopies also don’t set as many seeds at flowering and yield potential will be a lot less,” he adds.
Similarly, using high seed rates to dilute flea beetle damage also runs the risk of over-thick canopies and stifles yield.
If drilling later to avoid flea beetles, Dr Berry believes it is not as risky, but there is a cut-off.
“The window for drilling is bigger than people perceive, with mid-September crops often yielding as much as earlier-sown crops. However, if you go beyond mid-September you can usually expect yield to drop off,” he explains.
No silver bullet
Limagrain’s senior oilseed rape breeder Vasilis Gegas says, in flea beetle hotspots, manipulating drill date or increasing seed rate won’t provide a “silver bullet” for preventing crop failure.
He has seen early-drilled crops on heavy land struggle to grow away quickly in dry conditions and still being small and vulnerable to attack when adult beetles move into the crop in late August.
In addition, he has seen early-September sown crops – when flea beetle pressure is expected to be high – romp away from attack, as they went into a warm, moist seed-bed.
Subsequently, Dr Gegas says growers should ignore calendar date and apply three golden rules for getting oilseed rape safely established and growing away from flea beetle damage.
“Firstly, soil conditions are critical and having adequate moisture will allow the crop to grow away, provided the flea beetle attack isn’t extremely severe,” he adds.
Optimum seed rates
The second rule is to use the correct seed rate and Dr Gegas argues that even drilling a high number of home-saved conventional seed, high flea beetle pressure can still cause partial or complete crop failure in some seasons.
In addition, if that pressure doesn’t materialise, growers are then left with over-thick crops to manage.
“We are now breeding for varieties that suit wider row establishment systems. They branch lower and need light to reach down through the canopy, resulting in the formation of a deeper layer of pods, and therefore higher yields”.
“Target plant populations should be closer to 25-30 plants/m2, for both hybrid and conventional types, with 40-45 plants/m2 in the case of less vigorous varieties, or more challenging conditions,” he explains.
The last golden rule is variety choice, with autumn vigour being an important characteristic for giving crops a fighting chance of growing away from pest attack, with rapid early growth.
“A vigorous variety in a poor seed-bed won’t help, but if the variety is given the conditions it needs to grow well, it certainly can,” says Dr Gegas.
He points out that speed of germination and emergence, plus early rooting capacity, are all important for successful plant establishment, and while strong genetic influence on those characteristics and varietal differences does exist, environmental conditions can play a significant role.
“Conventional variety Anastasia is as vigorous as most hybrids and copes very well with late sowing, but it will need canopy management if sown too early”.
“Nikita on the other hand responds well to early sowing with quick establishment, but without getting too forward in the autumn,” notes Dr Gegas.