Tips on how to beat flea beetle when growing oilseed rape

A combination of only drilling when there is soil moisture, selecting the right variety and giving seedlings a nutritional boost are some of the measures helping crops cope better with cabbage stem flea beetle attacks.

In the six years since the ban on neonicotinoid seed treatments in rapeseed crops, cabbage stem flea beetle has become increasingly difficult to control.

The pest has proved to be particularly bad this season, with dry autumn conditions preventing crops from growing away from the adult grazing.

And for those that did survive, larval damage this spring has been much more prevalent than in previous years.

See also: Essex grower’s 7 steps to tackling a ryegrass problem

Agrii seed technical manager David Leaper explains that larval development stops when temperatures are below 3C, but last autumn it remained mild.

“Larvae continued to be active and that is why we are seeing more problems,” he says.

For the past six years, Agrii has been carrying out work to find ways of managing the pest. It has seen success, although Agrii research and development manager Jim Carswell says there is no single solution.

It is a whole package of incremental measures that help crops cope better with the pest. Here are 11 key elements:

1. Moisture, not calendar date

The most important factor is having adequate seed-bed moisture, so that seedlings are actively growing by the time the adults attack.

Therefore, Mr Carswell suggests farmers forget calendar dates and know when not to drill. “Go for conditions where you can get a good chit and early emergence. If there is no moisture, then wait.”

In the rare event it doesn’t rain in this six-week window, growers may be better off not drilling the crop at all, he adds.

2. Variety

It’s going to be a hybrid in most cases, says Mr Carswell, as good vigour in the autumn, as well as in the following spring, seems to help.

He points to new evidence from trials this season that suggest speed of early spring growth is as important as autumn vigour.

“Up to now, we have focused on autumn vigour to get crops away from adult damage.”

However, data from four Agrii trial sites in Kent, Wiltshire, north Lincolnshire and East Yorkshire shed new light on varietal differences with larval damage.

“There was a positive correlation between larval damage and vigour in the autumn and spring,” says Mr Carswell.

“Effectively those varieties that resulted in bigger plants in the autumn and had faster growth in the spring seemed to cope better.”

The varieties that showed particular resilience to larval attack included DK Exsteel, DK Expedient, Ambassador and Inv1035.

Drilling oilseed rape

© Tim Scrivener

3. Fooling the pest

The worst-case scenario is to plough and drill into clean seed-beds, as the adults seem to be able to detect the seedlings better, says Mr Carswell. “Green on brown [soil] is something that attracts beetles.”

Therefore, growers should look to retain crop residue and ensure it’s evenly spread to avoid increasing the slug risk. Leaving stubble longer (20cm) also seems to confuse the beetle.

Similarly, companion crops help camouflage the emerging crop and fool flea beetles.

Mr Leaper says buckwheat is proving to be the best companion crop species in Agrii trials, as it is fast-growing, but not too competitive, like turnip rape.

Frosts will naturally take it out. However, the one key limitation is that it requires a drill capable of sowing two different crops, and maybe also fertiliser.

4. Soil conditions

Check soil conditions are optimal for early growth. “Is the pH OK, is soil free of any compaction and are there adequate levels of macro- and micronutrients, including boron?”

Seed-bed nitrogen and phosphorous is seen as particularly beneficial, as both are essential for early growth and good rooting.

Placing 30kg/ha of nitrogen and 40kg/ha of phosphate in bands will stimulate roots. It is particularly important for phosphate, which is less mobile in the soil.

Again, this is why a combi drill is the best option. “If you don’t have one, consider getting a contractor in or apply the fertiliser to the seed-bed and incorporate ahead of drilling,” says Mr Leaper.

Spraying oilseed rape

© Tim Scrivener

5. Herbicide residues

Acetolactate synthase (ALS) inhibitor herbicide residues can be a problem, causing plant damage in a following oilseed rape crop – especially in a dry season like last year.

“If you think there is a possible risk, consider going for a Clearfield variety that has sulfonylurea tolerance,” says Mr Leaper.

He adds that some farmers are specifically growing Clearfield varieties this season because of residues. “Sulfonylurea herbicides went on later last year because of the season, and it then turned dry.”

6. Seed rates

Agrii’s advice is to ensure there is adequate seed-bed moisture and then stick with normal seed rates.

Some advocate going with high rates of conventional seed to compensate for plant losses, and then using plant growth regulators to manage canopies in overthick crops the following spring.

However, Mr Carswell says: “Opting for high seed rates is just setting the crop up for problems in future.”

Instead, he advises going for an optimum hybrid population, which means drilling 40-45 seeds/sq m in early August, rising to 50-55/sq m in the second half of the month and 60-70/sq m in September.

7. Seed dressings

While neonicotinoid seed treatments have been banned for some time, there are other seed dressings that can help with establishment, such as Take-off.

It is a root biostimulant containing phosphite and PGA, which can aid establishment and help metabolise early nitrogen.

Another is Scenic Gold, which is a new fungal seed treatment from Bayer that contains two different modes of action, with fluopicolide and fluoxastrobin. Mr Carswell says trials have shown improved rooting, and it also controls downy mildew.

However, it is not yet registered in the UK and is therefore only available on imported seed from Ireland, which is limited to certain Dekalb varieties.

Integral Pro from BASF is a biological treatment shown to help rooting. Containing Bacillus amyloquefaciens, it forms a film around roots to help promote health.

More importantly, trials showed it also helped reduce adult cabbage stem flea beetle damage. Mr Leaper adds that it costs £4-£5/ha, so is not an expensive option.

Lumiposa is an insecticide treatment from Corteva with a new mode of action. It has also not yet been registered in the UK and is only available on imported seed.

However, Mr Leaper says trial data shows variable results, depending on flea beetle pressure. With a cost of £20-£25/ha, farmers need to manage their expectations.

Mr Carswell adds that when using it, growers need to be prepared to use foliar follow-ups.

8. Risk share options

In recognition of the high upfront cost of growing oilseed rape, a number of breeders – including RAGT, KWS, Dekalb and BASF – have introduced risk-share options for seed.

These vary in the way they are structured, the varieties they apply to and their specific terms. Effectively, they involve either paying part of the seed cost upfront, then a second payment based on the area established, or partial refunds for areas that fail to establish by a certain date.

Mr Leaper says growers should look at them as a bonus, and not choose a variety simply because it is in a scheme.

9. Foliar nutrition

Applying a foliar mix of macro- and micronutrients at the two-leaf stage can give crops a boost, as a follow-up to any seed-bed fertiliser.

Mr Leaper explains that at about the two-leaf stage, crop growth pauses. This is when the crop switches from feed from seed to feed from roots.

Manganese, zinc and boron, combined with Nutriphite PGA, can help lift the crop. He acknowledges it is a busy time, but it is still worth going through with a foliar mix.

10. Monitor crops and use spray thresholds

Growers should monitor crops for the pest, which includes shot-holing at emergence, yellow trapping in autumn and dissecting crops in spring for larvae.

Where recognised thresholds are exceeded, growers should consider treating – for example, when there are five larvae a plant.

Use adjuvants to help get the most out of pyrethroids. There is some anecdotal evidence that spraying at night helps too, says Mr Leaper.

Larvae are more difficult to treat and the best control possible is typically only about 20%, so farmers need to rely on variety choice to overcome their effects.

11. Rotation

Finally, widening rotations is beneficial, with some anecdotal evidence that flea beetle attacks were less severe with longer rotations. Mr Leaper says oilseed rape should be grown no more than once in five years.