How to cut flea beetle risk in oilseed rape this autumn

The first season growing oilseed rape without the systemic protection from neonicotinoid seed treatments clothianidin, imidacloprid and thiamethoxam has probably been one to forget for many growers.

Although crops in some areas escaped the ravages of cabbage stem flea beetle (CSFB) and aphid attack relatively unscathed, there are numerous examples of severe crop damage or entire write-offs in hotspot areas such as Cambridgeshire, Hertfordshire and Bedfordshire.

See also: Backward oilseed rape crops suffer flea beetle larvae attacks

While neonicotinoids offered protection against a number of pests including flea beetle, peach potato aphid, turnip sawfly and cabbage root fly, it is flea beetle that has had by far the greatest effect.

AHDB Cereals and Oilseeds estimates flea beetle affects about two-thirds of the total oilseed rape area and its winter plantings survey says 5% of the crop in England and Wales was lost to flea beetle attack last autumn, with the eastern counties and Midlands worst affected.

Even after taking into account the 1.5% redrilled, an estimated 22,000ha has been lost and the wider effect on yield and the implications of larvae damage later in the season will not be known until after this harvest. An AHDB/Adas project is under way to assess the yield effect of larval populations.

So what can we learn from this difficult season and how can we reduce the risks of crop damage this autumn?

Focus on establishment

Neonicotinoid seed treatments were widely used to protect the vulnerable growing point during the early establishment phase, so experts are unanimous that the key to beating pests is to ensure crops grow away quickly by optimising seed-bed conditions.

It is a simple message, but one that is easier said than done when trying to manage the pressures of harvest, unpredictable weather and establishing next season’s crops.

Drilling

© Tim Scrivener

It also means there is no one-size-fits-all answer for every farm and growers must choose the establishment method, varieties, drilling date and agronomy that work in their situation.

“Seed-beds are by far the biggest factor for tackling flea beetle without neonicotinoids and growers have to get the fundamentals of establishing oilseed rape right,” says Frontier’s national technical and development manager Stuart Hill.

“Recent years have seen fixed costs driven out of oilseed rape growing with fewer passes, bigger kit and less soil movement, which hasn’t always been ideal for establishment.”

Whether using strip tillage, plough-based systems or sub-casting, oilseed rape must be sown into moist seed-beds, with good seed-to-soil contact, and consolidated well, he urges.

“The critical period is when oilseed rape is germinating, up to one or two true leaves. The quicker crops emerge and establish, the better they are able to get ahead of flea beetles and tolerate any damage that does occur.”

While early drilling is generally preferable to allow crops to take advantage of warmer weather, only do so if there is adequate moisture, he says. “Planting rape in early to mid-August and hoping for the best is false economy if it is too dry. You are better off waiting. Remain flexible and tailor what you do to conditions at the time.”

This was highlighted in Frontier trials last autumn where some later-sown oilseed rape into moist conditions emerged and established quicker than some August-drilled trials.

A survey of 63 farms covering 5,835ha of oilseed rape from Warwickshire to Kent by Strutt & Parker’s George Badger supports the need for good establishment.

It found flea beetle damage was highest on farms with either heavy clay or dry, cold, chalk-based soils prone to losing moisture when cultivated. The subsequent slow establishment led to the need for three pyrethroid sprays on average and nearly 23% of crops being written off or redrilled.

Light and medium soils with better soil moisture retention and firmer seed-beds were least affected by flea beetle, he says.

While the survey found that crop losses were less in earlier-drilled crops, early August drilling required more pyrethroid sprays than crops sown in mid- or late August. September drilling required the most pyrethroid sprays, although this was largely due to the weather and lack of rainfall causing a dry and slow-growing September, he says.

Plant populations

Growers are advised to aim for an established plant population of 30-35 plants/sq m and adjust seed rates according to field conditions to achieve this.

“In reality we haven’t found big differences in optimum seed rates between hybrids and conventional varieties, although the advice is 80-100 seeds/sq m for conventional and 50 seeds/sq m for hybrids, which is adequate taking into account expected establishment,” Mr Hill says.

Row width is the other dynamic to consider. Row spacing of 24-50cm is quite common, but growers may start to see a negative effect on yield when using the same seed rate on wider row spacings (typically above 70cm), as there are more plants within the row and increased competition, he adds.

Neonicotinoid policy

The two-year ban on the use of the neonicotinoids clothianidin, imidacloprid and thiamethoxam was enforced by the European Commission on 1 December 2013, after they were linked to a decline in bees.

The ban is currently under review by the commission, which will look at the effects of the moratorium and decide later in the year whether it will be extended or overturned.

In May this year, the NFU confirmed an application to allow oilseed rape growers to use neonicotinoids on oilseed rape crops this autumn had been submitted to government.

“Oilseed rape has an amazing ability to branch out and compensate for poor establishment, so it is possible to get a reasonable yield from populations down to seven to eight plants/sq m. This will be at the whim of the weather and it is down to growers’ attitude to risk as to whether to persevere with a crop that hasn’t established as well as expected.”

Starter fertiliser boost

Seed-bed fertiliser can be a useful way of boosting early establishment, especially as young plants have a limited “scavenging” range and need nutrients close by.

Mr Badger recommends a seed-bed starter fertiliser containing 30kg/ha nitrogen, plus phosphate to promote early growth and rooting.

“Phosphate in particular hardly moves in the soil, so needs to be placed as close as possible to the roots to be accessible,” adds Mr Hill, who says growers should also consider applying micronutrients where needed.

AHDB research suggests the most common micronutrient deficiencies limiting oilseed rape productivity are boron, manganese and molybdenum. Like other nutrients, their availability will be influenced by weather, soil type and pH.

Cultivation and consolidation

Mr Badger says there is little correlation between drill technique and flea beetle damage, although in his survey the best flea beetle control was achieved using low-disturbance subsoiler seeders or cultivation plus drill techniques. This was largely due to better consolidation and moisture conservation than was achieved using direct drilling.

Whichever method is used and however much soil is moved in the process, the key is to preserve soil moisture and consolidate seed-beds firmly after drilling, adds Mr Hill.

Anecdotal evidence suggests good seed-bed consolidation can help reduce flea beetle problems, either by improving seed-to-soil contact or by reducing the physical movement of pests through the soil.

Rolling twice, with the second pass at an angle or alternate direction to the first can improve consolidation, says Mr Badger.

Pyrethroids as last resort

Risk factors

AHDB Cereals and Oilseeds research and knowledge transfer manager Caroline Nicholls says early-sown crops are more susceptible to cabbage stem flea beetle attack and a warm, dry autumn will favour egg laying and early hatch of larvae, coinciding with smaller, more vulnerable plants.

Early sowing also puts crops at more risk of peach potato aphid colonisation during September and October and a warm autumn will encourage aphid flight, population development and movement within the crop. Aphids potentially transmit turnip yellows virus and widespread pyrethroid resistance means growers need to make the most of cultural options, such as varietal resistance.

The confirmation of widespread knock-down resistance (kdr) to pyrethroids last September means growers are urged to be extra careful about using pyrethroids to control flea beetle and to only do so as a last resort.

“Once you start applying a pyrethroid, you are already selecting for resistance,” says Mr Hill.

Where beetle numbers exceed the treatment threshold and a pyrethroid insecticide is deemed necessary, AHDB guidance says it should be applied at the full recommended rate. “If control remains poor, a pyrethroid-based product should not be used again and growers should switch to an insecticide with an alternate mode of action.”

Spray thresholds for adult cabbage stem flea beetle are when:

  • Adults have eaten more than 25% of leaf area at the one to two true leaf stage
  • Adults have eaten more than 50% of the leaf area at the three to four true leaf stage
  • The crop is growing more slowly than it is being destroyed.

Yellow water traps can also be used to monitor for adult beetles from early September to the end of October and the AHDB suggests considering a pyrethroid if more than 35 beetles are caught over the trapping period.

Dissection of plant petioles and stems during October/early November is useful for identifying larvae. AHDB advice is to dissect 25 plant samples per field and consider a pyrethroid if more than two larvae per plant are found or more than 50% of petioles are damaged.

Mr Hill says growers using a pyrethroid should select a better-quality active, such as lambda-cyhalothrin or zeta-cypermethrin to gain the most activity with the first application. “Cypermethrin-based products are currently being reregistered and coming with 18m buffer zones, so are not so attractive with regards to cross-compliance.”

Variety choice

Whether hybrid or conventional, all varieties tend to emerge at a similar rate up to the first one or two true leaf stage, so there is unlikely to be much difference in the ability of varieties to withstand heavy flea beetle attack early on, says Mr Hill.

However, once crops start to get roots down, differences in establishment and canopy growth become more apparent, with hybrids generally showing more vigorous growth later in the autumn and again when growth resumes in the spring, he says.

Mike Mann of breeder DSV UK says it is not just a case of selecting high-vigour varieties to beat pest attack, but growers need to match variety and growth habit to drilling date, soil temperature and plant population.

“Hybrids generally have better early vigour than conventional varieties, but they are not all the same. A variety that might be ideal for later drilling might not be the best choice for sowing earlier in the season, for example.”

For early drilling, he advises growers to select a variety with early vigour, to establish cleanly, which then exerts a “low-profile” growing habit to avoid crops getting too forward through winter and creating problems later.

In late September drilling scenarios, lower temperatures and shorter day length requires a variety that grows vigorously for the first 30-50 days before flattening to a lower growth rate before winter, he says.

“Cold, waterlogged and compacted soils are where the potential problems lie with late drilling, so you have to take all precautions to avoid this – soil temperature has more of an influence on seed establishment than air temperature.”

cabbage-stem-flea-beetle-larva-on-OSR-by-Blackthorn-Arable

© Blackthorn Arable

Vigour trials of DSV varieties in the UK and Germany found Popular was best suited to the early drilling slot, while Incentive45 was more suited to later drilling.

“Oilseed rape grows from its roots rather than its leaves, so the taproot’s development in the first 30-50 days is critical and actually dictates the crop’s final yield potential,” Mr Mann adds.

“Without neonicotinoids, poor root development will reduce yields and plants that don’t establish quickly are susceptible to pest attack.”

Seed treatment option?

Last year there was a limited supply of Mesurol (methiocarb) insecticide-treated seed on specific varieties. The availability of this option has yet to be confirmed for this season.

“Control from Mesurol is only about 50% of the activity offered by neonicotinoids, but it is a useful first line of defence,” says Mr Hill. “It doesn’t come with fungicide treatment, so this has to be considered with regards to diseases such as alternaria, damping off and early phoma.”

Where land is at particularly high risk of pest attack or serious blackgrass, he says it may be worth reviewing farm rotations and perhaps putting individual at-risk fields into alternative crops.

“This is the perfect opportunity to look at the bigger picture and diversify the rotation to make it work better for the farm at the same time as reducing pest damage in high-risk areas.”

Cabbage-stem-flea-beetle-lifecycle