Cabbage stem flea beetle (CSFB) damage has contributed to estimated losses of 3.2% of the winter oilseed rape (WOSR) area in England and Scotland, according to a ‘snapshot assessment’ report commissioned by HGCA.

Based on the period 22–29 September, the estimated 3.2% loss is equivalent to 17,000ha.

Of the area lost, the report estimated half of the area had been redrilled and half currently left bare.

The report also suggested there is significant regional variation in CSFB damage with some growers experiencing significant control issues. This was the case in Hampshire, Bedfordshire and Hertfordshire where over 40% of the WOSR was reported as having damage levels at or above treatment thresholds.

The regional picture of growth stages and treatment thresholds also meant that further crop losses post 30 September could not be ruled out.

HGCA Research Manager Caroline Nicholls said: "Faced with anecdotal reports of CSFB damage over the early autumn and an immediate lack of industry data on the possible extent of damage, HGCA funded ADAS to co-ordinate and gather evidence from an established network of agronomists covering England and Scotland.

"The report provides a regionalised ‘snapshot’ estimate of damage in WOSR crops walked in the last week of September."

Estimates were based on 32,000ha of WOSR walked by members of the ADAS network which comprised 23 county-level agronomists covering 30 counties.

The area walked was equivalent to 6% of the national area and was scaled up to give a national and regional picture.

No data was included from Wales which accounts for 1% of the WOSR area in Great Britain.

It was estimated that 55.4% of the WOSR area had either grown past the susceptible growth stages (34.9%) where adult CSFB, typically, causes the most damage or was currently unaffected by CSFB (20.5%).

Further crop losses could be anticipated as the report also estimated that 41.4% of currently affected WOSR was still at a growth stage considered to be particularly vulnerable to CSFB damage (4 true-leaf stage and earlier). Of these crops, 7.5% were considered to have already sustained damage above the control thresholds.

At this stage, due to the complexities associated with redrilling/crop compensating for damage, the impact of CSFB on yield has not been estimated.

Regional damage data

Miss Nicholls said: "The national crop loss figure of 3.2% masks considerable variation in the impact from CSFB at the local level."

The counties with the highest proportion of crops at or above control thresholds were Hampshire (46%), Bedfordshire (43%) and Hertfordshire (43%). Most crop losses were reported in the South East and Eastern counties.

Miss Nicholls continued: "The report results show some growers have been hit hard, with some needing to redrill or consider other cropping options.

"Due to the complexity of crop-pest interactions, there are likely to be many causes of this variation but there were some common observations."

Across the majority of counties, the view was that the earlier WOSR was drilled the less susceptible it was to CSFB.

It was reported that crops drilled in mid-August tended to have developed beyond the susceptible growth stage by the time the adult beetle migration started.

Some of the later September drilled crops were also mostly unaffected, possibly because the number of adult beetles migrating had decreased.

Where crops were drilled into dry/cloddy seed beds, crop development may have been slowed allowing beetles to consume more leaf than was being produced by the crop.

Pyrethroid usage

Another cause of variation in CSFB damage could be due to resistance to pyrethroid insecticides in CSFB.

On a national scale, it was estimated that 59% of the WOSR crop had been treated with at least one pyrethroid spray.

Regional reports of severe damage, such as in the South East and Eastern counties, tended to result in more intensive crop monitoring and repeat spray applications.

In many regions where CSFB pressure was reported to be lower – such as the West, East Midlands and Scotland – the use of pyrethroids was estimated to be similar to previous years.

In September, HGCA confirmed that knock-down resistance (kdr) to pyrethroids in CSFB was widespread, based on positive test results from all six counties tested.

Following a further call for CSFB samples to be sent to Rothamsted Research for analysis, researchers have also been able to confirm the presence of pyrethroid resistance in a sample of striped flea beetle mistakenly sent in.

Miss Nicholls said: "Unfortunately, it seems pyrethroid resistance can arise all too easily in crop pest species.

"Some of the initial CSFB test results also indicate that there may be a second resistance mechanism involved."

In order to help limit the spread of resistance, it is recommended that full pyrethroid field rates are used whenever control is sought.

If control remains poor, a pyrethroid-based product should not be used again and growers should switch to an insecticide with an alternative mode of action.

The recent Emergency Authorisation (until 23 January 2015) of InSyst (acetamiprid) for use in WOSR against CSFB will give growers crucial spraying flexibility and will play an important part in the management of pyrethroid resistance.

However, only one autumn foliar application of any neonicotinoid insecticide (eg acetamiprid and thiacloprid) is permitted.

Emerged WOSR will now be less vulnerable to damage to the precious growing point. However, CSFB will lay eggs at the base of the crop and, if conditions are mild (>3˚C), larvae may enter plants to feed from October to early April.

Research suggests that larvae are worth controlling if more than 35 beetles are caught in yellow water traps between crop emergence and the end of October or if two or more larvae per plant are found in late October/early November.

HGCA has updated its cabbage stem flea beetle, aphid, and oilseed rape publications to reflect the latest findings on resistance and changes in the authorisations of plant protection products.

www.hgca.com/pests

The report Cabbage stem flea beetle snapshot assessment – incidence and severity at end September 2014 was prepared for HGCA by Sarah Wynn, Steve Ellis and Lottie Alves (ADAS UK Ltd)