More vigorous oilseed rape varieties show less effects of flea beetle larvae damage

22.8.19

Yield losses from cabbage stem flea beetle (CSFB) are estimated to cost growers about £70 million a year in lost output, roughly equivalent to 5 -15% of total crop value, but with no easy means of control, what options are there to safeguard the future of Britain’s most popular, and for many, most profitable, break crop?

After the loss of neonicotinoid seed treatments most growers resorted to foliar insecticides to protect crops, but with resistance to pyrethroid insecticides already present among CSFB populations, its increased use served only to hasten the spread of resistance. A problem that began in East Anglia is now a concern across Great Britain.

The impact of this problem extends beyond growers; breeders need a solution too. KWS has variety development trials across Great Britain, but three trials at two locations – Old Weston, north-west of Huntingdon, and Worlingworth, east of Stowmarket in Suffolk – have been heavily affected.

These trials form an important part of our variety development programme, so when 782 plots, equivalent to about 50% of the plots in these trials, demonstrate some level of CSFB larvae damage, it raises concerns. It also leads to questions, first among them is: why roughly 50% and not 80% or more?

The level of detectable damage varied between the sites, but perhaps more significant is that some varieties showed more damage than others. The ability of CSFB to seek out certain varieties within a randomised trial is a significant observation and one that warrants further investigation.

As can be seen from the photos below Flamingo (in the plot pictured on the right) is a variety that exhibits low level damage from CSFB larvae. This observation was mirrored in the AICC Southern strip trials, where Flamingo looked better in the plots and at harvest produced the second highest yield in trial of 4.75t/ha.

Extremes in attack: A example of extensive damage in a plot of Elevation (left) compared with low level damage in Flamingo (right)

Both varieties pictured at the large field site at Worlingworth, Suffolk.
(Click to enlarge images)

As could be expected in such a trial, there is a range of characteristics between the varieties involved. There are restored hybrids and conventional types, KWS varieties and those from other breeders. There are also environmental factors to be considered such as drought, compaction, disease pressure, soil fertility and plot location. The variation in the damage, as highlighted in the heat maps below, suggests that environmental factors were not significant.

Larvae damage at the trial at Old Weston, Huntingdonshire

Green = extreme damage, white = high level of damage, pink and red = low level of damage 

(click to enlarge)

Larvae damage at the large field site at Worlingworth, Suffolk

Green = extreme damage, white = high level of damage, pink and red = low level of damage 

(click to enlarge)

Larvae damage at the small field site at Worlingworth, Suffolk

Green = extreme damage, white = high level of damage, pink and red = low level of damage 

(click to enlarge)

With the obvious environmental factors ruled out, investigations focussed on identifying the common features between affected varieties. It soon became apparent that the most heavily affected varieties were also late flowering types, followed by those considered mid-flowering with the least affected varieties being those regarded as early flowering. The significance of flowering timing is evident in the chart below.

Late-flowering-OSR-has-higher-level-of-damage.png

Late flowering varieties showed the greatest level of CSFB larvae damage

These observations support those first noted in 2016. At this time, the KWS variety trial at Duxford was abandoned at harvest due to severe flea beetle damage. The trial was scored for damage severity (as a % of plot damaged) and the scores considered alongside earlier assessments for flowering time and plant vigour (as a combined score for autumn and spring vigour assessments).

The 2016 trial was the first opportunity to draw parallels between flowering and the extent of damage caused by CSFB larvae

Flowering is a function of plant growth stage. Those varieties that demonstrate greater vigour tended to suffer less than varieties slower to develop.

The observations are far from conclusive, but there is a clear link between plant vigour and vulnerability to severe larvae damage. It is not determined by variety type as there are restored hybrids that are slow to develop just as there are conventional types that are fast to develop. Rather, it seems that more vigorous varieties suffer less.

The more vigorous varieties move earlier and faster both in the autumn and the spring, and overall, suffer less damage than those varieties acknowledged to be slower to develop.

Whether timing of flowering has a significant impact is not well understood. It has served as a suitable benchmark to categorise varieties and measure damage, but whether its significance extends beyond this, perhaps through changes in plant chemistry that serve to deter pests, is still to be considered.

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