Article featured in Farmers Guide, October 2018
Cabbage stem flea beetle has been widespread across my area of Wiltshire and neighbouring counties.
Most fields have been hit hard, and some crops will need to be replaced.
The good news is that crops sown into moisture, notably those that were direct drilled, had 2–4 leaves by mid-September and are growing away well.
Those established using more traditional cultivations have struggled, and have been sitting at the cotyledon stage through this dry weather, which has also been ideal for flea beetles.
Some crops have been sprayed with pyrethroids, but with up to 60 per cent of flea beetles now said to be resistant, the effect is limited.
Most crops will pull through, but growers faced with a few skeletal remains will need to rethink. The most obvious replacement is winter or spring beans to maintain the break from cereals.
These may not be as profitable as the crop they are replacing, but costed out over the rotation the decision is very likely to stack up.
Growers who have applied chemicals, mostly pre-emergence at this stage, should beware of falling foul of any following crop restrictions as described on the label.
For example, where clomazone has been used, manufacturers advise that winter beans cannot be sown until 6 weeks after application and soils must be ploughed to 25cm.
The same restrictions apply to winter cereals.
Some pre-ems are also subject to statutory restrictions, while others may offer some flexibility at growers’ own risk.
Please do check the label before ordering a replacement crop.
Freshly shed black-grass seed is said to have low dormancy this year, so using the most effective pre-emergence chemistry will be even more important to achieve the best kill.
We know from many trials over many years, as well as field experience, that tri-allate granules (Avadex Excel) deliver up to 30% additional activity when added to the pre-em programme.
Avadex Factor - Liquid formulation
However, Avadex Factor (the liquid formulation introduced last year) does not contain as much active at the full rate so delivers less additional control.
For that reason I’ll stick with granules, provided growers have access to an applicator, although Factor could still be useful for those that don’t.
Whichever version is used, the aim should be to drill, roll and apply Avadex within 48 hours.
The chemical (and other pre-ems) requires moisture to work, so a rethink may be required where soils are bone dry, perhaps even delaying drilling until conditions improve, rather than risking a massive flush that proves impossible to control in the growing crop.
Esme Shephard is an agronomist with Agrovista, based in Wiltshire