Crop Watch: Battling flea beetles and blackgrass

Oilseed rape crops coming under attack from flea beetle and keeping on top of blackgrass this autumn are two key issues for our agronomists this week.

There are hopes the recent rain will help OSR crops grow away from the relentless beetle attacks in some parts of the UK.

Blackgrass control is also proving to be a headache, as the dry conditions have resulted in limited germination of the weed in stale seed-beds and farmers are itching to drill because of an early finish to harvest.

One positive is that the recent rain will help boost the efficacy of residual herbicides. 

See also: 3 top varieties to consider as second wheats this autumn

David Martindale

David Martindale

North: David Martindale

Arable Alliance (Yorkshire)

The first heavy rain for quite some time finally arrived last week, with most areas receiving 40-60mm of rain in one day, although for some a mere 10mm landed, which still left desert-like soils. 

This rain has given a significant boost to oilseed rape, which previously had been struggling to grow in the dry conditions.  

Seed-beds have also softened by the rain, making them easier to cultivate. Grassweeds now have a chance to emerge, as stale seed-beds have so far been largely unsuccessful because of the dry soil conditions. 

The rain has also helped put the brakes on seed drills, albeit only for a few days. The annual discussions around delaying drilling on fields with grassweed issues have been made more difficult with such an early finish to harvest and seed-bed preparations are well advanced for the time of year. 

However, delaying sowing by just 10-14 days can make a real difference to the level of success of grassweed control.

The real danger now is that growers start drilling significant areas of cereals without waiting for grassweeds to emerge after the recent rainfall, so more pressure will be put on herbicides.

With the herbicide armoury weaker due to fewer active ingredients and resistance issues, trying to rescue the situation via the spray can is a high-risk strategy. 

Residual herbicides

With pre-emergence herbicides the cornerstone to successful blackgrass control, it is crucial to make every effort to achieve a good seed-bed in order for these residuals to work at their best. 

 To stop drilling and apply these key herbicides with good application techniques is also important. Ultimately, it is the attention to detail that will achieve the highest levels of blackgrass control from a given situation.

Cabbage stem flea beetle has been causing significant damage to oilseed rape, particularly on Wold land that historically suffers from this pest. 

Insecticide applications have given just enough flea beetle control to keep the crop alive, but it feels far from satisfactory to see limited efficacy from the pyrethroids.

Some heavy land growers have not had to worry too much about flea beetle yet as the crop has struggled to germinate in the dry soils. The rain from last week may just well have arrived in time to get these crops established.

Despite some difficulties establishing oilseed rape, the opposite has been true for barley volunteers, which have required a prompt graminicide to prevent them becoming too competitive against the host crop. 

Now there has been some rain and with soil temperatures remaining warm, oilseed rape will start to grow well.

Looking ahead, it will be a case of monitoring blackgrass emergence and deciding when to apply clethodim.

Ben Pledger

Ben Pledger

East: Ben Pledger

Farmacy (Beds/Herts)

The little oilseed rape that I have in the ground is growing well, mainly drilled around the middle of August after some welcome rains.

Flea beetle pressure this season (I’m touching wood as I write this) has been relatively low in the Hertfordshire area, with early attacks being nipped in the bud at cotyledon stage with a mix of lambda-cyhalothrin, sulphur and a latex-based sticker. On all bar one field this has been the only treatment needed for the pest.

The next issue is the large blackgrass populations flushing in the crop. Fortunately, there is enough of a flush to suggest it has germinated at the same time, and a well-timed application of clethodim will hopefully take it out.

The low dormancy of blackgrass is noted, with the weed flushing well in both uncultivated stubbles and seed-beds already prepared for cereal drilling.

Although some seed drills in the area have started to get going, it is still way too early to start when there are seed-beds only just showing one-leafed blackgrass, with others only getting rain at the weekend to get a chit of anything.

Seed treatments

As this is the last year for clothianidin-based seed dressings, we will have to push further toward delayed drilling, not only as part of our integrated blackgrass control, but also to reduce the potential of aphids bringing barley yellow dwarf virus into crops.

Further to this, I can find aphids quite easily in volunteer cereals growing in stubbles planned for spring cropping. It is important to spray these stubbles off sooner rather than later to eliminate any green bridge that may harbour pests for neighbouring winter crops.

On a final note, with the dry summer this year, seed quality is all over the place. I’ve heard of thousand grain weights for winter wheat ranging from 33g to 60g.

Please take this variance into account when calculating seed rates for drilling in the coming autumn and next spring.

Tod Hunnisett

Tod Hunnisett

South: Tod Hunnisett

AICC (Sussex)

It’s my first Crop Watch article of the new season and it is being written on the day after the first significant rain we have had for a month. I woke up to brilliant sunshine and a heavy frost – and it’s not even the end of September.

Many people, worried that once the weather broke it wouldn’t mend itself, have pushed on with drilling into very good, but dry seed-beds, to a point where some growers don’t have a lot left to do.

My fear is that what were supposed to have been stale seed-beds will turn into very fresh seed-beds and we will have to deal with grassweeds in the crop rather that out of it. My hope is that the recent rain and very good soil conditions will at least mean we get a reasonable effect from the residual herbicides.

It’s just a shame that the number one rule of managing difficult grassweeds – delaying drilling – has been overtaken by the worry that once it starts to rain it will never stop.

Flea beetle

Most oilseed rape crops around here are reluctantly dragging themselves away from the seemingly constant irritation of being bitten every few minutes by a flea beetle. I dread to think how much pyrethroid has been sprayed nationally to try to preserve livelihoods.

The worst-affected crops seem to be the ones that were drilled at textbook timing – at cotyledon stage when the very hot weather came back at the end of August. Those drilled very early – before mid-August – and those drilled (or redrilled) in early September look very much better.

It still irks me that we once had a system of putting a systemic active ingredient exactly – and only – where it was needed, that has disappeared in the interest of non-scientific politics.

Which in turn means we have gone back to a system that can only be worse for the environment.

Neil Potts

Neil Potts

West: Neil Potts

Matford Arable (Devon)

Dry conditions still dominate in the South West. At the time of writing we have just had the first significant rain in months, which has been fairly general over the entire region rather than the local thunderstorms we have had over the past six weeks.

Other than oilseed rape, there has been little drilling of combinable crops so far.

Winter OSR crops are establishing reasonably well, although the dry conditions have led to patchy emergence on some farms.

Flea beetle damage has not been a factor this far west, but crops are suffering from some fairly severe attacks by turnip sawfly larvae. On the whole, I think the acreage of rape planted this autumn is lower than last year.

Yields of all crops this year have been extremely variable. The main factor affecting yield has, unsurprisingly, been the availability of moisture, but other factors such as compaction and seed-bed preparation have also had an influence in places.

Takeaway lessons from this season are going to be difficult and of limited value, given that the lack of water has been the main limiting factor to production. As we have no influence over this, there is little to be learned about how we overcome the associated problems.

Perhaps a little surprising is the fact that despite the dry weather and the lack of significant foliar disease symptoms in cereals, yield responses to cereal fungicides has been larger than might have been expected.

Wheat varieties

Variety choices have been difficult where we have been considering whether to change to a new variety or not. Our starting point in the South West is usually and increasingly to look at the septoria resistance rating rather than the yield.

On that basis, Sundance should be an obvious choice, but there are question marks about its ability to produce a good enough specific weight, particularly in a wet season.

Skyfall is proving to be a dirtier variety than its ratings would suggest, but brings the benefits of early maturity and being unattractive to wildlife such as badgers, foxes and deer, all of which can cause significant damage to crops as they approach harvest.

Dunston has performed well in 2018 and Extase looks interesting for the South West considering its exceptional disease resistance.

Winter barleys are a little easier, as there is a much smaller choice to look at. The recent two-row additions to the Recommended List have proved to be disappointing compared with some of the older varieties.

With many farmers reluctant to grow six-row barleys, we are in desperate need of solid new two-row varieties to replace some of the older but reliable ones currently being grown.

With many crops likely to be planted earlier than usual this year, barley yellow dwarf virus is probably our most immediate and serious threat to yield.

Deter (clothianidin) seed treatment will not give as long a period of control in crops that are drilled early, so follow-up applications of aphicide may be required.