As featured in Arable Farming Magazine
Learning to live with flea beetle
by Arable Farming Magazine April issue
No chemical alternative to neonics has yet been found for cabbage stem flea beetle control, but research into cultural control methods which can be stacked continues at pace.
Growers are well versed in the marginal gains in black-grass control that can be achieved through Dr Stephen Moss’ ‘five for five’ initiative, but now it might be time to move the same concept to cabbage stem flea beetle (CSFB) control.
This is according to ADAS entomologist Dr Sacha White, who has spent the past eight years exploring ways to control the pest.
Speaking at the United Oilseeds and AHDB Agronomy meeting in Peterborough, he said: “Non chemical control strategies are often less reliable, more complex and sometimes more costly.
On their own, they are unlikely to control CSFB, but the key is these approaches can be combined.”
Taking a look at the wide range of methods that have been studied in recent years to combat flea beetle, he ranked them using a traffic light system (see panels over the next three pages), based on the amount of research carried out.
Pest management However, all of the approaches discussed that can be used to develop an integrated pest management programme should be almost solely set around the date the crop was sown, Dr White said.
This is because methods of control tend to combat either the adult beetle, or its larvae.
“We need to start treating CSFB as two different pests.
If you’re sowing early you should be thinking about a larval problem.
If you are sowing later, think about adult management.”
Target: Adults and larvae Earlier-sown crops tend to be able to cope better with adult flea beetle attack, but suffer more from larvae damage, whereas crops drilled after peak adult beetle migration in mid-September have reduced larval pressure
Crops sown during the migration period, typically in the first half of September, are at highest risk.
Dr White said: “It is possible earlier-sown crops that are larger may be able to tolerate larval feeding.
We don’t have data on this yet.
If that’s the case, it’s a mitigation strategy.
“The later you sow, the risk from adults increases and risk of larvae decreases.
Once you sow towards late August to the first half of September, you are in a high-risk window for adult flea beetle.
After that, risk from adults begins to tail off and this is where you see the lowest risk from larvae.
“If your main focus is to establish a crop, sow early, but if your main focus is to reduce larvae because you’ve maybe had a bad experience, consider sowing late.
However, the key caveat is always wait for good conditions.
“The link between adult feeding damage and larvae load is tenuous.
We’ve seen sites with low adult feeding damage but still seen very high larval pressure.
Don’t assume you’re not going to have larval problems because you’re not seeing feeding damage.”
Suited to all drilling dates but mainly start at mid/late August Target: Adults and larvae There is some evidence to suggest increasing seed rates can dilute flea beetle activity, but trials have shown mixed results, Dr White said.
“The reductions weren’t huge, but this has potential in poor establishment, so when it’s dry.”
However, recent trials data shows that crops drilled at a high seed rate tend to have lower levels of larvae.
Conversely, it’s possible plants sown at a low seed rate will be able to tolerate flea beetle attack because they will be larger, Dr White said.
One of the farms ADAS is working with on the study drilled at a very low seed rate aiming for 15 plants/sq.m, and although larvae numbers were 40-50 per plant, the crop still yielded four to five tonnes per hectare, Dr White added.
“We think these plants were just well able to tolerate damage.”
Suited to drilling in early to mid/late August Target: Larvae Dr White said: “We know grazing or topping reduces larval load, but it is risky.
We have done lots of trials and the majority leads to yield penalty.”
However, some farmers are seeing successes and persevering with the approach, he added.
“Timing is critical.
Do it in November, December at the latest.
Crop condition is important – forward crops have a high larval load and are best suited.
Defoliation doesn’t need to be very severe, you just need to take the leaves off and go no further.
Pay attention to other pests.
“Pigeon damage might be worse, pollen beetle might be worse because flowering is likely to be delayed and poor spring weather will hinder recovery.”
Suited to mid-late August Target: Adult flea beetle Any tillage method that minimises loss of soil moisture and encourages rapid establishment will help manage adult flea beetle problems, Dr White said.
“There is also tantalising evidence that suggests cultivations that minimally disturbed soil resulted in a lower larval population several weeks later.
“We think it is because these approaches are having less of a negative impact on natural enemies in the soil like ground beetles and parasitoids, but we are doing more trials on this.
Beneficials “These beneficials can help to manage adults and larvae, but if you are interested in reducing larvae this is particularly useful for crops in August.”
Suited to drilling in mid-August to mid-September Target: Adult flea beetle There are two types of companion crops – sacrificial species, such as mustards that flea beetle will eat, and deterrent crops that help to mask OSR like buckwheat and clover.
“We have seen some encouraging controls in reducing adult damage – not huge, but they seem to be reducing pressure,” Dr White said.
“A companion crop either needs to be frost sensitive, not competitive with OSR or removed with a herbicide.”
Companion crops should be drilled a week or so before the OSR to provide protection from CSFB as the crop emerges.
Suited to mid-August to mid-September Target: Adult beetle Similar to companion crops, leaving stubble long may interfere with the pest’s ability to seek out the crop.
Dr White said: “We’ve done a few trials on this and had some instances where there’s been no effect, but some where there’s a 30-40% reduction in damage.
“However, we still have a lot of questions to answer, such as whether the length of the stubble plays a part.”
Suited to mid-August to mid-September Target: Adult beetle Applying organic amendments such as chicken muck or digestate to the seedbed may mask the smell of the crop from the beetle, or actively deter it.
It may also improve provisional nutrition, giving the crop a growth boost, Dr White said.
“We still need to look at whether different amendments work better than others and the timing – application may need to coincide with flea beetle migration.”
Suited to all drilling dates Target: Adults and larvae Work is ongoing to determine if certain commercial breeding lines of oilseed rape have reduced palatability.
“So far we are not seeing any great differences,” Dr White said.
“Varieties with autumn vigour might well help manage the adult problem and spring vigour may also help crops to grow away from larvae.
If you’ve got a mid-August sowing date, use one with good autumn vigour.”
Meanwhile, the John Innes Centre has identified resistance to CSFB in breeding lines of OSR and work is ongoing to see if this could be developed.
to mid-August onwards Target: Adults and larvae There is good evidence to suggest that trap crops like volunteer OSR can reduce adult and larvae pressure.
Dr White said: “By doing this you are trapping the migrating beetles in those volunteer crops.
Where we’ve done this we’ve seen significant reductions in larvae and improved plant populations.”
He advises leaving a whole field of volunteer OSR in the ground as late as possible to lure the pest in.
“I would say late September at the earliest, probably the later the better.
It is suited to any drilling date from mid-August because crops drilled earlier will be emerging at the same time as the volunteers.”