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Crop Advice & Expertise

Beware CSFB pressures despite much improved OSR establishment, warns ADAS

Better oilseed rape establishment across the country this season doesn’t necessarily mean cabbage stem flea beetle pressures have been any lower in many parts of the country, warns ADAS entomologist, Dr Sacha White.

Very much better oilseed rape establishment across the country this season doesn’t necessarily mean cabbage stem flea beetle pressures have been any lower in many parts of the country, warns ADAS entomologist, Dr Sacha White. This means growers need to be alert to the possibility of significant larval damage in the spring.

The extended monitoring ADAS, Harper Adams University and several industry partners have been conducting on 14 farms from Devon to North Yorkshire this autumn as part of the latest AHDB and industry funded research project shows adult beetle numbers building to between 200 and almost 500 in a single trap by the end of September in traditionally problematic areas (Figure).

“We have certainly been receiving far fewer reports of establishment disasters this autumn,” Dr White points out. “And in other parts of the country, eight-week cumulative beetle levels in our monitor traps have been below 100.

“Because we haven’t previously monitored CSFB migration in so much detail over such an extended period, it’s difficult to compare this season’s results with those of recent years. However, the levels we have been recording are clearly substantial in the areas that generally see greater problems from the pest. What’s more, overall, they are not noticeably lower than we have been expecting.

“Visually, adult flea beetle damage may appear to be much less than the last two seasons in many cases,” he agrees. “But because leaf damage is far less noticeable in better-growing crops, in reality CSFB populations may be every bit as high.

“While the unsettled weather in August could have played a part in delaying the peak of migrations, there was the usual build-up of adults in our traps from early September, with a big jump in the middle of the month coinciding with drier, more settled conditions. Earlier sown crops were able to take advantage of good levels of soil moisture so they were well beyond their most vulnerable stage by this time.

“However, our previous studies show that early sown crops tend to suffer much higher larval burdens that later sown-ones. Equally, we know that having to write-off a crop in the spring has a far greater economic impact that losing it in the autumn. So, we are certainly not out of the flea beetle woods yet.”

On the positive side, the ADAS team suggests that better-grown crops are likely to able to tolerate greater larval damage. At the same time, of course, the fact that many growers have reported much lower insecticide use this autumn – either as policy or through a lower perceived need – may help predation by encouraging the build-up of beneficials like ground beetles and parasitic wasps.

Under these circumstances, ADAS crop physiologist, Dr Sarah Kendall urges growers to keep a careful eye on their crops as they come through the winter so they can apply the most appropriate – and cost-effective – agronomy over the spring and early summer.

“On the one hand, we know even crops with very high levels of CSFB larvae in the spring can yield surprisingly well if they are robust enough to compensate sufficiently and their plant populations and growing conditions allow them to do so,” she notes. “On the other, as we saw last season, crops that are in the wrong place coming out of the winter or don’t have the right conditions can be devastated by relatively modest larval populations.

“More than anything else, we need to avoid two things – either abandoning a crop which has every chance of going on to yield reasonably, or ploughing on regardless with one that is most unlikely to do so.

“Instead, we need to make the best-informed field-by-field decisions on the extent of inputs warranted at a time when we can most effectively adjust them to give ourselves the best margin-earning opportunities.

“To do so our management has to be as much about understanding the essentials of the crop’s performance ability as dealing with CSFB,” stresses Dr Kendall. “Which means dispassionately assessing each crop as it comes into the spring for a number of key factors well beyond larval levels. These include rooting, soil condition, plant population, GAI and, crucially, the sort of OSR potential our experience tells us is possible from our ground.”

Figure 1:   Adult Cabbage Stem Flea Beetle Levels in ADAS Monitoring - 2020

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Source: AHDB Project 21120185

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