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Oilseed rape growers are being warned of significant larval damage from cabbage stem flea beetle this coming spring. Watch out for flea beetle pressure as spring beckons

Oilseed rape growers are being warned of significant larval damage from cabbage stem flea beetle this coming spring.

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

Some 200-500 adult beetle numbers were being found in a single trap by the end of September in traditionally problematic areas, according to ADAS monitoring of 14 farmers in partnership with Harper Adams University

Dr White said there had been far fewer reports of establishment disasters last autumn. And in some parts of the country, eight-week cumulative beetle levels in monitor traps had been below 100.

But he added: “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. But because leaf damage is far less noticeable in better-growing crops, in reality flea beetle populations may be every bit as high.”

Unsettled weather in August could have played a part in delaying the peak of migrations. But there was the usual build-up of adults in 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. But early sown crops tend to suffer much higher larval burdens that later sown-ones.

Dr White said: “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.”

Beneficial insects

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, lower insecticide use last autumn may help predation by encouraging the build-up of beneficial insects.

Under these circumstances, ADAS crop physiologist 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 flea beetle 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.

“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, Dr Kendall says growers 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 flea beetle – which means dispassionately assessing each crop as it comes into the spring.”

Growers should look for a number of key factors well beyond larval levels. These include rooting, soil condition, plant population, green area index and the sort of oilseed rape potential experience suggests is possible from each soil type.