Oilseed rape growers and agronomists can now calculate the optimum timing to target cabbage stem flea beetle larvae hatch, with the chance to limit stem infestation.
Dr Max Newbert, Syngenta technical manager, reports that female cabbage stem flea beetle (CSFB) typically start egg laying in soil around the base of OSR plants two weeks after migrating into the crop. Larvae then emerge after 240 degree days (DD), before crawling up the stem and burrowing into leaf petioles.
“The aim is to target Hallmark Zeon sprays during the key period of larvae movement,” he advises. Trials have shown that a well-timed application can have a good effect in limiting initial infestation, with a 0.3t/ha yield advantage, according to the company.
Growers can identify migration dates for CSFB into crops from results of water trap monitoring. From that, it is possible to make a prediction of egg hatch and larvae movement, based on historical weather data or, ideally, in field weather station information for the current season, advises Dr Newbert.
Degree days, for CSFB, can be calculated by taking a site’s daily high temperature, plus the low, dividing by two and subtracting a base of 3.2degC - the point at which CSFB larvae stop emerging. If the high was 16degC and the low 6degC, the calculation would be (16 + 6) ÷ 2 – 3.2 = 7.8 DD. More advanced DD calculators work on the same basis, but assessing hourly temperatures.
“It is important to recognise that in warmer conditions larvae will emerge sooner, but if conditions turn cooler that would be delayed. It would also be different for locations across the UK. Application timing needs to be tailored accordingly,” he advises.
Recommended thresholds for treatment are currently five larvae per plant or 50 per cent of petioles with scarring. However, research on Syngenta’s Norfolk-based iOSR site has shown larvae numbers can increase very rapidly, with in excess of 40 larvae per plant frequently recorded – with a serious impact on the crop.
“The earlier the infestation, the greater the degree of damage on the plant,” says Dr Newbert. “In mild autumn conditions adults will continue to lay eggs and new larvae continue to emerge whilst temperatures are above 3.2degC. With warmer winter weather we have seen egg laying continue into February or March.”