Most oilseed rape crops have been drilled or sown into exceptional growing conditions of seedbed moisture and warm soils so far this season. The major threat to establishment now is Flea Beetle damage, nipping off growing points before plants have time to produce cotyledons or leaves. 

Growers and agronomists need to consider the threat of early attack from Flea Beetle – the generic term for a vast array of small beetles that attack the earliest seedling stages. 

Whilst the larger and more noticeable Cabbage Stem Flea Beetle is often associated with establishment issues, on some farms the much smaller species can be the culprit, warned Syngenta Field Technical Manager and entomologist, Pete Saunders.

He pointed out that much of the Flea Beetles’ damage occurs before the crop has fully emerged and may go unnoticed – until reports of poor or patchy establishment. 

“With good soil conditions this season, growers are reported to have trimmed seed rates right back to help with canopy management later,” he said. “But there is a real danger for future crop potential if they do not get sufficient plants established now.”

Pete advocated growers inspect seedbeds and chitting seed for signs of beetle activity; if seedling shoots and cotyledons are being damaged, crops will need immediate treatment with Hallmark Zeon. “Some small species are very difficult to find so a thorough detailed visual inspection is needed to see what’s happening during emergence,” he advised. 

For the more difficult to control Flea Beetle, Hallmark Zeon has a label approval of 75ml/ha, whilst for Cabbage Stem Flea Beetle the rate is 50ml/ha.

“Trials have shown that growers should always use the full permissible rate to achieve the best possible control and the longest lasting results,” he advised. “It is important to identify the target beetle, in order to use the appropriate rate in the field.”

Importantly, the formulation of Hallmark Zeon has been shown to provide insect repellent activity that could help to deter further damage. And the built-in UV light stabilisation would help to ensure it remains more active in longer daylight hours and sunlight of early autumn applications, he added.  

Targeted sprays

If good growing conditions remain, then once seedlings reach the true leaf stage they may be able to grow away from typical Cabbage Stem Flea Beetle shot-holing damage, advised Pete. “At 1-2 leaves plants will generally recover from up to 25% leaf damage, and at 3-4 leaf stage up to 50% leaf damage may be tolerable. However, dry conditions or high volunteer cereal competition, for example, can slow the oilseed rape growth and make plants more susceptible to damage.  

“It is important that growers stick to the spraying thresholds to minimize the use of insecticides,” he said. “But, prior to the true leaf stage, its important growers protect emerging seedlings to ensure the target plant population is achieved.”

Whilst there had been reports of pyrethroid resistance among Cabbage Stem Flea Beetle populations, he reported its distribution appeared sporadic. In Syngenta’s own testing in the past two years there was a trend for high levels of susceptibility in the South, and indications of more resistance in Yorkshire and Humberside, for example. 

“In East Anglia it’s a mixed bag; with some farms having populations that remain highly susceptible to control, and a few farms with high levels of resistance,” he commented. Where partial resistance had been encountered, application at the full rate still achieved a useful level of control.

“If a grower does not achieve adequate control from a pyrethroid, then resistance may be suspected and a follow up pyrethroid should not be applied,” he advised. “However, growers need to ensure that the appropriate dose is hitting the target. Use full rate of Hallmark Zeon, applied with the appropriate nozzle choice and in the best available spraying conditions.

“Furthermore, fine seed beds avoid beetles hiding under clods, whilst applications should be timed for when beetles are most active in the day. All these factors influence how much of the spray beetles are exposed to, and therefore the level of successful control,” he added.