22 December 1995

New flea beetle advice aims to remedy too-late sprays

INSECTICIDE seed treatments should be used routinely to control flea beetle attack on linseed and flax crops, says ADAS.

In severe cases, a post-emergence insecticide should also be applied, with choice depending on air temperature, says entomologist Jon Oakley.

The new recommendations replace previous advice to spray two or three times to control the pests. Often sprays are too late to prevent substantial or even total crop loss, he warns.

Sheer pest numbers are to blame. Both large and small flax flea beetle populations have increased in line with crop area, Mr Oakley explains. Significant damage was first noted in 1992 in southern England. Last year, total crop loss was recorded as far north as the Vale of Pickering.

"Crops are frequently destroyed despite farmers attempts at control. No approved treatments are available, only off-label extrapolations from treatments recommended for flea beetle control on oilseed rape. These have often proved inadequate."

Work last season at four ADAS centres across England highlighted the effectiveness of treated seed, says Mr Oakley.

Highest field losses occur when germinating seed is attacked before it emerges, when the pest is out of reach of sprays. But seed treated with Vitavax RS (carboxin + gamma-HCH + thiram) gave as good a yield as soil-treated control plots in such cases.

In severe attacks, a post-emergence insecticide spray was also needed. Which spray to use appears to depend on temperature, although further work is needed to verify this, Mr Oakley stresses.

"It seems pyrethroids work better below 8íC, whereas gamma-HCH is not very effective at these temperatures. My recommendation for now is to use Sumi-Alpha (esfenvalerate) at 0.3litres/ha to treat early sown crops in April, and Gamma-Col (gamma-HCH) at 0.7litres/ha on later sown ones in May."

In some cases, flax flea beetle arrives after plants have emerged, and sprays alone will control it, Mr Oakley admits. But given the difficulty of predicting the time and level of attack, and the cheapness of the seed treatment, he believes growers should adopt a "belt and braces" approach.

"Due to the continued high incidence of flea beetle attack, routine use of Vitavax RS seed treatment is recommended, followed by a further spray as needed. This should suffice in all but the very worst cases, showing an overall saving on the current two and three spray programmes."


Benefits of treatments averaged over four

ADAS sites

£/ha

seed trt alone+£19.10

spray:(gamma-HCH) x1+£11.50

x2+£21.50

(esfenvalerate)x1+£10.80

x2+£4.10

seed trt + spray:

(gamma HCH)x1+£22.10

x2 +£9.60

(esfenvalerate)x1+£19.40

x2+£21.20

Assumes linseed price of £120/t;

seed treatment @ £6.40/ha;

gamma-HCH @ £11.50/ha;

esfenvalerate @ £7.20/ha.

ADAS trials highlight the value of seed treatment in checking flea beetle damage (inset) to linseed. Sprays often go on too late to save pest-hit crops.