9 July 1999

Consider implications of reducing drilling rates

By Charles Abel

Winter oilseed rape growers should think carefully before reducing their drilling rates below those recommended by breeders following MAFFs decision to ban the insecticide seed treatment lindane, says NKs sales and marketing manager Nigel Padbury.

Lindane, or gamma – HCH, is the active ingredient in the three common organochlorine insecticides incorporated in oilseed rape seed dressings. Its ban from last week leaves growers without any routine pre-emergence treatment against cabbage stem flea beetles.

"Flea beetles are a widespread problem and the majority of the UKs winter oilseed rape crops will be at risk from these insects in their first few weeks," says ADAS Boxworth entomologist John Young. "The adults summer dormancy comes to an end in late August when they migrate to newly established rape crops to feed on the emerging plant and its cotyledons.

Damage varies according to growing conditions and weather. If the crop is slow to emerge in dry conditions and suffers a growth check, it is put at additional risk and, in severe cases, the cotyledons can be completely removed," he says.

"Growers can attempt to reduce the risk of flea beetle attack by cultural control. For starters, they can increase the number of seeds drilled in an attempt to compensate for damage. They can also attempt to drill earlier so the crop will be established before the flea beetles summer dormancy ends and ahead of invasion by the beetles.

"They also need to be vigilant in monitoring the crop throughout the emergence period, and if plants are suffering damage, then it may be necessary to make an early application of an approved synthetic pyrethroid during what is a very busy period on most farms," says Mr Young.

"This additional spray will go towards preventing further attack from flea beetle larvae which tunnel into the plant stems leaving them exposed to fungal attack. However, an early spray targeted against adult flea beetle will not necessarily remove the need for a later spray to control larval attack at the more usual October/November timing."

Mr Padbury acknowledges the desire of growers to cut seed costs plus the fact that reduced seed rates can help towards achieving stronger healthier plants which over winter more successfully, have improved standing power, and ultimately achieve a higher yielding crop.

But NK has already taken such factors into account in the sowing rates that it recommends for Madrigal, he says.

"Now that insecticide seed treatments arent available, wed urge growers not to cut corners and make compromises. Rather, they should stick with a varietys recommended drilling rate in order to minimise risk from flea beetle attack, optimise crop establishment and ultimately total harvest yield." &#42