05 May 1998
Varroa threat to OSR crops
BEE losses as high as 90% in some areas – 60-70% on average – because of the parasitic Varroa mite have been reported. Could there be implications for oilseed rape growers from a shortage of pollinating bees?
Varroa mite is the biggest problem that we have ever faced in the history of UK beekeeping. England, Wales and south west Scotland have been declared statutory infected areas. The mite affects both wild and commercially-kept bees. So farmers relying on bees for the pollination of their crops – such as oilseed rape – have a big problem on their hands this year.
Using bees for pollination has been shown to increase oilseed rape yields by 5%-15%. An average oilseed rape crop requires about 2.5 bee colonies per hectare.
Previously when bee numbers have been low, growers had the option of introducing commercially-kept bee colonies. This may no longer be an option with the decline in commercially-managed colonies.
The onus is on growers to take responsibility for the bees which pollinate the crops in their area. They can do this by using pesticides which achieve good pest control without adversely upsetting the balance, in numbers, of beneficial insects such as bees. Fastac is the only pesticide endorsed by the British Beekeepers Association for use on oilseed rape.
As a matter of good practice, spraying oilseed rape should be carried out in the late evening or early morning, or in dull weather if at all possible.
Local beekeepers should be advised of the intention to spray. At these times, pollinators are less active, and late evening application allows dispersal of the pesticide before bees become active the following day.
When used as recommended, Fastac presents only a minimal hazard to foraging honeybees and has no long-term effects on other beneficial arthropods.
Every year there is unnecessary use of insecticides on winter rape. Apart from being uneconomic, this can be harmful by killing bees, particularly when applied to crops during early flowering. Only apply an insecticide for pollen beetle control if the spraying thresholds are exceeded during green-yellow bud stage throughout the crop, not just at the crop edges where pollen beetles initially infest the crop.
Spray threshold for a healthy crop is 15 beetles per plant at green-yellow bud throughout the crop, falling to five beetles per plant for backward crops.