Resistenta rapsbaggar
Oilseed rape crops are an important in southern Sweden because of the high intensity
of wheat growing. Pollen beetles are the most important pest in oilseed rape and
causes often economic losses. Therefore it?s of great importance that we can control
the population density, e g by using insecticides if necessary, to avoid mayor yield
losses.
In year 2000 pollen beetles were numerous in the province of Östergötland. The
farmers sprayed with insecticides several times, but with low effects on the beetles.
Field trials and laboratory experiments showed that pollen beetles had developed
resistance against pyrethroids. Between 2001 and 2003 several field trials were laid
out in Östergötland to investigate pollen beetle mortality from different groups of
insecticides.
The results shows that only the highest recommended dose (72 g/ha) of tau-fluvalinat
and OP-compounds had acceptable effects on pollen beetles, giving more than 80 %
mortality. It might seem strange that tau-fluvalinat had good effects, because it?s also
a pyrethroide. There is no explanation for this.
Inventories among 32 farmers in Östergötland indicate that 78 % of the treatments
were made with tau-fluvalinat and 22 % with OP-compounds. The widespread use of
tau-fluvalinate has resulted in the first indications of resistance also against this
insecticide.
Interviews with farm advisers in mayor agricultural districts were held to find out
what kind of treatments against pollen beetles they recommended next year to their
farmers and how to minimize the risk of more problems with resistance.
Recommendations are not to grow spring- and winter oilseed rape crops in the same
area, to use winter oilseed rape crops instead of spring varieties and only to use the
highest recommended dose. These recommendations will lead to fewer treatments and
therefore less risk of resistance development against insecticides.