The purpose of this paper is to examine what problems cabbage growers have with the
Swede Midge. The last few years the Swede Midge, Contarinia nasturtii, has become
a larger problem for cabbage growers in several countries in Europe as well as in
Sweden. In Canada the problems are so severe that the midge is classified as a
quarantine pest and therefore certain rules have to be followed. I have collected facts
of the biology, host plants and reproduction of the Swede Midge, as well as the
growers experiences and the occurrence of the midge in Canada, Europe and Sweden.
The Swede Midge is hard to control because of its short life cycle and the larvae are
protected between the heartleaves of the plant. When the larva feeds on the growth
point the plant becomes distorted, in some cases several heads develop or none at all.
When the midge infests spring rape its flower buds become undeveloped and rosette
like. By severe infestations in spring rape the fields can re-flower later during the
season. In Sweden the midge has two to three generations per year. The larvae
overwinter in the soil for about 1-2 years.
I have counted midges caught in the pheromone traps, placed in fields of winter
wheat, spring rape and cabbage by Växtskyddscentralen, Alnarp, 2005, to get a
picture of the generations and reproduction of the Swede Midge. The results show that
a large amount of midges overwinter in the spring rape fields and hatch the following
year. Then the midges fly to new hostplants and start a new generation. If there are
cabbage fields nearby, the next generation of the Swede Midge can cause severe
damage. An investigation was made in the 1970’s in Skåne and Blekinge, which
showed that the infestations were severe. Already then it was established that a large
increase of the midge population in spring rape could lead to huge consequences for
the cabbage grower. The pheromone traps in the cabbage fields show that if cabbage
is grown year after year or on a field nearby, the population is kept alive.
The chemical control should be directed towards the adult midges. However, today
there is no good method of actually knowing when the midge flies. The pheromon
traps have made it possible to start the development of a monitoring system. Research
and trials are in progress to optimize the pesticide treatment to the flight peaks and to
prevent egg-laying. Still there are some preventive measures to be taken, such as crop
rotation, controlling weeds and field selection.
Except developing ...
