Biological control of Scotch Thistle
In the late 1990’s a local farmer – the late John Foster – led a campaign, via the local beef and lamb monitor farm group, to introduce into New Zealand an insect parasite to control scotch thistles. John obtain funding from several national agencies along with contributions from local farmers, including myself.
This insect, known as the scotch thistle gall fly – “Urophora Stylata” – is somewhat smaller than a house fly, deposits its eggs o the buds and flowers of the thistle. Upon hatching, the larvae bore into the centre of the seed head and devour the developing seed.
This insect parasite was trialed in Switzerland, Canada and Australia, where it was found to infect 90% of thistle seed heads destroying 60% of seed. From observations, I suspect that 60% is very conservative here.
About 15 years ago, John brought some thistle seed heads, containing larvae, onto my farm, where I have watched their progress over the years. They seem to spread quickly and I have found them on neighbouring farms about 7kms distant and against the prevailing winds. Originally, I was advised not to spray the thistles so that the insects would spread. Nevertheless, I continued spraying but missing the release area. As some thistles are often missed during spraying and many germinate later, the gall fly’s spread is not impeded.
Today I do not see thistles in full bloom. The vast majority of seed heads do not develop to the flowering stage, and those that do are malformed and on being cut open, the larvae are present. We have also observed a great reduction in thistle down drifting past in the wind over recent years, as this parasite has spread to farms where thistles are not controlled.
Where biological endeavours are pursued, the parasites need their hosts to survive. Therefore the control of the thistle problem is not absolute. However, as the gall fly spreads nationwide, this problem should be considerably reduced.
Yes, I continue to use a helicopter to spray my steep hill country on the back of the farm where the wing thistle is the worst problem. I also use my quad to control the scotches on the more undulating third of my property. The number of scotch thistles seem to be decreasing year by year.
At this stage, it seems possible to reduce the incidence of scotch thistles in pasture by 50% to 70%. This in turn should translate to less chemicals being used, a saving in time and less vegetable contamination in our wool clip.
How do I obtain the Gall Fly?
I am able to supply scotch thistle seed heads containing the larvae of the Gall Fly. There are two windows of opportunity for transferring the larvae. At the present time, the larvae are in the dead thistle heads and you can tell if they are present if the seed heading has a “gall” or hard stone like centre to the flower head. Seed heads that have no hard core will not contain the larvae. At this time of year, the larvae will “winter over” and emerge when the spring thistles are in flower. The other opportunity for relocation is over the summer when the larvae will develop into adult flies probably in a very short time.
If you wish me to send you thistle heads containing larvae, the cost will be $60.00 - $30.00 of which will be donated to the Red Cross. The remainder is to cover the costs of collection, packaging and postage. The thistle heads will be despatched in mesh bags ready to attach to a fence or tree. Instructions will be included.
In the late 1990’s a local farmer – the late John Foster – led a campaign, via the local beef and lamb monitor farm group, to introduce into New Zealand an insect parasite to control scotch thistles. John obtain funding from several national agencies along with contributions from local farmers, including myself.
This insect, known as the scotch thistle gall fly – “Urophora Stylata” – is somewhat smaller than a house fly, deposits its eggs o the buds and flowers of the thistle. Upon hatching, the larvae bore into the centre of the seed head and devour the developing seed.
This insect parasite was trialed in Switzerland, Canada and Australia, where it was found to infect 90% of thistle seed heads destroying 60% of seed. From observations, I suspect that 60% is very conservative here.
About 15 years ago, John brought some thistle seed heads, containing larvae, onto my farm, where I have watched their progress over the years. They seem to spread quickly and I have found them on neighbouring farms about 7kms distant and against the prevailing winds. Originally, I was advised not to spray the thistles so that the insects would spread. Nevertheless, I continued spraying but missing the release area. As some thistles are often missed during spraying and many germinate later, the gall fly’s spread is not impeded.
Today I do not see thistles in full bloom. The vast majority of seed heads do not develop to the flowering stage, and those that do are malformed and on being cut open, the larvae are present. We have also observed a great reduction in thistle down drifting past in the wind over recent years, as this parasite has spread to farms where thistles are not controlled.
Where biological endeavours are pursued, the parasites need their hosts to survive. Therefore the control of the thistle problem is not absolute. However, as the gall fly spreads nationwide, this problem should be considerably reduced.
Yes, I continue to use a helicopter to spray my steep hill country on the back of the farm where the wing thistle is the worst problem. I also use my quad to control the scotches on the more undulating third of my property. The number of scotch thistles seem to be decreasing year by year.
At this stage, it seems possible to reduce the incidence of scotch thistles in pasture by 50% to 70%. This in turn should translate to less chemicals being used, a saving in time and less vegetable contamination in our wool clip.
How do I obtain the Gall Fly?
I am able to supply scotch thistle seed heads containing the larvae of the Gall Fly. There are two windows of opportunity for transferring the larvae. At the present time, the larvae are in the dead thistle heads and you can tell if they are present if the seed heading has a “gall” or hard stone like centre to the flower head. Seed heads that have no hard core will not contain the larvae. At this time of year, the larvae will “winter over” and emerge when the spring thistles are in flower. The other opportunity for relocation is over the summer when the larvae will develop into adult flies probably in a very short time.
If you wish me to send you thistle heads containing larvae, the cost will be $60.00 - $30.00 of which will be donated to the Red Cross. The remainder is to cover the costs of collection, packaging and postage. The thistle heads will be despatched in mesh bags ready to attach to a fence or tree. Instructions will be included.