The Field Trip headed off towards Mannum at 8.30 and the first stop was at Reedy Creek to demonstrate the effectiveness of mechanical removal of cactus in the area. Other short stops occurred to look at examples of different invasive cactus and methods of control, for example Opuntia ficus-indica, the Indian fig and Austrocylindropuntia cylindrical, the cane cactus. I recognized cane cactus from the Maldon area and enquired about control methods using spray. Daconate (MSMA) spray had been trialled but it was not very successful and required follow-up. The only other method was mechanical removal, ensuring all broken bits of the plant were collected. Another site indicated clearly the need for the right cochineal biotype for the right host for effective biocontrol.
Cochineal Insects – a selective biocontrol
A biotype of cochineal insect had infested Opuntia streptacantha (similar to Opuntia robusta and sometimes confused with it but distinguished by its low creeping habit and pads without the milky blue green appearance) but not Opuntia robusta, which was growing in the same area. There were small dots of cochineal on the Opuntia robusta plants but it had not flourished nor damaged any other the plants. However, some Opuntia streptacantha were damaged and others dead or dying.
At another location large stands of Opuntia streptacantha were infested with cochineal insects and the population was flourishing. It had taken 8 years to develop such a large population of cochineal insects after releasing it on one single pad on one mature cactus.
Cochineal insects infesting Optunti streptacantha
The original host Opuntia streptacantha
We then stopped at a creek bed at Walkers Flat, where Opuntia robusta (wheel cactus) spread for about 2km along the creek. The area was heavily infested with cochineal insects, originally sourced from Burra in SA. After a quick demonstration the group harvested young healthy pads that were infested with cochineal insects. Once 10 tubs were full we set off for Nildottie to an extensive and very dense stand of Opuntia robusta.
Stem injection of Daconate
We then had a demonstration of how to do a stem injection with Daconate (MSMA) stem injection method where a hole was drilled in each stem with a handheld power drill. An injector was then used to inject 4-6ml neat Daconate per stem. We were shown a plant that had been injected 3 weeks earlier and there was complete knockdown. Apparently there is no regrowth. Juvenile plants have a scratch made in the plant surface with the injector and a drop of Daconate is put on the scratch. There is complete knockdown and no regrowth.
Kitted up for Daconate stem injection
Complete knockdown in 3 weeks
Releasing cochineal insects
It was then time to release the 5 tubs of cochineal insects (the other 5 were destined for Victoria). The infected pads needed to be placed lower down on larger plants, on the lee (sheltered) side to protect them from prevailing winds. The infested pads also needed to be protected from rain. The pads were distributed every 50-100 metres, one pad per plant. The cochineal insect population is typically denser on the sheltered side of the pad and this side is laid on a healthy pad on the host plant. One method was to put a small hole in both the host pad and the cochineal infested pad and then join them together with a stick. Helmuth Zimmerman, who has 50 years experience in cactus biocontrol, suggested it was better to find a place on the sheltered side where the cochineal infested pad could be securely wedged against a host pad. This prevented damaged to the healthy host pad.
