A promising biocontrol method for wheel cactus (Opuntia robusta) is cochineal scale insects (Dactylopius species), cactus herbivores that attach to the outside of the host plant and feed by sucking moisture from it. From 1927-39, survey teams in Texas, Argentina and Mexico identified 56 biotypes of cochineal insects and only 17 of these established in Australia. All look very similar to the naked eye but they are host specific, meaning they differ in terms of what cactus they feed on. So biocontrol of wheel cactus is like match making, pairing the right cochineal biotype to the right cactus for a successful relationship. However, even pairing up two that seem right for each does not necessarily mean success relationship. Matching an Opuntiae cochineal insect with an Opuntia cactus, such as wheel cactus, can fail because relationships may be so highly evolved that local cochineal populations only feed on local variants of a single Opuntia species. Harvesting cochineal insects from one area and shifting it to another area may not result in successful colonization.
Despite these difficulties, Tarrangower Cactus Control Group is exploring biological control of local wheel cactus with cochineal insects.
Cochineal insect release 2011
Ian Grenda released the cochineal beetle to an area of Wheel cactus in November 2011. He brought some Wheel Cactus pads infected with cochineal insects from South Australia in the hope that the population of cochineal would flourish in the Maldon Baringhup region. However, two years on the population numbers have not reached the critical mass required to have any noticeable impact.
Cochineal insect release 2013
In 2013 two group members attended the Australian Invasive Cacti Network Field Day in SA in May. They visited a number of sites showing different cactus control methods. An internationally recognized biocontrol expert, Helmuth Zimmerman, shared his 50 years of experience to demonstrate the best way to harvest cochineal insects and then release them into a new area. Biosecurity SA arranged permits to harvest a large number of wheel cactus pads infested with cochineal insects, which were slowly but surely damaging and killing the wheel cactus in the area. These have been released on three heavily infested sites in the Maldon and Baringhup region. The pads were placed lower down on the sheltered side of larger plants to protect the cochineal insects from prevailing winds and rain. The cochineal population is typically denser on the sheltered side of the pad so this side was laid on a healthy pad on the host cactus. The pads were distributed every 50 metres or so, one pad per plant. The three sites will be monitored regularly and only time will tell if it is a perfect match. It can take a couple of years or more for the cochineal population to build up and infest new plants. The SA experience demonstrates the importance of regularly moving infected pads to new cactus host plants in the area to build up the critical mass needed for effective biocontrol.