Biological Control – using Cochineal
A promising biological control method for Wheel Cactus (Opuntia robusta) is Cochineal insects (Dactylopius species). These herbivores attach to the outside of the host cactus plant and feed by sucking moisture from it.
From 1927-39, survey teams in Texas, Argentina and Mexico identified 56 biotypes of cochineal insects but only 17 of these became established in Australia. All look very similar to the naked eye but they are highly host specific, meaning they differ in terms of what species of cactus they will feed on. Hence, biological control of Wheel Cactus requires pairing the right Cochineal biotype to the right cactus species for a successful effect.
However, even pairing up a specific Cochineal with a specific cactus does not necessarily guarantee a successful relationship. Matching an Opuntiae Cochineal insect with an Opuntia cactus, such as Wheel Cactus, can fail because biotypes can 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. However it is vital that all mature, fruiting Wheel Cactus plants continue to be killed using the herbicide injection technique. The Cochineal biological control may take 5 or more years to become established enough to have any significant effect. Therefore infestations producing new seed need to be controlled using current techniques. Hopefully in the future the use of Cochineal biological control will be an additional tool that can be used in combination with our current methods.
Cochineal insect Release 2011
Ian Grenda released some Cochineal insects (biotype unknown) 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 and Baringhup region. However, two years later, the population numbers had not reached the critical mass required to have any significant impact.
Cochineal insect release 2013
In May 2013, TCCG members attended the Australian Invasive Cacti Network Field Day in SA. They visited a number of sites showing different cactus control methods. Internationally recognized biological control expert, Helmuth Zimmerman, shared his 50 years of experience to demonstrate the most effective way to harvest Cochineal insects and then transfer them into new cactus plants.
‘Biosecurity SA’ arranged permits to harvest Wheel Cactus pads infected with Cochineal insects and transport these to Victoria. These have been released in 3 sites with dense Wheel Cactus infestations in the Maldon and Baringhup region. The infected pads were placed lower down on the sheltered side of large Wheel Cactus plants, to protect the cochineal insects from prevailing winds and rain. The side of the Cochineal infested pad with the heavier colonies was laid directly in contact with a healthy pad on the host Wheel Cactus plant. The Cochineal infested pads were distributed every 50 metres or so, one pad per host plant.
The 3 sites will be monitored regularly, however it may take up to 5 years or more for the Cochineal population to increase and spread to infect new Wheel Cactus plants. The SA experience demonstrates the importance of regularly moving infected pads to new Wheel Cactus host plants in the area to build up the critical mass needed for effective biological control.
Cochineal release sites (May 2013)