The performance of Dactylopius opuntiae as a biological control agent on two invasive Opuntia cactus species in South Africa

Many biological control projects targeting weeds in the Cactaceae have been noteworthy successes. Recently, populations of a prickly pear, Opuntia humifusa, have spread across South Africa, endangering both grazing lands and natural biodiversity. A biotype of the cochineal insect, Dactylopius opuntiae 'stricta', which has been successfully used in South Africa to control Opuntia stricta, has been observed to use O. humifusa as a host. However, it does not appear to effectively control O. humifusa infestations. To investigate the possible reasons for this, we tested two hypotheses: firstly, that O. humifusa is a sub-optimal host of D. opuntiae 'stricta' compared to O. stricta; and, secondly, that the underground tubers characteristic of O. humifusa enable it to regenerate after sustaining cochineal damage. We compared the survival, fecundity and development of D. opuntiae on the two host plant species under controlled conditions. Host plant had no significant effect on the survival rates and development of the insects. In addition, O. stricta plants generated more new growth after sustaining damage from D. opuntiae than O. humifusa under the same conditions. These results show that O. humifusa and O. stricta are equally suitable hosts for D. opuntiae 'stricta' and that the underground tubers of O. humifusa do not increase its resistance to D. opuntiae damage. Further ecological observations may elucidate other possible reasons for the failure of cochineal insects to control invasive O. humifusa populations in South Africa.