Dactylopius opuntiae 'stricta' lineage was released more than 20 years ago in South Africa where it has successfully controlled Opuntia stricta. However, its host preference had not been tested against all invasive cacti in South Africa. Therefore, the possibility exists that it might successfully control other invasive cacti. We compared the performance of the 'stricta' lineage on Opuntia stricta, Opuntia humifusa and two lineages of Opuntia engelmannii. Additionally, we assessed if morphology of these Opuntia taxa could be used as an indicator of host selection by the 'stricta' lineage of D. opuntiae. Acceptability and suitability of the host taxa were measured by D. opuntiae life history parameters such as crawler development time and female weight. Principal component analysis of six morphometric characters of the Opuntia taxa was used to assess similarities between the taxa. Opuntia stricta is morphologically more similar to the O. engelmannii lineages than to O. humifusa, which showed little similarity to any of the other taxa. Additionally, the two O. engelmannii lineages showed some morphological similarity to each other. The 'stricta' lineage completed its development on O. humifusa such that the cochineal could be considered as a potential biological control agent for O. humifusa. Conversely, D. opuntiae 'stricta' was not able to complete development on O. engelmannii. Consequently, it would not be a suitable biological control agent for either lineage of O. engelmannii. The results show that O. humifusa is a suitable host for D. opuntiae 'stricta' and that Opuntia morphology is not an obvious indicator of host suitability.

Opuntia engelmannii Salm-Dyck (Cactaceae:Opuntioideae) is a morphologically variable cactus that has invaded pasture lands in South Africa and Kenya. Biological control of invasive Opuntia species can be very successful when the correct agent is matched with the appropriate host. This research investigated the similarity (morphological and genetic) between several populations of O. engelmannii and a morphologically uniform, O. stricta, that occur in South Africa and Kenya to help understand the identity of the target weed. Spine and cladode morphology showed that the Limpopo (South African) and Kenyan populations were most similar, while the spiny Eastern Cape (South African) population was more similar to Opuntia stricta. Multivariate analysis showed that spine morphometry was a useful diagnostic character in identifying different putative O. engelmannii populations in the field. In addition, we used amplified fragments length polymorphisms to estimate genetic similarity and genetic structure among populations that represent the morphologically variable populations.There was relatively little differentiation in the population structure between the Limpopo and Kenyan populations. The spiny Eastern Cape population showed moderate genetic differentiation (F ST = 0.05 À 0.15) relative to other O. engelmannii populations and was genetically more similar to the O. stricta population. These results will facilitate the search for suitable biocontrol agents by directing future collections of the appropriate biotype of Dactylopius opuntiae Cockerell (Hemiptera: Dactylopiidae) to relevant populations of O. engelmannii in its home range. In addition, given the genetic similarity of the spiny Eastern Cape population to O. stricta, we recommend the efficacy of D. opuntiae 'stricta' biotype be tested on the spiny Eastern Cape population as a potential biocontrol agent.

No abstract