Confirming the identity of the Hypogeococcus species (Hemiptera: Pseudococcidae) associated with Harrisia martinii (Labour.) Britton (Cactaceae) in Australia: implications for biological control

Determining the identity of potential control agents is critical to successful biological control and can contribute to our understanding of the failures of previous introductions, especially in cases where host-associated cryptic species may be present. In 1975, a mealybug was introduced into Australia from Argentina for the classical biological control of the invasive cactus Harrisia martinii (Cactaceae). This cactus also originates from Argentina and is an environmental and agricultural weed in parts of Australia. Since its release, the imported mealybug species has been incorrectly referred to as Hypogeococcus festerianus (Hemiptera: Pseudococcidae) in the applied literature, and its performance as a biological control agent has been considered poor in some locations. In this study, the identities of mealybug specimens collected from 10 locations in Queensland and New South Wales, Australia, were assessed. The genetic, morphological and reproductive characteristics of these specimens were compared with those of two congeneric mealybug species, Hypogeococcus pungens sensu stricto (Hemiptera: Pseudococcidae) and Hypogeococcus festerianus. Specimens from the different Australian localities examined were all very similar to each other morphologically and genetically, based on comparisons of mitochondrial and nuclear DNA sequence data. The morphological features of all the specimens were typical of Hypogeococcus pungens sensu stricto. H. pungens is now considered to constitute a species complex, and the specimens from Australia are genetically similar to the Cactaceae clade of this species complex from Argentina. In common with H. pungens s. s., the insects collected in Australia can also reproduce parthenogenetically. These findings help confirm that all populations of the mealybug in Australia are not H. festerianus, but part of the H. pungens cryptic species complex. There is no mismatch between this agent and the host plant in Australia, as H. martinii is one of the host plants of the most closely related cryptic species of H. pungens in the native range in Argentina. Thus, despite the original confusion around its identity, the variable performance of the introduced mealybug as a biological control agent of H. martini in Australia is likely due to other factors, and these require further investigation.