Biocontrol of Harrisia cactus Harrisia martinii by the mealybug Hypogeococcus festerianus (Hemiptera: Pseudococcidae) in salt-influenced habitats in Australia

This study provides new information on the efficacy of Harrisia mealybug Hypogeococcus festerianus Lizer y Trelles control of Harrisia cactus Harrisia martinii, a major weed of grazing lands, after the last reported field assessments in 1985. In particular, the effectiveness of H. festerianus in a salt-influenced environment, on the marine plains of tropical Australia, was evaluated, under both high-density and low-density Harrisia cactus cover conditions. Effectiveness of the biocontrol agent was evaluated by assessing mealybug occurrence levels and groundcover estimates of Harrisia cactus over time. Similar to studies from the 1970s, mealybugs reduced dense stands of Harrisia cactus to low levels within 4 years, although plants were only killed after being heavily infected for a few years. New plants were also infected with mealybugs and did not reach more than one or two small unhealthy stems in size. For the first time, successful inoculation of relatively sparse infestations of Harrisia cactus infestations was demonstrated. The mealybug established in ~70% of treated clumps and, as in dense infestations, substantial mortality and reduction in area of cactus cover was apparent 4 years post-inoculation. The Harrisia mealybug also dispersed to adjacent uninfested areas at two locations, indicating that sustainable long-term biocontrol may be possible even in low-density infestations. This study confirmed the efficacy of H. festerianus as a control agent in these physically extreme marine plain environments in both dense and scattered populations. The relatively low effort (a few ad hoc releases) to achieve establishment and natural dispersal to low-density situations as compared with annual herbicide control efforts augurs well for real-world situations where grazers are time constrained. © 2019 Australian Entomological Society