Biological control of an exotic scale, Orthezia insignis Browne (Homoptera: Ortheziidae), saves the endemic gumwood tree, Commidendrum robustum (Roxb.) DC. (Asteraceae) on the island of St. Helena

Abstract

Abstract St. Helena is a small South Atlantic island with a highly degraded, but internationally significant, terrestrial flora. The 2500 remaining gumwoods, Commidendrum robustum , are an important part of this remnant flora. In 1991, a South American scale insect, Orthezia insignis , was identified attacking the gumwood trees. By 1993, severe infestations had killed over 100 trees. If the number of dead trees had continued to increase exponentially, all 2500 trees would have died by 1995. This outcome was likely given abundant alternative host plants, and lack of natural enemies of O. insignis . Host range tests showed that the other members of the endemic genus Commidendrum were also at risk. The coccinellid predator, Hyperaspis pantherina , was released on St. Helena in 1993. H. pantherina numbers increased from 1994, coinciding with a 30× decrease in scale numbers. Scale outbreaks have not been reported on St Helena since 1995, and culturing of H. pantherina was discontinued because insufficient O. insignis prey could be found in the field. Extensive blackening from sooty molds on the surviving trees in 1995 suggested that the predator was effective just in time to prevent death of most trees. H. pantherina appears to have saved the field population of a rare endemic plant from extinction.