BIOLOGICAL CONTROL OF ARTHROPODS OF CONSERVATION IMPORTANCE

The small South Atlantic island of St. Helena has a highly degraded but internationally significant terrestrial flora, now covering only 1% of its land area. The 2500 gumwood trees, Commidendrum robustum, in the last two natural stands, are an important part of this remnant flora. In 1991, a scale insect infesting the gumwoods was identified as Orthezia insignis. This South American pest is widespread in the tropics, but this was the first record from St. Helena. By 1993, there were severe patches of infestation of the scale, and over 100 gumwood trees were dead. If the exponential increase in the number of dead trees had continued, all 2500 trees would have been killed by 1995. This was a likely outcome given the lack of natural enemies, and abundance of alternative host plant species for the scale. Fortunately, O. insignis had a history of successful biological control in Hawaii, and several African countries, through the introduction between 1908 and 1959 of the predatory South American coccinellid beetle, Hyperaspis pantherina. The life history and environmental safety of the predator were studied in quarantine in the U.K., and in 1993 the St. Helena government gave permission for its introduction onto the island. In May 1993, 80 H. pantherina survived the 6-day journey to St. Helena, and were used to establish a laboratory colony, from which over 5000 beetles were released from June 1993 to February 1994. Monitoring was undertaken using visual counts of O. insignis and H. pantherina on 300 labelled branchlets on the gumwood trees. Although the cause of tree death was visually obvious, monitoring demonstrated significant correlations between the levels of attack by the scale and tree mortality. H. pantherina was detected on the labelled shoots in February 1994, and numbers then increased, coinciding with a 30× decrease in mean scale numbers. This measured reduction is conservative, because the number of live scales tended to be underestimated when debris from recent feeding by the coccinellid was present. There have been no further problems reported with the scale on St. Helena since 1995. Laboratory rearing of H. pantherina was discontinued in July 1995 because insufficient O. insignis could be found anywhere on the island. Biological control of O. insignis was successful, but the extensive blackening from sooty moulds on all surviving gumwood trees in February 1995, suggested that the predator was effective only just in time to prevent most of ______________________________________________ Control of Orthezia insignis on St. Helena Island Second International Symposium on Biological Control of Arthropods 53 the trees being killed. Experimental transfers of O. insignis showed that the other three members of the endemic genus Commidendrum could also be at risk from the scale. The deliberate introduction of H. pantherina into St. Helena is an early example of biological control being initiated solely for conservation of indigenous biodiversity. It appears that this successful programme has saved the field population of a rare endemic plant from extinction. INTRODUCTION The 122-km2 island of St. Helena is situated in the South Atlantic Ocean (15o 56 ́ S, 5o 42 ́ W). Despite widespread environmental degradation since the 16th century, the extant biota of the island is of international significance (Pearce-Kelly and Cronk 1990). St. Helena’s flowering plants, for example, include 30 endemic species in 23 genera, and 10 of these genera are also endemic (Pearce-Kelly and Cronk 1990). Native vegetation covers less than 1% of the land area, and many of the indigenous plant species exist in only very small numbers (Cronk 1989). On the positive side, plant species thought to be extinct for over 100 years have been rediscovered, and ambitious restoration programs have been started (Cronk 1989). The endemic genus Commidendrum contains four species, including C. robustum (Roxb.) DC. (St. Helena gumwood) (Asteraceae), the island’s national tree. The once extensive forests of gumwoods are now represented by 2500 trees in two small stands. In 1991, an insect was noticed attacking gumwood trees at Peak Dale (G. Benjamin, pers. comm.) (Fig. 1). This was identified as the South American scale, Orthezia insignis Browne (Homoptera: Ortheziidae), a polyphagous pest that has been accidentally introduced into many tropical countries on imported plants. The first gumwood deaths attributed to O. insignis occurred in 1992. Control of O. insignis using insecticides was not an option because of the steep terrain, strong winds and risk to indigenous insects. CAB International suggested biological control as an option. A predatory beetle, now known as Hyperaspis pantherina Fürsch (Coleoptera: Coccinellidae) (Fig. 2), was a recognised biological control agent for O. insignis, but because of poor past monitoring it’s success record was uncertain, and little was known about it’s biology. Consequently, the first steps were to review the past record of the agent, and study its life history with an emphasis on assessing its host specificity and optimising methods for rearing and transportation. Figure 2. Adult Hyperaspis pantherina. UGA1390006 Figure 1. Gumwood branch heavily infested with Orthezia insignis, Peak Dale, St. Helena, May 1993. Note the blackening from sooty molds. UGA1390005