Monitoring the effects of Rodolia cardinalis on Icerya purchasi populations on the Galapagos Islands

Abstract

In January 2002, the first biological control program was implemented on the Galapagos Islands with the release of the Australian coccinellid Rodolia cardinalis Mulsant to control the invasive cottony cushion scale, Icerya purchasi Maskell. This was the first time that Galapagos authorities had approved the introduction of a biological control agent to this iconic archipelago and, because of this precedent, it was important to monitor and evaluate its behaviour soon after its introduction. Surveys were carried out after the release of R. cardinalis in 2002 to confirm establishment on Santa Cruz Island. In 2009, seven years post-release, a broader survey was done to document spread throughout the archipelago. Directly after the release of R. cardinalis in 2002, a predator exclusion study and field observations were carried out on scale insect populations on white mangrove (Laguncularia racemosa [L.] Gaertn. F.) on Santa Cruz Island to document impact. In less than three months after R. cardinalis was released in 2002, populations of I. purchasi on white mangrove that were exposed to the predator in the exclusion experiment, or were monitored in the field, had declined by 99–100%. Results suggest that R. cardinalis played a key role in this decline, possibly in combination with high rainfall. Rodolia cardinalis dispersed quickly after its release and by 2009 was found in a wide variety of habitats on seven of the eight islands surveyed that had records of I. purchasi. Two of these were self-introductions. Further monitoring is recommended to determine whether this biological control agent has successfully reduced scale insect numbers on other valued plant species.