Problem Statement

Bocconia frutescens are medium-size shrubby trees up to about 6 m tall, native to Neotropics (Central and South America and West Indies) (Wagner et al. 1990). This tree species is now established, spreading, and highly invasive in a variety of native Hawaiian ecosystems (dryland forest, mesic koa forests and wet Metrosideros forests). It currently occurs on East Maui and Hawai’i islands but with its abundance in disturbed sites will likely be inadvertently spread to other Hawaiian islands.

Bocconia grows extremely rapidly and, with its large (to 60 cm long) leaves and colonial habit, develops dense groves that quickly shade and displace native plant species. It develops such thick stands on ‘a’a lava on East Maui that in the regional absence of species like fountaingrass, Bocconia is now considered the primary threat to dryland forest. In restoration efforts at Auwahi on East Maui, after years of protection and restoration where most non-native species have declined, Bocconia is the only non-native species that alarmingly has increased in both cover and frequency (Medeiros et al. 2003; Medeiros et al. 2007). On the Big Island, the species is an invader of wet Metrosideros-dominated rain forests (Benitez and Saulibio 2007).

This species is also now spreading and progressively dominating fern-filled gulches of koa forests of Kahikinui and Makawao districts. The Kahikinui area has been identified as a key site targeted for koa restoration and as the prime Maui parrotbill (Pseudonestor xanthophyrs) release site. If unchecked in the next decade, the single species Bocconia has the potential to highly retard if not untrack efforts to restore these forests.

One troubling ecological trait of the species is its large seeds which may be a key trait in its ability to germinate and become established in dense shade. Seeds are prolifically dispersed by non-native passerines such as Japanese white eyes (Zosterops japonica) and Red-billed leiothrix (Leiothrix lutea) for distances of apparently >1km. It is listed by the Hawai’I Department of Agriculture as a noxious species subject to legal restrictions for sale and distribution (HDOA 1992).

The control of rapidly spreading populations of Bocconia is clearly beyond mechanical and/or chemical methodologies. At this point, populations on Maui alone exceed several hundred thousand individuals and infestations appear to be spreading rapidly. It is likely in the next 3 to 5 years without control, the species will invade Kipahulu Valley facilitated by bird dispersal and progressively dominant native rain forest, degrading the sole habitat of numerous Endangered Hawaiian honeycreepers.

Despite this formidable profile, Bocconia appears to be an excellent candidate for classical biological control in Hawai’i, as there is only a single native representative of the family, the Hawaiian poppy, puakala, Argemone glauca, so that non-target concerns are limited and relatively easily addressed. Bocconia frutescens is common in a variety of habitats in Costa Rica. It does not display invasive characteristics there, which suggests that co-evolved natural enemies may be keeping it in check. Costa Rican habitats of this plant are not expected to differ substantially in climate compared to its potential Hawaiian range, so that ecological requirements of natural enemies should not be a barrier to their establishment in Hawai’i.

Tracy Johnson & Kenji Nishida, 2008