Ecological role of honeydew-producing scale insects (Coelostomidiidae) in native forests of northern New Zealand

Abstract

Phloem-feeding scale insects often produce honeydew, a sugar-rich waste product that can be a major source of carbohydrate in both natural and production ecosystems, hi New Zealand endemic scale insects in the Coelostomidiidae family are widespread, but their ecological role within native forests in northern New Zealand is poorly understood. A survey of mainland forests in the Auckland region detected three Coelostomidiid species: Coelostomidia zealandica was uncommon, C. pilosa was widespread in broadleaved-podocarp forest but only formed light infestations, while C. wairoensis was present in all tea treestands examined and often formed heavy infestations on kanuka trees (Kunzea ericoides). The scarcity of C. zealandica infestation suggested this species may require conservation attention and this was confirmed by further survey work (Chapter 3). The extent of C. wairoensis infestation on kanuka suggested it was likely to have community-level impacts and this was investigated in three further studies (Chapters 4-6). A year-long study in the Karamatura Valley (Auckland region) found that kanuka honeydew was highly seasonal, with both standing crop and 24 hr production peaking in summer. Vespula wasps fed directly on honeydew by day, a suite of moths (both native and exotic) fed on honeydew by night and infested trees had higher abundance of native fungivorous beetles than uninfested trees. A study of floral visitation patterns at the same site suggested that the additional sugar provided by C. wairoensis has the potential to negatively impact on pollination in kanuka stands by reducing the abundance of nocturnal floral visitors to infested trees and by provisioning the predatory Vespula wasps with sugar. A study of below-ground effects found that infestation by C. wairoensis increased throughfall C-content and litterfall N-content. No changes in soil chemistry were detected, but tree seedlings grown in soils collected from beneath infested trees developed more rapidly than those grown in soils from uninfested trees. Together these results indicate that C. wairoensis honeydew shapes community composition and processes in both direct and indirect ways. Net effects in kanuka stands are still not clear but it is likely that simple bottom-up models will not be adequate to characterise honeydew effects.