The gorse pod moth (Cydia succedana Denis & Schiffermüller); is it a successful gorse (Ulex europaeus L.) biological control agent?

Abstract

Most weeds that cause issue for agricultural production in New Zealand have been imported. One such imported and well-established weed, Ulex europaeus L. (gorse), was declared a noxious weed in New Zealand in 1900. Since the 1950s, much of the control of this weed has been through herbicide application. However, as the cost of chemicals has risen, along with public concerns about the environmental sustainability of chemical use, there has been an increasing demand for non-chemical control methods. Beginning in the 1980s, a concerted effort was made to find biological control agents for U. europaeus suitable for New Zealand’s conditions. One of those agents, Cydia succedana (Denis & Schiffermüller) (gorse pod moth), was released in New Zealand in 1992. This agent feeds on green U. europaeus seeds and, as it has two reproductive cycles each season, it was thought that it would assist in controlling U. europaeus by reducing seed production. This study investigated the effectiveness of this agent in central New Zealand. Nine South Island sites from Canterbury, Nelson and Marlborough and six North Island sites from Wellington, Manawatu, Taranaki, Hawkes Bay and Bay of Plenty, with differences in climatic conditions and altitude, were used for this research. Aspects investigated included the phenology of U. europaeus, phenology of C. succedana, host-specificity of C. succedana, levels of U. europaeus seed damaged or destroyed by C. succedana and determination of Cydia species attacking gorse in New Zealand. At sites just south of Christchurch, plus Murchison and Conway Flat, Kaikoura U. europaeus only had one reproductive cycle per season. At all other sites there were two reproductive cycles per season. In non-damaged pods, the seed viability averaged 90% over all sites, and there was an overall mean germination of 80%. Phenology of C. succedana was determined by pheromone trapping of male moths at all the sites. C. succedana was bivoltine at all sites, even at sites where U. europaeus was limited to one reproductive cycle. Synchronisation between U. europaeus and C. succedana was poor at most sites, as the peak trapping occurred 2-3 months after the peak of flowering. The host specificity of C. succedana was tested using U. europaeus, Cytisus scoparius L. (Scotch broom), Carmichaelia petriei Kirk (Desert broom), Sophora molloyi Heenan et de Lange (Cook Strait kowhai or Dragons Gold) and Lupinus polyphyllus Lindl. (Russell lupin) Only flowers of U. europaeus were entered by C. succedana in a field cage trial. A small number of Lupinus arboreus Sims pods (2/450) were entered by larvae at one site, but their survival until pupation was not known. Pod damage as a result of C. succedana attack varied markedly both within and among sites, ranging from 3% to 55% of all pods produced over one year, with an overall mean of 21%. The seed damage ranged from 1% to 65%, with an overall mean of 27%, although most intact seeds within damaged pods were also dead. At the sites studied, C. succedana was not able to destroy or damage sufficient seeds to have a significant impact on control of this weed. A small region of the mitochondrial DNA was sequenced from samples of larvae, obtained from mature U. europaeus pods from the sites, in order to determine if there was more than one Cydia species attacking gorse in New Zealand. C. succedana was the only Cydia species present. For biological control of U. europaeus in New Zealand to succeed, additional biological agents will be required, especially in the South Island, where C. succedana was less successful than in the North Island. The implications of these results are discussed.