An investigation of the life history of the gorse pod moth (Cydia succedana) and its effectiveness at reducing gorse (Ulex europaeus) seed production

Abstract

Gorse (Ulex europaeus L.) has been a major weed in New Zealand and other temperate countries throughout the world for over 100 years. In 1931, Exapion ulicis (Forster) (gorse seed weevil) was introduced to New Zealand in an attempt to control the spread of gorse. However, gorse in much of New Zealand has two reproductive cycles per year and E. ulicis is active only in spring. In 1992, Cydia succedana (gorse pod moth), which has two generations per year, was introduced into New Zealand to improve the biological control of gorse.

Moths were collected from McLeans Island, Canterbury, and placed under laboratory conditions for egg and larval development experiments. Lower temperature threshold for egg and larval development of C. succedana was 11.5°C, which is similar to other species in the same genus.

The limited results obtained indicated that C. succedana has six larval instars, again similar to other insects of the same genus. Cydia succedana larvae damaged up to three gorse pods.

Moth phenology was studied at only two of the sites. At these sites, McLeans Island and Hinewai Reserve, the phenology of C. succedana was synchronised with the phenology of gorse, especially for the first flowering. However, there was less synchronicity for the second flowering. Cydia succedana was more active at the warmer site of McLeans Island had a larger population of C. succedana male moths. In addition, C. succedana was active for longer and became active earlier there.

Seven field sites were chosen in the South Island of New Zealand, with different altitudes and climatic conditions. Monthly inspections were made of the reproductive stage of the gorse and gorse pods were sampled when ripe.

Gorse at Golden Bay sites produced mature seeds throughout the study period. Sites further south had much shorter reproductive seasons with only one gorse reproductive season per year as confirmed by gorse seed collected in seed trays.

There was a wide variation in the number of seeds in the seed trays, 0 - 304 seeds/m². This represented a big reduction in the number of gorse seeds compared with previous reports. Viability of the seed from the seed trays and sample pods was tested. Seed from the seed trays had a viability of 60% whereas seed from sampled pods had a viability of 80%.

The effectiveness of both seed feeding insects at reducing the amount of viable gorse seed produced varied from site to site, with gorse seed weevil being more effective at the southern sites. At the northern sites the gorse pod moth was more effective. However, the gorse bushes still produced a significant amount of seed at all sites.

The results indicate that further gorse seed feeding biological agents could be required to assist in the biological control of gorse, especially in southern regions where C. succedana does not appear to be as successful. The implications of these results are briefly discussed.