Establishment of a beneficial invader, broom twig miner (Leucoptera spartifoliella Hübner ), on broom (Cytisus scoparius (L.) link) in New Zealand

Abstract

Broom twig miner, Leucoptera spartifoliella Hübner (Lepidoptera: Lyonetiidae), an accidentally introduced species, shows considerable potential as a biological control agent for broom (Cytisus scoparius (L.) Link) in New Zealand. Since its arrival, this species has successfully colonised and dispersed throughout the country. In contrast to deliberately released biological control agents, nothing specific is known about the individuals that originally arrived in New Zealand. This study has employed molecular techniques towards determining the geographic origin of New Zealand populations of broom twig miner, and understanding the genetic effects following colonisation. In addition, fecundity and dispersal were measured as indicators of the biotic performance of this species as an example of a successful coloniser (or invader).

Amplified fragment length polymorphism (AFLP) analysis of populations from the native range of broom twig miner (western Europe) showed low differentiation, which implies widespread gene flow. Most of the genetic variance (96%) was accounted for by differences within rather than among populations. Differentiation among populations was, however, statistically significant and this is reflected in the presence of private fragments in each of the populations sampled. Some genetic separation of populations is also indicated by the presence of unique mitochondrial DNA (mtDNA) haplotypes in populations in the native range. These data suggest that although populations may have some genetic separation through both distance and isolation, there is probably sufficient migration to maintain gene flow.

Some colonisation effects on genetic diversity can be seen in the genetic data from New Zealand broom twig miner when they are compared with data from individuals from the native range. There is low mtDNA haplotype diversity in New Zealand populations with neither of the geographically and numerical1y common haplotypes appearing from the native range. The AFLP study also showed some loss of genetic diversity in New Zealand populations compared with those from the native range. However, this could largely be attributed to the loss of less frequent alleles. When using the measures of proportion polymorphism (P) and average heterozygosity (H), the New Zealand populations in this study showed very similar levels of diversity to those from broom twig miner's native range. Therefore, despite that the data suggest that the genetic profile of current populations has been influenced by colonisation events; the New Zealand populations still show considerable genetic diversity.

Mitochondrial DNA data suggest that there was a single founder event for the introduction of broom twig miner into New Zealand. The data also suggest that the propagule size was small. Although no matches for the New Zealand mtDNA haplotypes were found in any of the other populations surveyed, the New Zealand haplotypes were most similar to a haplotype detected in a UK population (Keighley). The exact geographic origin of the New Zealand populations was not established, but the work in this thesis provides starting point for future studies. The original hypothesis, from the literature, that broom twig miner arrived in New Zealand on ornamental cultivars of broom imported from North America seems less likely than previously.

Fecundity studies, and estimates of per capita rate of increase (r) show that broom twig miner populations have the capacity to increase very rapidly in New Zealand conditions. Therefore, any bottlenecks that might occur through colonisation events are likely to be short resulting in little loss of genetic variation, especially heterogeneity. Both inter- and intra-population variability were noted for fecundity parameters. Differences in fecundity, in particular, were explained largely by adult female weight. Such differences, whether caused by environmental or genetic factors, would result in different population dynamics at different locations. Consequently, the rate of spread and gene flow will vary in different areas.

The estimate of the asymptotic velocity of dispersal (around 20 km/yr), from observations of changes in the front of its southern distribution, reinforces observations that broom twig miner is a good disperser. It would still appear, however, that dispersal through New Zealand involved more than one focal point. It is likely that dispersal has involved both a combination of diffusion through short range flitting and migratory flight, and longer distance jump dispersal through wind-assisted flight and the transfer of host material infested with immature life stages of the moth.

Broom twig miner has overcome both environmental and demographic stochasticity, the greatest threats to its establishment, and has proved an effective biological control agent over a wide range of environmental conditions throughout New Zealand. This occurred despite that the initial colonisers were likely to be of small propagule size(s).

The results presented in this thesis have implications for the selection of populations of new biological control agents. If there is low differentiation among populations of a species through its native range, it may not be critical where specimens for introduction to a new environment are sourced. With the numbers that are normally imported and released, it is likely that a large representation of the true genetic variability in the native range could be picked up from a single or a few populations. Presumably this degree of variability would include the most common and widely successful attributes of the species.