Punto de partida para entender las Representaciones Sociales.

Figure I . Logistic regression o f the risk of parasitism by T. brevifacies dependent on host location. Sampling transects started 5 m outside the forest boundary (denoted by the upper l imit line) and extended into the forest centre. Dashed lines are 95 % confidence bands.

DISCUSSION

The arrangement of habitat structures may limit parasitoid dispersal, and consequently, patterns of parasitism on hosts. Patterns of natural enemy distribution and parasitism have been linked to both host distribution and the spatial structure ofhabitats (yvaage, 1 983; Kruess, & Tscharntke, 1 994; Roland & Taylor, 1 997). Dispersal of other flying insects is also limited by the structure of habitats. Hedgerows were found to restrict the dispersal of the hoverfly Melanostomajasciatum (Macquart) (Diptera: Syrphidae) between fields (S.Wratten, pers. comm., 1 999). The present study determined that host refuges from T brevifacies parasitism exist at the centre of native forest patches.

Host location and the probability of parasitism

Trigonospila brevifacies attacked on average 8 % of the C. obliquana trap hosts placed at the three forest sites. However, the overall level of T brevifacies parasitism on non-target hosts may be less significant when determining undesirable impacts on non-target hosts, than the distribution of parasitism through forest patches. A clear distance effect emerged of declining parasitism on trap hosts along transects from forest edges to centres, and this concurs with patterns of adult T brevifacies distribution on sticky traps comparing edge and centre samples in similar forest patches (Chapter 6).

Data modeled in the present study confirm that hosts occurring on forest edges have a

significantly higher probability of parasitism by T brevifacies, than hosts located in other forest

zones. Therefore, not all non-target hosts within a forest patch are equally exposed to the same level of parasitism, and forest centres provide a host refuge to some individuals ofthe host population.

Factors influencing T. brevifacies distribution

Several factors have been found to determine the patterns of distribution and parasitism observed in tachinid species including host abundance (Roland & Taylor, 1 997), limitations on dispersal imposed by the structure of habitats (Roland & Taylor, 1 997) and preference of rnicrohabitat and certain types of plant architecture (yveseloh, 1 98 1 ). Two ofthese factors appear dominant in determining the distribution of T brevifacies parasitism in forest patches. There is some support for plant architecture and rnicrohabitat determining the distribution of lekking T brevifacies males along forest edges (Chapter 6). However, this is unlikely to limit the distribution of ovipositing females (which live for 30 days) to forest edges once mating has

taken place. The two most likely explanations for the observed patterns of T. brevifacies

parasitism are response to host density and/or limitations of habitat structure.

Data comparing the distribution of larval tortricid hosts, adult T. brevifacies and T.

brevifacies parasitism within small forest patches shows that parasitoid and host abundances were both higher on forest edges (Chapter 6).

However, the forest edge may also provide conditions preferred by T. brevifacies as well as a richer source of hosts. Forest edges in broadleaflpodocarp forests have different rnicroclirnates (measured as photosynthetically active radiation, air temperature and vapour pressure deficit) to forest centres (Young & Mitchell, 1994) and a particular rnicroclirnate has been demonstrated to be preferred by some parasitoids (Weseloh, 1 972). Forest structure may also prevent movement of T brevifacies further into forests. Recently, Roland and Taylor

( 1 997) advanced the hypothesis of habitat structure limiting parasitoid movement when they found evidence of Tachinidae occurring in specific habitat types (fragmented forest and edge zones) with inverse density dependence on host abundance in North American forests.

T. brevifacies

Roberts ( 1 986) speculated that T. brevifacies is responsible for a suspected decline in the abundance of Planotortrix avicenniae Dugdale (Lepidoptera: Tortricidae) in mangroves around Auckland. The present research suggests that species occupying habitats without continuous canopy, such as P. avicenniae in mangrove swamp, may be subject to a more consistent risk of parasitism throughout the population, than the edge-concentrated parasitism of host populations in the broadleaflpodocarp forests.

The mechanism that concentrates T. brevifacies parasitism on forest edges may either be in response to host density or habitat structure. Both parasitoids and hosts were more abundant on forest edges. But the ratio of hosts to parasitoids was similar between edges and centres of forests, indicating that higher parasitoid abundance at the forest edge could be related to it's host abundance (Chapter 6). However, when trap hosts were offered for parasitism at a constant density along edge to centre transects at forests in the present study, parasitism was highest at the forest edge. This implies that habitat structure rather than host density may determine where T. brevifacies parasitism occurs in forests or that the distribution of the wild host population concentrated T. brevifacies parasitism at forest edges in the sites studied.

In conclusion, this work has clarified the implications of T brevifacies for non-target

hosts in the native forest habitats it has invaded. Trigonospila brevifacies has been shown to

parasitise significantly more hosts on forest edges. The model also predicts areas of continuous canopy found within forest patches provide a form of host refuge. Survey data of host and parasitoid distribution patterns indicate that both occurred at higher densities on edge zones in broadleaflpodocarp forests (Chapter 6). This finding also implies that non-target Lepidoptera occurring in habitats with similar habitat structure to edge zones, such as shrubland, have a

higher probability of parasitism by T brevifacies across the entire population as the refugia

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New Zealand Vol.

Eutorna phaulocosma Meyrick ( Lepidoptera: