Individual study: Impact of the mite Aculus hyperici released as a biocontrol of perforate St John's wort Hypericum perforatum, on growth of non-target grassy St. Johns-wort Hypericum gramineum, Mt Ainslie, Smith's Paddock and Pierce's Creek, New South Wales, Australia
Willis A.J., Berentson P.R. & Ash J.E. (2003) Impacts of a weed biocontrol agent on recovery from water stress in a target and a non-target Hypericum species. Journal of Applied Ecology, 40, 320-333
A mite Aculus hyperici (Acari: Eriophyidae) was introduced into Australia to help control the invasive weed, perforate St John's wort Hypericum perforatum, despite indications from pre-release trials that this mite could survive and reproduce on a non-target native species, grassy St. Johns wort Hypericum gramineum. Field experiments were undertaken to explore A.hyperici colonization of these plants in sympatric populations (see Case 631), and to determine the impact of the mite on the growth H. gramineum as summarised here.
Study sites: The impact of Aculus hyperici mites on H.gramineum growth was determined at three sites in the Australian Capital Territory (Mt Ainslie, Smith's Paddock and Pierce's Creek) over 8 months commencing in winter (July) 1992. At each site 40 H.gramineum plants were selected. Due to lack of resources to exclude mites from control replicates during the experiment, the number of cross-contaminations was minimised by selecting plants no closer than 15 m to any adjacent H.gramineum or H.perforatum individual. Plant size was standardized using individuals comprising 3–4 shoots of 1–2 cm length. Twenty plants were allocated randomly for infestation with mites, 20 were retained as mite-free controls.
Mite inoculation: Inoculation of St John's wort plants was achieved by tying with cotton thread to each shoot, one H.perforatum bud infested with about 10 mites (c. 40 adult mites per plant). At the end of the trial, a 40× dissecting microscope was used to scan plants for evidence of A.hyperici. Infested controls (≥ 1 mite or cast skin) and treated plants that had not become infested were excluded from analyses. Pooled across sites, there were a total of 40 infested individuals and 43 mite-free controls.
In early autumn (March) 1993, plants were relocated and the number of flowering shoots, total number of shoots, average shoot height (cm) and the number of fruits scored. Plants were ranked from 1 to 5 for damage caused by grazing vertebrate herbivores (1 = no apparent damage to 5 = severe damage) before harvesting the shoots at the soil surface and oven-drying at 60 °C for 5 days to determine total shoot mass (g).
St John's wort growth: Mites had no significant impact on any index of H.gramineum growth (i.e. shoot mass, height, number of flowering shoots, number of fruit (see Table 1, attached). In fact, there was a tendency towards slight increases in these variables associated with A.hyperici presence.
Conclusions: Despite the ability of A.hyperici to infest H. gramineum, the mite had negligible impacts on all measured indices of growth and reproduction.
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