Impacts of a weed biocontrol agent on recovery from water stress in a target and a non-target Hypericum species

Summary

Study 1

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. Johnswort Hypericum gramineum. Field experiments were undertaken to explore A.hyperici colonization of these plants in sympatric populations, and (see Case 632) to determine the impact of the mite on H.gramineum growth.

Host colonization study sites: The probability of the mite Aculus hyperici infesting grassy St. Johns wort Hypericum gramineum plants was investigated at three field sites. Two sites, separated from each other by 2 km, were situated in north-eastern Victoria, Australia (36°23'S, 146°44'E), beneath a Eucalyptus woodland canopy. The third site was in a 1 ha clearing adjacent to a Monterey pine Pinus radiata plantation in the Australian Capital Territory (35°20'S, 148°55'E). The understorey of each site was dominated by H.perforatum, interspersed with H.gramineum and several other native forbs and grasses. A survey prior to the experiment confirmed that A.hyperici was not present.

Mite introduction: One hundred vegetative H.perforatum buds, each infested with about 10 adult mites, were used to inoculate a release plot (3 × 3 m) at each site in winter (July) 1992. Within a radius of up to 50 m from the release plot, 30 H.gramineum plants were selected randomly and paired with the nearest H.perforatum. If no H.perforatum occurred within 30 cm of the selected H.gramineum, it was discarded in favour of another.

Monitoring: In March 1993, after the main mite dispersal period in spring/summer, the plant pairs were scored for mite presence. Plants were rated as 'infested' if one or more mites were detected on any of the shoots. Occasionally, live mites were not detected but characteristic cast skins were present. These plants were also scored as infested. Infestation severity was rated from 1 to 4 (no mites present to heavy infestation). Where no live mites were found, the infestation rating was based on the density of cast skins. It was assumed that rating the infestation in this way reflected total mite abundance and did not bias for plant size.

Mite colonization: The probability of H.gramineum being colonized by A.hyperici was significantly less (31% chance of infestation) than the probability of H.perforatum being colonized (51% chance). Study site did not affect the likelihood of colonization. There was a significant difference between the average infestation ratings on H.perforatum and H.gramineum, with infestation levels on the target (2.38 ± 0.12) being more than double those on the non-target native (0.98 ± 0.12).

Conclusions: The probability of A.hyperici colonizing grassy St John's wort H.gramineum was significantly lower than the probability of it colonizing the target invasive, perforate St John's wort H.perforatum.

A further field study was undertaken to look at the consequences of mite infestation on H.gramineum growth (see Case 632).

Study 2

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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