Control Mite
2.4 Discussion
The significant reduction in dry matter production caused by T. lintearius in this study indicates that this species has the potential to be a useful agent. Preliminary studies by others on the impact of T. lintearius were conducted over different time frames, under different environmental conditions using different assessments of growth. Despite these differences, studies in New Zealand (Fowler and Griffin, 1995; T.R. Partridge unpublished data cited by Richardson and Hill 1998; Landcare Research, unpublished data) and in Oregon, USA (Rice, 2004) also found that T. lintearius can cause
significant reductions in growth. In this study, the measured reduction in growth of 36% on the T. lintearius treated plants after two and a half years could have been greater (ie. approximately 44%) if it had been possible to prevent T. lintearius damage to the controls.
Although the death of gorse plants attributed to T. lintearius attack was observed by Zwolfer (1963), T. lintearius did not cause the death of any gorse plants in this study. However, the measured growth reduction of 36% (or greater) could benefit attempts to re-establish a gorse infested area with more desirable plant species. Pasture species can compete strongly with gorse seedlings (Ivens, 1979; Ivens and Mlowe, 1980) so pasture competition could be an important component of an integrated control
strategy incorporating biological control agents. For instance, pasture competition was shown to reduce both the survival and shoot dry weight of gorse seedlings when combined with another biocontrol agent of gorse, Sericothrips staphylinus (Haliday) (Thysanoptera: Thripidae), in a glasshouse experiment (Davies et al., 2005, see Chapter 5).
Herbivory and competition are two of the main forces shaping the composition of plant communities (del-Val and Crawley, 2005). It is widely accepted that even small reductions in growth of one plant species can result in major shifts in the competitive balance of plant communities (Crawley, 1989). Therefore the reduction in growth caused by T. lintearius may reduce the competitive ability of gorse and be beneficial to more desirable plant species by enabling them to compete with gorse more
effectively. Furthermore, the feeding of T. lintearius and other biological control agents could have an effect on the longevity of gorse, which can exist in some plant stands for up to 33 years (Richardson and Hill, 1998).
Although T. lintearius had no apparent impact on pod production such an effect may take a more extended time period to develop. The effect of T. lintearius on flowering was not clear in this study but flower production was enormously variable between plants irrespective of treatment. In spring 2002, 18 months after T. lintearius
introduction there was a increase in flowering due to T. lintearius. However, in spring 2003, 30 months after introduction, the mean numbers of flowers on the T. lintearius
treated plants was not significantly different to the control plants. Furthermore, there was no correlation between T. lintearius damage levels and either flower or pod production in 2003. The allocation of resources within a plant can be dramatically altered following herbivore damage, including either decreasing or increasing the production of flowers or fruit (Trumble et al., 1993). However, little evidence either
This study assessed the impact of T. lintearius on gorse during the first 2.5 years following its introduction to a site. However, on an ecological time scale, this is a relatively short time period. T. lintearius populations and the subsequent damage to gorse may fluctuate considerably over more extended time periods. Tetranychid mite populations are notoriously patchy in time and space due to ecological factors such as temperature, exposure to rainfall, host plant quality and natural enemies (Sabelis, 1985). This patchy distribution of T. lintearius populations and the subsequent localised damage to gorse has been observed at Tasmanian field sites (J. E. Ireson, unpublished data). Following release and initial establishment, T. lintearius can cause severe damage to gorse in localised areas. Eventually, however, numbers start to decrease probably as a result of predation or migration triggered by the presence of predators, colony size and the decline in food quality (Ireson et al., 2004).
Determination of the impact of the predators' Stethorus sp. and P. persimilis, which were recorded eight and ten months into the study respectively, and were active on T. lintearius colonies throughout the site, was beyond the scope of this study. However, a predator exclusion study in the USA (Pratt et al., 2003) found that predation by P. persimilis significantly reduced the volume and number of T. lintearius colonies. It is therefore probable that predation would ultimately have caused a significant decline in the numbers and effectiveness of T. lintearius at the Bothwell site after the completion of the experiment. Studies in Victoria and Tasmania have shown that predation of T. lintearius by Stethorus sp. and P. persimilis is already widespread (Ireson et al., 2003) and probably a key factor in restricting its usefulness as a
biological control agent. Furthermore, a controlled temperature development study in Tasmania as part of this investigation (Davies et al., 2004, see Chapter 3), found that
lintearius. Interestingly, although Stethorus sp. has been recorded in gorse infestations in the high rainfall areas of the West Coast and parts of the North West of Tasmania,
P. persimilis has not (J. E. Ireson, pers. comm.). However, in these locations, high rainfall may limit the establishment of T. lintearius populations more than predation (Ireson et. al., 2003).
Although the results from this study suggest that T. lintearius did not negatively interfere with the seed feeding agent E. ulicis, such an interference may only occur in certain circumstances. For instance, E. ulicis only oviposits into young green gorse pods in spring (Hill et al., 1991), but in this study T. lintearius activity peaked in late summer and autumn. It is possible that a mild, dry winter could allow a build up of larger numbers of T. lintearius, resulting in extensive webbing on the plants during the spring and early summer thus coinciding with gorse flowering. Perhaps an interference effect could be observed under these circumstances.
Although predation and other factors may be restricting the efficacy of T. lintearius, its usefulness must be considered as part of the overall biological control program for gorse involving established and additional biological control agents. If T. lintearius
can reduce the growth rate of gorse while not interfering with other agents, then it could be a useful component of a guild of agents that attack gorse at different stages of its lifecycle. If these agents are successful at reducing the vigour and