8.1 Introduction
The primary aim of this study was to assess the individual impact of Exapion ulicis
(gorse seed weevil), Tetranychus lintearius (gorse spider mite) and Sericothrips staphylinus (gorse thrips) on the growth and development of gorse. This chapter summarises the major findings of this study and discusses the future of biological control of gorse in Australia and its role within an integrated weed management strategy.
8.2 Impact of the gorse spider mite
In a field experiment conducted over two and a half years, T. lintearius caused a reduction in dry matter production of approximately 36% (see Chapter 2), which suggests that this species has the potential to be a useful agent. However, a threat to the effectiveness of this agent is predation by natural enemies including Stethorus spp. and the Chilean predatory mite, Phytoseiulus persimilis. An experiment showed P. persimilis will develop at a similar rate on a diet of T. lintearius as it would on the two-spotted mite and that its generation time relative to its host is fast (see Chapter 3). This confirms the ability of this species to have a negative impact on T. lintearius
populations. As larger T. lintearius populations will presumably increase the damage to gorse, it is highly likely that P. persimilis will reduce the effectiveness of T. lintearius as a biological control agent of gorse.
T. lintearius attacks foliage and reduces growth on mature plants. However, the damage sustained in this study was recorded in the early stages of T. lintearius
introduction into the area and it is likely that predation will limit the impact of this agent resulting in low levels of patchy damage to gorse in the long term.
As T. lintearius populations are likely to fluctuate considerably over extended time periods, further research, such as predator and prey exclusion experiments, are needed to investigate the tri-trophic interactions between gorse, T. lintearius and its predators to determine the resulting long term impact of T. lintearius on gorse.
8.3 Impact of the gorse seed weevil
E. ulicis was found to reduce seed production of gorse at two sites in Tasmania (see Chapter 4). Due to differences in the patterns of pod production between the sites, the percentage of mature seed damaged in black pods for the whole 20 months of
sampling was 2.7 times higher at Stonehenge (45.5%) than at Lymington (16.7%). Rees and Hill (2001) determined that to reduce seed production of gorse below replacement levels, a reduction in seed production in excess of 90% would be
required. Although E. ulicis reduced seed production, the levels obtained in this study are not considered enough to have an impact on gorse populations. Therefore, an additional seed feeding biological control agent (or agents) would be required if seed feeding agents are to have any impact on gorse populations in Australia.
8.4 Impact of the gorse thrips
In a glasshouse experiment, S. staphylinus, ryegrass competition and simulated grazing individually reduced the growth of gorse seedlings. It was also demonstrated that S. staphylinus could reduce gorse seedling survival when used in conjunction with ryegrass competition and simulated grazing (see Chapter 5). If similar combinations of S. staphylinus, competition from pasture species, and grazing management also affect gorse seedling survival under field conditions, then the S. staphylinus could become an important component of an integrated management strategy, especially in the earlier stages of gorse establishment.
However, in a field environment, significant damage to gorse plants has not yet occurred five years after introduction into Australia (J. E. Ireson, pers.comm., 2006). As population build up in the field has been slow, further studies on the population ecology of this species are required.
A field study was conducted to identify potential natural enemies of S. staphylinus
within the arthropod fauna inhabiting gorse (see Chapter 6). The Phlaeothripid
Haplothrips victoriensis and mites in the family Phytoseiidae were the most abundant predatory arthropods present on gorse throughout the study and are also reported to be natural enemies of other members of the family Thripidae. Further research, such as a predator exclusion experiment, is needed to determine the impact of these predators on S. staphylinus populations.
A preliminary bioassay was conducted to determine the toxicity of herbicides and adjuvants commonly used to control gorse on adult and juvenile S. staphylinus (see Chapter 7). The herbicides triclopyr/picloram, triclopyr and glyphosate were found to
research is required to determine the toxicity of these chemicals to S. staphylinus in a field situation. These studies could investigate the timing of herbicide applications, dispersal of mobile stages from dying gorse plants and maintaining small patches of unsprayed gorse to allow reinfestation by S. staphylinus to re-established gorse. However, an assessment of the impact of S. staphylinus in a field environment should be conducted before any further recommendations regarding the use of herbicides is made.
8.5 The role of biological control within an integrated management strategy for gorse in Australia
The three agents investigated in this study all had a measurable impact on the performance of gorse and exert their impact at different stages of the gorse lifecycle (Fig. 8.1). T. lintearius attacks foliage and reduces growth on mature plants, E. ulicis
reduces seed production and S. staphylinus possibly has an impact on younger plants. Although all three agents had an impact on gorse, a measurable impact on plant performance does not necessarily translate into an impact at the plant population level (Crawley, 1989). Earlier findings now coupled with those found within this thesis suggest that additional biological control agents will be required if biological control is to be considered an important long-term component of an integrated management strategy for gorse in Australia.
Figure 8.1. Schematic representation of the lifecycle of gorse (U. europaeus) displaying the points in the lifecycle where the biological control agents established (black ovals) and not established (grey ovals) in Australia have their greatest impact. Key: Eu = Exapion ulicis,
Cs = Cydia sp., Tl = Tetranychus lintearius, Au = Agonopterix ulicetella, Ss = Sericothrips staphylinus.