Effects of natural enemy exclusion in the native and

With regard to our first hypothesis that exclusion of natural enemies in the native range would have a positive effect on plant growth, we found mixed evidence. Natural enemy exclusion had no effect on the performance of shoot height and relative growth rate of shoot height, but in some cases did have a positive effect on plant population growth and shoot development. Population growth remained positive in the native range where natural enemies were excluded, but went negative in the control where an ambient level of natural enemy pressure was present. However, not all exclusion treatments were always significantly greater than the control, and there was no consistent pattern in which enemy exclusion treatment had a greater affect on population growth compared to the control. With regard to shoot development, only exclusion of insect herbivory had a positive affect on transitions from the vegetative to budding and budding to reproductive growth stages. This effect on shoot development is also supported by the visual damage ratings, which indicated significantly reduced herbivory on insecticide treated plots in 2007. In 2008 there were no significant differences among the treatments in the visual ratings of herbivory, which corresponds well with the reduced impact on shoot transitions in that year. Therefore, studies from the native range indicate that population growth of C. arvense benefits from natural enemy release, and individual shoot development benefits more from insect herbivore release compared to release from pathogens. Our prediction that dual exclusion of herbivores and pathogens would have an increased positive effect was not supported.

In the native range the population growth of C. arvense was followed over two years. It has been hypothesised that reducing shoot biomass will cause a reduction in root biomass, and thereby reduce the number of shoot emerging the following season (Donald

1993; Bourdôt et al. 1998). Thus, we might have expected the total number of shoots to decrease where natural enemies were present, and possibly to increase where natural enemies were excluced, from one year to the next. However, there was no evidence of a year effect on the number of shoots in any of the treatments. It is possible that the ambient natural enemy pressure was not sufficient to reduce shoot biomass enough to cause a reduction in the overwintering root biomass. Precisely how much natural enemy pressure is necessary to cause an impact root biomass uncertain. Therefore, in the second year we conducted additional experiments with the biocontrol agent, Cassida rubiginosa, to test if realistic outbreak densities of this beetle might have an impact on C. arvense (Chapter 6).

In accordance with our second hypothesis that natural enemy exclusion would not affect plant growth in NZ we found no evidence that exclusion of insect herbivores or pathogens had an affect on plant growth, development or population growth, relative to controls. In the exotic range of a plant, we might expect natural enemies to have an impact if: (i) there are host shifts by native insects or pathogens, or (ii) if generalist insects and pathogens were able to utilize the plant, or (iii) if introduced biological control agents have established. The fact that insect exclusion had no effect is not surprising since it has been well documented that both specialist and generalist insect herbivory is almost completely absent on C. arvense in NZ (Spiller & Wise 1982; Fenner & Lee 2001; Chapter 3). However, in terms of pathogens, both specialist and generalist attack on C. arvense is present in NZ. A recent comparative survey showed that the specialised rust fungus, P. punctiformis, occurs at similar levels in NZ and Europe (Chapter 4; Cripps et al. 2009); and several generalist pathogens are also known to attack C. arvense in NZ (Cunningham 1927; Pennycook 1989; Waipara et al. 1993; Bithell & Stewart 2001). Shoots infected with P. punctiformis usually die before flowering (Watson & Keogh 1980; Thomas et al. 1994), but even artificial manipulation with this pathogen has failed to have population level impacts on C. arvense (Kluth et al. 2003). Inundative treatment with the generalist pathogen Sclerotinia sclerotiorum (Lib) de Bary has been shown to have detrimental effects on the population growth of C. arvense in NZ (Bourdôt et al. 1995; Bourdôt et al. 2006), which suggest that under some circumstances (e.g. epidemic conditions) generalist pathogens might be able to have a regulating influence on the demography of C. arvense. Nevertheless, in this study, the exclusion of pathogens with the use of fungicides had no affect on C. arvense in NZ, which might point towards a

greater regulating influence of herbivores, since natural enemies did have an effect in the native range.

Similar to our study, DeWalt et al. (2004) investigated the affects of natural enemies on Clidemia hirta in its native and introduced ranges with the use of pesticide exclosure experiments. DeWalt et al. (2004) did not investigate the impact of natural enemies on population growth of C. hirta, but did find that exclusion of natural enemies had a positive effect on growth and survival in its native range, but no effect in its introduced range, thus providing supportive evidence for the ERH. They found that insecticide or fungicide applied alone had a positive affect on C. hirta growth and survival, and that the dual application of both had an additive positive affect on plant growth and survival. Similarly, we also found that exclusion of natural enemies benefited C. arvense shoot development in the native range, but in contrast, this was only evident with insecticide treatment, and not with the fungicide or dual treatment.

Probably the most extensively studied invasive weed in its native and exotic ranges is broom, Cytisus scoparius (Fowler et al. 1996; Paynter et al. 1998; Memmott et al. 2000; Paynter et al. 2000; Sheppard et al. 2002; Paynter et al. 2003). In the case of broom, there was no clear signal that natural enemies were a key population regulating factor for the species in the native range, but their influence could not be ruled out (Paynter et al. 2000; Paynter et al. 2003). Exclusion of natural enemies in its native range with the use of pesticides had no effect on the survival, growth, reproductive age, or number of seeds produced per pod; but rather disturbance appeared to be the most important factor contributing to the increase in these traits of broom in its native range (Paynter et al. 1998). However, earlier research in the native range of broom showed that long-term exclusion of natural enemies had significant affects on plant survival, size and vigour (Waloff & Richards 1977), which in theory should translate into reduced population growth of broom (Rees & Paynter 1997). In the introduced range of NZ an insect exclusion study with broom indicated that insect herbivores had a significant impact on broom growth and both native insects and introduced biological control agents were implicated in the effect (Syrett et al. 1999).