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Classical biocontrol systems are sometimes treated as an exercise in community assembly. As such, they include multiple species interactions. This thesis explores multi-species aspects in classical weed biocontrol, using thistles as a case study. The abundance, phenology and impact of three biocontrol agents were followed on their target host, Carduus nutans L. and are described, for the first time in New Zealand for two of them (Urophora solstitialis L. and Trichosirocalus horridus sensu (Panzer)). Composition in New Zealand of the recently revised Trichosirocalus weevil species complex was surveyed nation-wide. One species only was found, albeit exhibiting a wider host range than anticipated from the published revision. Interspecific interactions and individual and combined effect of multiple biocontrol agents on C. nutans were tested in cage setups; the effect on the weed population was then estimated by manipulations of an existing matrix population model for this weed in New Zealand. The potentially better seed predator (U. solstitialis) was outcompeted by the worse seed predator (Rhinocyllus conicus (Froehlich)) which has similar niche preference. Urophora solstitialis was also adversely impacted by the crown-root feeder (T. horridus). Trichosirocalus horridus affected C. nutans survival, even at the medium density used, and significantly reduced potential seed production by 33%; in field densities, T. horridus is likely to affect C. nutans even more. Urophora solstitialis was estimated to destroy about 28% of the remaining seed in the absence of the other agents, and about 17% in the presence of T. horridus. The estimated combined effect of T. horridus and U. solstitalis on C. nutans population growth rate was greater than the effect of either agent alone. In the face of growing weed invasions, multiple thistle species were used to test ‘multi-targeting’ as a novel approach to target groups of ‘sleeper weeds’. Both in a field experiment and in a field survey, the seed predator R. conicus was found to attack and damage some ‘non-target’ thistle species more in the presence of the target species (C. nutans) than in its absence; however, levels of attack on non-target species were always modest. The ultimate goal of biocontrol is to reduce weed populations. A field survey revealed that current population densities of multiple thistle species in Canterbury are not obviously lower than in the mid 1980s, when only R. conicus was present. This may be because successful biocontrol has reduced the management input required to maintain the same thistle density.

Classical biocontrol systems are sometimes treated as an exercise in community assembly. As such, they include multiple species interactions. This thesis explores multi-species aspects in classical weed biocontrol, using thistles as a case study. The abundance, phenology and impact of three biocontrol agents were followed on their target host, Carduus nutans L. and are described, for the first time in New Zealand for two of them (Urophora solstitialis L. and Trichosirocalus horridus sensu (Panzer)). Composition in New Zealand of the recently revised Trichosirocalus weevil species complex was surveyed nation-wide. One species only was found, albeit exhibiting a wider host range than anticipated from the published revision. Interspecific interactions and individual and combined effect of multiple biocontrol agents on C. nutans were tested in cage setups; the effect on the weed population was then estimated by manipulations of an existing matrix population model for this weed in New Zealand. The potentially better seed predator (U. solstitialis) was outcompeted by the worse seed predator (Rhinocyllus conicus (Froehlich)) which has similar niche preference. Urophora solstitialis was also adversely impacted by the crown-root feeder (T. horridus). Trichosirocalus horridus affected C. nutans survival, even at the medium density used, and significantly reduced potential seed production by 33%; in field densities, T. horridus is likely to affect C. nutans even more. Urophora solstitialis was estimated to destroy about 28% of the remaining seed in the absence of the other agents, and about 17% in the presence of T. horridus. The estimated combined effect of T. horridus and U. solstitalis on C. nutans population growth rate was greater than the effect of either agent alone. In the face of growing weed invasions, multiple thistle species were used to test ‘multi-targeting’ as a novel approach to target groups of ‘sleeper weeds’. Both in a field experiment and in a field survey, the seed predator R. conicus was found to attack and damage some ‘non-target’ thistle species more in the presence of the target species (C. nutans) than in its absence; however, levels of attack on non-target species were always modest. The ultimate goal of biocontrol is to reduce weed populations. A field survey revealed that current population densities of multiple thistle species in Canterbury are not obviously lower than in the mid 1980s, when only R. conicus was present. This may be because successful biocontrol has reduced the management input required to maintain the same thistle density.

Thesis--Virginia Polytechnic Institute and State University. / Also available via the Internet.

A computer simulation model written in PL/I of the flowering phenology of C. thoermeri and the seasonal population dynamics of R. conicus was constructed based on a synthesis of the literature, and on laboratory experiments which determined developmental periods, mortality for immature stages and the ovipositional pattern of R. conicus. The model successfully simulated the ovipositional pattern of R. conicus and the flowering phenology of C. thoermeri. Larval population dynamics of R. conicus, in the model however, did not fit the validation data. Though the model was temporally synchronized with the field data, the model results were one degree of magnitude less. The poor fit was attributed to sampling errors in the validation data. Studies of the overwintering mortalities of both organisms were performed. R. conicus was found to overwinter with lowest mortality on musk thistle rosettes; mortality was 73.6% in 1981, higher than previously reported but possibly more representative of field conditions. No overwintering mortality was observed for established rosettes of C. thoermeri. Studies of seed dispersal of C. thoermeri were performed to determine the percentage of seeds removed from the vicinity of the plant where they originated. Seed dispersal observed for different wind velocities demonstrated that less than 1% of seeds are blown further than 100m and most seeds are deposited within 50m of the release site. A Gaussian model of seed dispersal based on experimentally determined parameters and literature values corroborated these results. / Master of Science

Musk thistle (Carduus nutans) seed production from early developing (terminal and first lateral) heads was reduced by 10% in 1973 and 75% in 1974 due to larval feeding of Rhinocyllus conicus. This was caused by heavy infestation of the early heads (45% in 1973, 70% in 1974) with a subsequent 5-fold increase in the number of weevil pupation chambers in these heads. Total seed production was reduced by 35-36% in both years despite increased plant vigor due to better growing conditions in 1974. Larval feeding reduced viability of mature musk thistle seeds. Plumeless thistle (C. acanthoides) seed production from early heads decreased only 5% in 1973 and 4% in 1974 due to low rates of infestation (9% in 1973, 5% in 1974). Total seed production was reduced 0.2% in 1973 and 0.1% in 1974. Weevil larvae developing in plumeless thistle heads were more heavily parasitized than those in musk thistles. Parasitoids suffered high mortality due to entrapment in weevil pupation chambers (Aliolus curculionis, 837; Bracon mellitor, 10%), and did not inhibit weevil population buildup. These combined factors resulted in successful control of musk thistle at 1 Virginia site. R. conicus showed ovipositional preference for C. nutans over C. acanthoides heads both in the laboratory and under field conditions. Oviposition of an established weevil colony was better synchronized with musk than with plumeless thistles at a site where both thistles were present. Sequentially released, overwintered R. conicus were better synchronized with Carduus thistle development because of an extended ovipositional period, but the weevils suffered reduced fecundity. Ovipositional preference was for the earlier stages of musk thistle heads. Intermediate stages of plumeless thistle heads were preferred, and larval survival was reduced because of insufficient time for weevil development and subsequent depletion of food supply. Musk thistles subjected to mechanical and R. conicus-induced damage to terminal heads were shorter than control plants and suffered reduced seed production from terminal heads. Total head and seed production per thistle were not significantly reduced. Emerging adult weevils decreased in weight relative to crowding; no difference was detected between sexes (female, 13.4mg; male, 13.8mg). / Ph. D.

Master of Science