Resprouting plants provide an interesting test to the generality of plant allometric relationships. The ability to rapidly resprout after disturbance also makes weeds more difficult to control. We performed a glasshouse experiment to investigate regrowth of an invasive plant (alligator weed, Alternanthera philoxeroides (Martius) Grisebach) after an experimental mowing treatment. Nutrient levels and biomass accumulation were measured weekly for five weeks, and carbohydrate levels were measured when regrowth was quickest. After five weeks, the biomass of treatment plants was similar to that expected from a growth curve fitted to undamaged control plants. Treatment plants, however, had a higher below-ground biomass, and a higher ratio of stem to leaf biomass than expected. The regrowing material also had a lower nutrient concentration. Both the rapid regrowth and the change in morphology make the mechanical control of alligator weed more difficult, and may, in part, be responsible for the invasiveness of this plant.

Weeds of Australian rangelands have been the target for biological control since 1914. There have been several spectacular successes, e.g. prickly pear cactus (Opuntia spp.), with extraordinary ecological and commercial benefits. There have also been projects where no positive outcome has occurred, e.g. prickly acacia [Acacia nilotica (L.) Willd. ex Delile ssp. indica (Benth.) Brenan], parkinsonia (Parkinsonia aculeata L.), and creeping lantana [Lantana montevidensis (Spreng.) Briq.]. Numerous other projects are currently underway and require continuing support to achieve the stated aims. There is considerable potential to increase investment in biological control of rangeland weeds to improve outcomes of current projects and to implement new projects. There is promise of excellent benefits to cost ratios based on past performance. However, there is a worrying trend of loss of capacity in Australia to conduct biological control. Since resources will always be limited, it is important that the weeds that have the greatest impact on ecosystem function and commercial production are identified and targeted for management. Biological control should be part of the management strategies for those priority weeds when conflict over weediness v. commercial value can be resolved in favour of biological control, and when it is considered that biological control could offer benefits.

Weeds are serious threats to Australia's primary production and biodiversity conservation. For example, a recent Australia Bureau of Statistics survey found that 47% of farmers across Australia have a significant weed problem. A literature review revealed that legumes represent a significant proportion of the national weed problem and most serious Australian legume weeds are exotic thicket-forming species that were deliberately introduced for their perceived beneficial properties, such as for shade and fodder, or even quite trivial reasons, such as garden ornamentals. The low economic value of the rangelands most of these species infest, compared with control costs, hinders chemical and mechanical control of these weeds, such that biological control, which takes time, is expensive to implement and has no guarantee of success, may represent the only economically viable alternative to abandoning vast tracts of land. We argue that, because the behaviour of an introduced species in a novel environment is so hard to forecast, better predictive techniques should be developed prior to further introductions of plant species into novel environments. We also discuss the potential of legumes currently being promoted in Australia to become weeds and suggest the recent trend of exporting Australian Acacia spp. to semiarid regions of Africa risks history repeating itself and the development of new weed problems that mirror those posed by Australian Acacia spp. in southern Africa.

Secondary side branching in Salvinia molesta plants grown in nutrient solution was dependent on the availability of nitrogen. The compensatory response of S. molesta to damage by complete manual disbudding was the production of extensive secondary side branching at nitrogen levels below those required for this type of branching in undamaged plants. After 28 days damaged plants had the same number of ramets as undamaged plants but the dry weight was considerably reduced.After 7 days there was no change in dry weight with partial or complete disbudding. However completely disbudded plants had no new ramets and plants with all terminal buds removed had numbers of ramets significantly reduced. The concentration of nitrogen in the tissue of new growth was higher in damaged than undamaged plants. The results are discussed in relation to the biological control of S. molesta by the bud-eating weevil, Cyrtobagous salviniae.

The Old World grass, Arundo donax L. (giant reed), is a serious invader of California riparian areas, and its purported ecosystem impacts led to its consideration as a target for biological control development. However, the herbivore complex in the Arundo adventive range has not been characterized, so there is little information regarding insects that may hinder biological control efforts by interfering with the release of new agents or that could be promoted as augmentative biological control agents if they have a substantial impact on the target weed. Here we report the results of surveys in California, with emphasis on three presumably non-indigenous insects that inflict significant damage to the host. One is a shoot-boring wasp, Tetramesa romana (Walker) (Hymenoptera: Eurytomidae), with a range limited to southern California and that damages shoots generally less than 1 cm in diameter. A shoot fly, Cryptonevra sp., is also associated with shoot damage and often mortality of secondary stems. A third herbivore, the aphid Melanaphis donacis (Passerini), is widespread in the southern and central parts of the state but has less apparent impact to the host. T. romana and Cryptonevra sp. are currently candidates for biological control development and introduction from overseas locations. Their established presence in California suggests that efforts could be revised to focus on documentation of host ranges and impacts under field rather than in quarantine conditions, in anticipation of future re distribution in North America.