Mic H. Julien

Alternative stable states explain unpredictable biological control of Salvinia molesta in Kakadu

Suppression of the invasive plant Salvinia molesta by the salvinia weevil is an iconic example of successful biological control. However, in the billabongs (oxbow lakes) of Kakadu National Park, Australia, control is fitful and incomplete. By fitting a process-based nonlinear model to thirteen-year data sets from four billabongs, here we show that incomplete control can be explained by alternative stable states—one state in which salvinia is suppressed and the other in which salvinia escapes weevil control. The shifts between states are associated with annual flooding events. In some years, high water flow reduces weevil populations, allowing the shift from a controlled to an uncontrolled state; in other years, benign conditions for weevils promote the return shift to the controlled state. In most described ecological examples, transitions between alternative stable states are relatively rare, facilitated by slow-moving environmental changes, such as accumulated nutrient loading or climate change. The billabongs of Kakadu give a different manifestation of alternative stable states that generate complex and seemingly unpredictable dynamics. Because shifts between alternative stable states are stochastic, they present a potential management strategy to maximize effective biological control: when the domain of attraction to the state of salvinia control is approached, augmentation of the weevil population or reduction of the salvinia biomass may allow the lower state to trap the system.

Phenotypic divergence during the invasion of Phyla canescens in Australia and France: evidence for selection-driven evolution

Rapid adaptive evolution has been advocated as a mechanism that promotes invasion. Demonstrating adaptive evolution in invasive species requires rigorous analysis of phenotypic shifts driven by selection. Here, we document selection-driven evolution of Phyla canescens, an Argentine weed, in two invaded regions (Australia and France). Invasive populations possessed similar or higher diversity than native populations, and displayed mixed lineages from different sources, suggesting that genetic bottlenecks in both countries might have been alleviated by multiple introductions. Compared to native populations, Australian populations displayed more investment in sexual reproduction, whereas French populations possessed enhanced vegetative reproduction and growth. We partitioned evolutionary forces (selection vs. stochastic events) using two independent methods. Results of both analyses suggest that the pattern of molecular and phenotypic variability among regions was consistent with selection-driven evolution, rather than stochastic events. Our findings indicate that selection has shaped the evolution of P. canescens in two different invaded regions.

Selective Herbicides Reduce Alligator Weed (Alternanthera Philoxeroides) Biomass by Enhancing Competition

Abstract Physical and chemical methods of managing invasive plants (weeds) create disturbances that paradoxically often promote these species because weeds tend to have traits that confer competitive advantages over desired species in disturbed habitats. A more holistic and sustainable method of managing invasive plants is to design disturbance regimes to favor desired species over weeds. This study investigated how the biomass of a herbicide-tolerant plant, alligator weed, and its competitors respond to different chemical disturbances over a 2-yr period. We compared the response of alligator weed and its monocotyledon competitors to 16 different herbicide treatments in a blocked 4 by 2 by 2 factorial design. Treatments included broad spectrum (nonselective) and dicotyledon specific (selective) herbicides applied at two concentrations (variable depending on herbicide) and two frequency regimes (three or four applications). Belowground biomass of alligator weed in unmanipulated control plots was 10 times greater than aboveground biomass, highlighting the need to reduce belowground material if control is to be achieved. All herbicide treatments reduced belowground alligator weed biomass when compared with controls; however in the short term (8 d after the final treatment), even four applications at the highest listed concentration were not sufficient to eliminate alligator weed from study plots. Over the long term (15 mo after the final treatment), selective herbicide application resulted in a sustained reduction in alligator weed biomass and an increase in monocot biomass. Nomenclature: Alligator weed, Alternanthera philoxeroides (Mart.) Griseb

Biological Control of Tropical Weeds Using Arthropods: Pistia stratiotes L. (Araceae)

Taxonomy Earliest descriptions of Pistia stratiotes L. ( Araceae ) were by the ancient Egyptians and by the Greek philosophers Dioscorides and Theophrastus. This plant has also been mentioned by Plinius (Stoddard, 1989). According to Bogner and Nicolson (1991) P. stratiotes is the solitary member of the subfamily Pistioidea in Araceae . However, USDA (2008) places it in the subfamily Aroideae along with the numerous other genera. The many synonyms and obsolete subspecific names ( Plantatlas , 2006) attest to the variability of this taxonomically isolated species, which is the only free-floating aroid. The plant is known as water lettuce; other common names are available in Randall (2002). Description Pistia consists of a rosette of obovate to spatulate, velvety, light-green leaves (up to 40 cm long in African and American clones) (Fig. 17.1a, b), covered by short hairs, which trap air bubbles and thus enable buoyancy. The underside of leaves is densely hairy and almost white, with longitudinal ribs with embedded veins. The long feathery roots hang freely in the water. A clonal plant forms small colonies through stolons. Inflorescences are inconspicuous (7–12 × 5 mm) with short peduncles in the center of the rosette, growing on a stem. The spadix, enclosed in a whitish spathe, is pale green, hairy outside and glabrous inside. The spathe generally shows a constriction between the groups of male and the female flowers. The spathe below the constriction opens first in the morning hours to expose the wet stigma, whereas the male flowers remain enclosed.

Biological Control of Tropical Weeds Using Arthropods: Cabomba caroliniana Gray (Cabombaceae)

Introduction Cabomba ( Cabomba caroliniana Gray, Cabombaceae), or water fanwort, is a fast-growing submerged aquatic plant that has the potential to infest permanent water bodies in a range of regions – from tropical to cool temperate – throughout the world. It is considered a serious pest in the United States, Canada, the Netherlands, Japan, India, China, and Australia, and is present in Hungary, South Africa, and the United Kingdom. Cabomba grows well in slow-moving water bodies, preferring areas of permanent standing water less than 4 m deep; however, it can also grow at depths up to 6 m in Australia (Schooler and Julien, 2006). The weed is recognized by its opposing pairs of finely dissected underwater leaves that are feathery or fan-like in appearance (Fig. 6.1 and Fig. 6.2). Small white flowers bearing three petals and three sepals extend above the water surface, making infestations more visible in summer months. Reproduction is almost entirely vegetative throughout most of the introduced localities and any fragment that includes nodes can grow into a new plant (Sanders, 1979). Cabomba originates from South America (Orgaard, 1991). The plants tolerance of fragmentation and delicate appearance make it a desirable aquarium plant (Hiscock, 2003) and consequently it was brought into many countries through the aquarium trade. Cabomba was subsequently introduced into lakes and streams both accidentally, through the dumping of aquarium water, and on purpose, to enable cultivation for later collection and sale.

Biological Control of Tropical Weeds Using Arthropods: Salvinia molesta D. S. Mitchell (Salviniaceae)

Introduction Salvinia molesta D. S. Mitchell (Salviniaceae) (salvinia) is a floating water fern of tropical and subtropical distribution worldwide. Its center of origin is southeastern Brazil. It is an extremely important invasive species and its biological control is an extraordinary, contemporary, success story. Salvinia molesta is named after Antonio Maria Salvini (1633-1729), University of Florence. The specific epithet molesta originates from the Latin molestus meaning ‘troublesome,’ ‘annoying,’ referring to its weediness (Parsons and Cuthbertson, 2001). Taxonomy Salviniaceae in Hydropteridales comprises the monotypic taxon Salvinia with 10-12 species (Hassler and Swale, 2002): S. minima Baker, S. oblongifolia Martius, and four species in the S. auriculata complex originating in the tropical Americas. The S. auriculata complex comprises species in which the upper section of each leaf hair forms an ‘egg-beater’ or ‘cage’ shape by splitting apart below the tip and joining at the tip (Fig. 19.1) (Forno, 1983) and includes S. auriculata Aublet, S. biloba Raddi, S. herzogii de la Sota, and Salvinia molesta D. S. Mitchell. Salvinia molesta was separated from S. auriculata by Mitchell (1972). Most literature that refers to S. auriculata as a pest species outside South America and Trinidad actually refers to S. molesta . Herzog (1935) recognized S. auriculata Aublet, and this name was applied to the invasive species that occurred outside South America. De la Sota (1962) recognized that S. auriculata comprised a number of species and described S. herzogii .

Flood-induced recruitment of the invasive perennial herb Phyla canescens (lippia).

Phyla canescens (Kunth) Greene, lippia, (Verbenaceae) is an important invasive species in the Murray–Darling Basin, Australia. The general lack of quantitative information on aspects of the life-history of P. canescens is a substantial impediment to the sustainable management of this species and the communities it invades. Complementary laboratory and field experiments investigated P. canescens germination. A thermogradient plate was used to examine its germination response to a variety of temperature regimes. Recruitment in the field was investigated at four sites across two catchments following four season disturbances. In the laboratory trials, seeds required temperatures that alternated by at least 5°C, light, and to be covered by a thin film of water. Field germination occurred only at the one site that experienced a flood, despite periods of high rainfall that stimulated germination of other species. In this site seedling density and survivorship were reduced in the presence of existing vegetation. A survey of P. canescens following flooding of a temporary billabong revealed recruitment from both seed and vegetative fragments. This recruitment was almost exclusively restricted to the area that had been flooded. The results suggest that P. canescens requires inundation for successful recruitment and that these germination requirements are typical of species from disturbance-prone environments.

Determining the growth responses of Phyla canescens to shoot and root damage as a platform to better-informed weed-management decisions

Understanding the responses of invasive plants to control methods is important in developing effective management strategies. Lippia (Phyla canescens (Kunth) Greene : Verbenaceae) is an invasive, perennial, clonal forb for which few control options exist for use in the Australian natural and agro-ecosystems it threatens. To help inform management decisions, lippia’s growth responses to damage it may experience during proposed control measures, i.e. cutting, crushing, twisting, were assessed in three glasshouse experiments using either whole plants or plant pieces. Plants quickly recovered from severe damage through growth from shoot and root buds at stem nodes. After shoot and root removal, the relative growth rate of the remaining plant was twice that of controls, suggesting tolerance to damage. Lacking buds, root pieces and isolated stem internodes were incapable of responding. Crushing and cutting individual ramets and plant pieces induced the largest responses, including release of axillary buds on damage or removal of apical buds, but full recovery was not achieved. Lippia will be difficult to control because of its ability to rapidly propagate from stem fragments possessing undamaged or damaged nodes; thus, the full impact of control methods that increase fragmentation (e.g. grazing) should be assessed before implementation. Our results also suggest that the most effective biological agents will be those that limit lippia’s vegetative growth and spread, such as shoot- or crown-feeding insects.

Preliminary results of a survey on the role of arthropod rearing in classical weed biological control.

The rearing of arthropods is an essential but sometimes neglected and underestimated part of a classical weed biological control programme. Success in rearing is usually a pre-requisite to conducting host-specificity tests, obtaining enough individuals for initial field release or, later, for large-scale implementation. Although most biological control researchers can list situations where agent development has been stopped or slowed due to rearing difficulties, failures seldom get reported in the literature, thus preventing us from gauging the extent and relevance of rearing issues. To rectify this, a questionnaire was developed to investigate the prevalence of rearing problems in weed biological control programmes and to classify their occurrence according to a list of variables (e.g. taxonomy, biological features, genetic issues and researcher/programme attributes). The questionnaire was sent to 80 researchers from eight countries; 65% responded, generating 79 useful responses. Results confirm that, of the challenges faced in programmes, rearing is the most prevalent (56% out of ten possible general problem categories). The most common rearing problems encountered were conditions that were not conducive to mating and/or oviposition (30% of reported arthropod cases) or development (22% of reported arthropod cases). Our results identify key areas for rearing improvement, thus contributing to increased weed biological control project successes.