Shon S. Schooler

Variability and Parasitoid Foraging Efficiency: A Case Study of Pea Aphids and Aphidius ervi

When a parasitoid is searching for hosts, not all hosts are equally likely to be attacked. This variability in attack probability may affect the parasitoid functional response. Using a collection of experiments, we quantified the functional response of Aphidius ervi (Hymenoptera: Braconidae), an insect parasitoid of the pea aphid Acyrthosiphon pisum (Homoptera: Aphididae). We measured variability in the number of hosts attacked by a foraging parasitoid both among plants and within plants. At the first scale, A. ervi, searching among plants containing different numbers of aphids, showed both aphid‐density‐dependent and aphid‐density‐independent variability in the number of aphids attacked per plant. Within plants, A. ervi selectively attacked second and third instar aphids relative to other instars. Furthermore, there was variability in the susceptibility of attack among aphids independent of instar. Variability in attack rates among aphids both among and within plants decreased parasitoid foraging efficiency, with the greatest decrease caused by among‐plant variability. Furthermore, the decrease in foraging efficiency was greatest when the average number of aphids per plant was low, thereby transforming a strong Type II functional response into one approaching Type I.

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.

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 as an invasion process: disturbance and propagule pressure affect the invasion success of Lythrum salicaria biological control agents

Understanding the mechanisms behind the successful colonization and establishment of introduced species is important for both preventing the invasion of unwanted species and improving release programs for biological control agents. However, it is often not possible to determine important introduction details, such as date, number of organisms, and introduction location when examining factors affecting invasion success. Here we use biological control introduction data to assess the role of propagule pressure, disturbance, and residence time on invasion success of four herbivorous insect species introduced for the control of the invasive wetland plant, Lythrum salicaria, in the Columbia River Estuary. Two sets of field surveys determined persistence at prior release sites, colonization of new sites, and abundance within colonized sites. We quantified propagule pressure in four ways to examine the effect of different measurements. These included three measurements of introduction size (proximity to introduction site, introduction size at a local scale, and introduction size at a regional scale) and one measure of introduction number (number of introduction events in a region). Disturbance was examined along a tidal inundation gradient (distance from river mouth) and as habitat (island or mainland). Statistical models and model averaging were used to determine which factors were driving invasion success. In this study we found: (1) sparse evidence for the positive influence of propagule pressure on invasion success; (2) disturbance can negatively affect the invasion success of herbivorous insects; (3) the effects of disturbance and propagule pressure are species specific and vary among invasion stages, and (4) not all measures of propagule pressure show the same results, therefore single measures and proxies should be used cautiously.

Differential influence of clonal integration on morphological and growth responses to light in two invasive herbs.

Background and aims In contrast to seeds, high sensitivity of vegetative fragments to unfavourable environments may limit the expansion of clonal invasive plants. However, clonal integration promotes the establishment of propagules in less suitable habitats and may facilitate the expansion of clonal invaders into intact native communities. Here, we examine the influence of clonal integration on the morphology and growth of ramets in two invasive plants, Alternanthera philoxeroides and Phyla canescens, under varying light conditions. Methods In a greenhouse experiment, branches, connected ramets and severed ramets of the same mother plant were exposed under full sun and 85% shade and their morphological and growth responses were assessed. Key results The influence of clonal integration on the light reaction norm (connection×light interaction) of daughter ramets was species-specific. For A. philoxeroides, clonal integration evened out the light response (total biomass, leaf mass per area, and stem number, diameter and length) displayed in severed ramets, but these connection×light interactions were largely absent for P. canescens. Nevertheless, for both species, clonal integration overwhelmed light effect in promoting the growth of juvenile ramets during early development. Also, vertical growth, as an apparent shade acclimation response, was more prevalent in severed ramets than in connected ramets. Finally, unrooted branches displayed smaller organ size and slower growth than connected ramets, but the pattern of light reaction was similar, suggesting mother plants invest in daughter ramets prior to their own branches. Conclusions Clonal integration modifies light reaction norms of morphological and growth traits in a species-specific manner for A. philoxeroides and P. canescens, but it improves the establishment of juvenile ramets of both species in light-limiting environments by promoting their growth during early development. This factor may be partially responsible for their ability to successfully colonize native plant communities.

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.

Adult insect assemblages as a conservation planning tool

Summary Our study found differences in insect populations collected from three regions in Texas. We showed that three to four light trap samples taken on consecutive nights are sufficient to capture the pooled insect diversity of the wetlands we studied. While we were able to show differences across these broad regions using insects identified to the order level, we found no patterns when we examined differences between specific wetland types, using the same taxonomie level. Within a single region, however, we were able to use insects from one order (Tri- choptera), identified to the family level, to discriminate between different wetland types: Trichopteran family-level cumulative diversity measures were significantly different between sites classified by abiotic ecosystem properties (geographic location, hydrology) and by biotic ecosystem properties (vegetation type). Our study focuses on the responses of assemblages rather than individuals. We chose aquatic insects as potential indicators because insects are rarely, if ever, directly manipulated by ecosystem managers. Insect data are collected and analyzed using well-known methodologies that do not rely on professional judgment, as in the case of some assessment techniques (i.e. HGM, HEP, WET). Our method looks at the response of an entire suite of organisms, some with very strict requirements or ecological tolerances that are quick to respond to changes in their environment. We recognize that there are situations where it is entirely appropriate to measure effects on a specific target species. However, if the goal is to manage functions rather than species, then a single species approach may not be appropriate.