Alison M. Fox

Predicting invasive plants in Florida using the Australian weed risk assessment.

Abstract Screening tools that effectively predict which nonnative species are likely to become invasive are necessary because of the disproportionate ecological and economic costs associated with invaders. We tested the effectiveness of the Australian Weed Risk Assessment system (WRA) in distinguishing plant species that are major invaders, minor invaders, and noninvaders in Florida. The test included 158 annuals and perennials in six growth forms from 52 families in 27 orders. The WRA with a secondary screen met all hypothesized accuracy levels: it correctly rejected 92% of test species that have been documented to be invasive in Florida and correctly accepted 73% of the noninvaders. The incorrect rejection of noninvaders was 8% with the remaining 19% of noninvaders falling into the “evaluate further” outcome. Only 10% of the 158 species required further evaluation. Invaders of natural areas and agricultural systems were identified with equal accuracy. Receiver operating characteristic analysis demonstrated high separation of invaders from noninvaders. The degree to which the WRA is precautionary may be adjusted by altering the cutoff scores that define the “accept, evaluate further,” and “reject” outcomes. This approach could be adopted in Florida as a screening mechanism to reduce importation of new invaders.

Temperature-dependent development, survival, and potential distribution of Ischnodemus variegatus (Hemiptera: Blissidae), a herbivore of West Indian marsh grass.

Abstract The bug Ischnodemus variegatus (Signoret) (Hemiptera: Blissidae) is an adventive herbivore, native to South America that feeds in the invasive grass Hymenachne amplexicaulis (Rudge) Nees (Poaceae). This grass is a problematic weed in Florida and Australia, but it is a highly valued forage in Mexico, Cuba, and Venezuela. We studied the influence of nine constant temperatures (8–38°C) on the developmental time and survival of I. variegatus. Complete egg and nymphal mortality occurred at temperatures ≤20.5°C and at 38°C. Developmental time decreased linearly with temperature until 28–30°C and then increased at 33°C. Mortality of first, second, and third instars was high across all temperatures. Developmental time across all temperatures was greatest for eggs, first and fifth instars compared with other stages. Linear and Brière-1 nonlinear models were used to determine the lower temperature threshold at which the developmental rate (1/D) approached zero. The lower thresholds to complete development (egg to adult) estimated with the linear and nonlinear model were 14.6 and 17.4°C, respectively. The total degree-days required to complete development estimated by the linear model was 588. Using temperature data from Florida, a map was generated to project a prediction grid of I. variegatus generations per yr. Based on these predictions, the insect can complete three to five generations per year in areas currently invaded in Florida. Results of this study will be used to understand the potential distribution and population growth of I. variegatus in H. amplexicaulis infested regions.

Hydrilla control with split treatments of fluridone in Lake Harris, Florida

After several unsuccessful management efforts, a split treatment of fluridone was applied to the 6700 ha Lake Harris in March and June 1987, at a rate of 3.4 kg ha−1 (680 and 340 kg fluridone, respectively) to two 3 m deep, hydrilla-infested bays. Fluridone concentrations in the water were sampled following the June treatment. Average fluridone concentrations were 2.1 µg l−1 prior to this second application, and a maximum concentration of 30.2 µg l−1 was detected in the treated area on the day following application. Fluridone residues dissipated out of the plot quickly due to dilution but concentrations declined lake-wide more slowly, following a logarithmic model, with an estimated fluridone half-life of 97 days. Control of hydrilla in Lake Harris resulted from the long exposure (over 25 weeks due to the split application) to fluridone concentrations of 2 µg l−1, well below the maximum labelled rate of 150 µg l−1.

Approaches for Assessing the Status of Nonnative Plants: A Comparative Analysis

Abstract The magnitude of the invasive plant species problem necessitates prioritization of species for control, regulatory, and public-education programs. Many such priority lists exist but few have been developed according to specified procedures and criteria. We reviewed approaches to assessing the status of nonnative plant species currently occurring in natural areas (status assessments). We identify four generalized types of status assessments, which reflect a gradation from those that simply adopt existing lists from elsewhere (type 1), to those with relatively easy and rapid development and implementation (type 2), to those that are more time-consuming and costly but may be more robust in the face of challenges (types 3 and 4). These latter assessments explicitly have greater transparency, objectivity, and consistency than the other types. We use a matrix of assessment characteristics to distinguish the types of 17 example status assessments. We also review the factors related to assessment intent, scope, structure, content, and implementation that must be considered during the development of new status assessments so that the resulting tool and its products are appropriate for the users purposes. These analyses should facilitate evaluation of different assessment methods and provide a basis for development of improved assessments. Identification of the relatively low percentage of nonnative plant species that are inflicting ecological and economic harm using well-understood and accepted assessment methods should facilitate a more comprehensive, collective approach to implementation of effective management efforts. Nomenclature: Invasive plants, alien, exotic, introduced species, status assessments.

Host specificity of Ischnodemus variegatus , an herbivore of West Indian marsh grass ( Hymenachne amplexicaulis )

West Indian marsh grass, Hymenachne amplexicaulis Rudge (Nees) (Poaceae), is an emergent wetland plant that is native to South and Central America as well as portions of the Caribbean, but is considered invasive in Florida USA. The neotropical bug, Ischnodemus variegatus (Signoret) (Hemiptera: Lygaeoidea: Blissidae) was observed feeding on H. amplexicaulis in Florida in 2000. To assess whether this insect could be considered as a specialist biological control agent or potential threat to native and cultivated grasses, the host specificity of I. variegatus was studied under laboratory and field conditions. Developmental host range was examined on 57 plant species across seven plant families. Complete development was obtained on H. amplexicaulis (23.4% survivorship), Paspalum repens (0.4%), Panicum anceps (2.2%) and Thalia geniculata (0.3%). Adults survived 1.6 times longer and laid 6.6 times more eggs on H. amplexicaulis than the other species. Oviposition on suboptimal host species was positively related to I. variegatus density under multiple choice conditions. Results from field experiments indicated that H. amplexicaulis had higher densities of I. variegatus than other species. Spill-over to suboptimal hosts occurred in an area where H. amplexicaulis was growing in poor conditions and there was a high density of I. variegatus. Thus, laboratory and field studies demonstrate that I. variegatus had higher performance on H. amplexicaulis compared to any other host, and that suboptimal hosts could be colonized temporarily.

Production and survivorship of the functional stolons of giant cutgrass, Zizaniopsis miliacea (Poaceae)

Giant cutgrass [Zizaniopsis miliacea], a tall emergent grass native to the southeastern United States, was studied in two Florida lakes. In Lake Seminole (15 176 ha) giant cutgrass forms large expanding stands, but in Lake Alice (9 ha) it is confined to a stable narrow fringe. By monitoring individual plants in Lake Seminole, it was found that an average decumbent flowering stem produced three flowers and ten nodes, 80% of which became rooted in the substrate. Such flowering stem development could potentially result in stand expansion of 2.2-2.7 m/yr, depending upon water levels and rates of node rooting. Once flowering stems became decumbent in Lake Alice, they typically broke, producing no more than two flowers with four nodes in a growing season. While still attached to the parent plant, few of these nodes were able to become rooted in the substrate, limiting the rate of stand expansion in Lake Alice. Sections of flowering stems bearing axillary shoots that were detached from the parent plant and free-floating could become rooted on reaching shallow water and produce robust, new, flowering plants. This interesting mode of population dispersal and spread has important implications for the distribution and management of giant cutgrass.

Factors that influence water exchange in spring-fed tidal canals

Environmental factors that influenced the rates of water exchange in a series of tidal, dead-end canals in Crystal River, Florida, were investigated by measuring the rates of dilution of the fluorescent dye Rhodamine WT. Water temperature regime was the most influential of these factors, with type of tidal cycle (spring or neap) and density of submersed vegetation (dominated byHydrilla verticillata) only affecting dye dilution rates in winter treatments. Dye half-lives were shortest in the summer (12–24 h), longest in the fall and spring (60–120 h), and most sensitive to water temperature patterns in the winter (26–60 h). These data provide useful information relating to the timing of hydrilla management using aquatic herbicides in spring-fed tidal systems. A mechanism is proposed that relates the canal water temperatures to tidal water movement and outputs from the adjacent Three Sisters Springs.

Criteria for Listing Invasive Plants

Abstract The Weed List Criteria Workshop consisted of four invited speakers and an animated discussion about the value of developing a standardized criteria system for developing lists of invasive plants. The presentations addressed the importance of having lists and criteria that are sufficiently robust to withstand legal challenges, the sophistication of a national prioritization system, some alternative strategies to the use of “one-size-fits-all” invasive plant lists, and the variety of lists that are already in use. Participants agreed that a standardized criteria system might need a modular format (e.g., agriculture or natural areas modules) with options for different thresholds for some criteria (e.g., rigor of documentation) to satisfy diverse listing objectives. Many participants expressed interest in future participation in this endeavor. Additional index words: List standardization, status assessment, weed lists.

Wetland Nightshade (Solanum tampicense) Growth Response to Temperature, and Winter Survival, in Relation to Potential Spread1

Greenhouse, growth chamber, and winter survival studies were conducted at Stoneville, MS from 1996 to 2002 to determine growth, time to first flower, and winter survival of wetland nightshade. At 12 wk after emergence, wetland nightshade plants had 58-, 45-, 48-, and 4-cm heights, respectively; 24, 21, 21, and 12 nodes/plant, respectively; 62, 31, 36, and 21 leaves/plant, respectively; and 7.1, 3.9, 5.1, and 0.3 g/plant dry weights, respectively, at temperatures of 26/36, 20/30, 14/24, and 8/18 (±0.5) C at the 14/10 day/night length. Flowering occurred at 79, 85, and 85 days after emergence at 26/36, 20/30, and 14/24 C night/day, respectively at the 14/10 day/night cycle. Wetland nightshade plants did not flower at 8/18 C. Wetland nightshade growth was adequate for flowering and fruit production in additional areas of the southeastern United States with night/day temperatures greater than or equal to 14/24 C. Winter survival was greater than or equal to 33% for established wetland nightshade plants in 5 of 6(1996 to 2002) above water levels and 82% from 20 cm below the water surface. Based on these results, wetland nightshade has the potential to continue to spread in the United States. Nomenclature: Wetland nightshade, Solanum tampicense Dunal #3. Additional index words: Solanum tampicense Dunal.