Invasive species research in the United States Department of Agriculture-Agricultural Research Service.

Invasive pests cause huge losses both to agricultural production systems and to the natural environment through displacing native species and decreasing biodiversity. It is now estimated that many thousand exotic insect, weed and pathogen species have been established in the USA and that these invasive species are responsible for a large portion of the $130 billion losses estimated to be caused by pests each year. The Agricultural Research Service (ARS) has responded with extensive research and action programs aimed at understanding these problems and developing new management approaches for their control. This paper provides an overview of some of the ARS research that has been conducted on invasive species over the past few years and addresses both different categories of research and some specific pest systems of high interest to the US Department of Agriculture.

Age estimation of Mexican fruit fly (Diptera: Tephritidae) based on accumulation of pterins.

A common method of aging adult flies, fluorescence spectrometry, was used to monitor the increase of overall pterine titer in head extracts of Anastrepha ludens (Loew). Accumulation of fluorescent compounds was measured as a function of chronological age of flies maintained at 17 and 27 degrees C. Although relative fluorescence increased with age, field studies revealed that this phenomenon could not be used for accurate age estimation, as relative fluorescence did not increase predictably with age over the entire life span. Accumulation of individual pterins, deoxysepiapterin and sepiapterin, were studied in a similar manner. These two specific compounds were separated by high-pressure liquid chromatography and their accumulation was followed at 15 and 30 degrees C in the laboratory and under caged field conditions. While titer of deoxysepiapterin increased steadily in a curvilinear fashion, sepiapterin quickly reached a maximum and then maintained a constant level for the rest of the life of the flies. Based on the physiological response of deoxysepiapterin to chronological time and ambient thermal conditions, this compound was determined to be an age specific biological parameter for the Mexican fruit fly and should allow age estimation in field-collected flies.

Field testing Diorhabda elongata (Coleoptera: Chrysomelidae) from Crete, Greece, to assess potential impact on nontarget native California plants in the genus Frankenia.

When laboratory host specificity tests on weed biological control agents produce ambiguous results or are suspected of producing false-positive findings, field cage or open field tests can be used to help determine the true ecological host range of the agent. The leaf beetle Diorhabda elongata (Brullé) from Crete, imported to the United States for the control of saltcedar (Tamarix spp., Tamaricaceae), showed a low but variable ovipositional response to nontarget Frankenia spp. (Frankeniaceae) in previous laboratory tests conducted in small cages, where up to 11.4% of eggs were laid on these native plants. Results from field tests presented in this article show that no eggs were laid on Frankenia palmeri S. Watson and significantly more eggs were always laid on Tamarix ramosissima Ledebour than Frankenia salina (Molina) I. M. Johnston. Furthermore, the ovipositional response to F. salina was substantially lower than that recorded in laboratory tests. The percent of eggs laid on F. salina in field tests was 3.7 in a paired choice cage test, 4.3 in a multiple choice cage test, and 2.5 in a multiple choice open field test, suggesting that the true acceptance rate of the nontarget by D. elongata in the field will be lower than laboratory tests predicted. However, some damage was caused to F. salina by adult and larval feeding in the field, although this occurred only at the very end of the open field test, when D. elongata densities were extremely high, and all of the surrounding saltcedar had been totally defoliated. Scientific representatives from various stakeholder organizations (state, county, university, and environmental groups) viewed the open field test when in progress and reviewed the final results before advising State regulatory agencies on beetle redistribution. These test results, and the open review process, led regulators to conclude that redistribution of D. elongata in California was warranted owing to its significant ability to defoliate saltcedar, and its low rate of feeding on nontarget Frankenia spp. The introduction of D. elongata provides an interesting case study for risk assessment of a potentially efficacious weed biocontrol agent that may also be capable of using nontarget native plants.

Canopy assessment of biochemical features by ground-based hyperspectral data for an invasive species, giant reed (Arundo donax).

This study explored the potential use of hyperspectral data in the non-destructive assessment of chlorophyll, carbon, and nitrogen content of giant reed at the canopy level. We found that pseudoabsorption and derivatives of original hyperspectral data were able to describe the relationship between spectral data and measured biochemical characteristics. Based on correlogram analyses of ground-based hyperspectral data, we found that derivatives of pseudoabsorption were the best predictors of chlorophyll, carbon, and nitrogen content of giant reed canopies. Within the visible region, spectral data significantly correlated with chlorophyll content at both 461 nm and 693 nm wavelengths. Within the near-infrared region, carbon levels correlated with hyperspectral data at five causal wavelengths: 1038 nm, 1945 nm, 1132 nm, 1525 nm, and 1704 nm. The best spectral wavelength for estimating nitrogen content was 1542 nm. Such relationships between nutrient content and spectral data were best represented by exponential functions in most situations.

Invasive species change detection using artificial neural networks and CASI hyperspectral imagery.

For monitoring and controlling the extent and intensity of an invasive species, a direct multi-date image classification method was applied in invasive species (salt cedar) change detection in the study area of Lovelock, Nevada. With multidate Compact Airborne Spectrographic Imager (CASI) hyperspectral data sets, two types of hyperspectral CASI input data and two classifiers have been examined and compared for mapping and monitoring the salt cedar change. The two types of input data are all two-date original CASI bands and 12 principal component images (PCs) derived from the two-date CASI images. The two classifiers are an artificial neural network (ANN) and linear discriminant analysis (LDA). The experimental results indicate that (1) the direct multitemporal image classification method applied in land cover change detection is feasible either with original CASI bands or PCs, but a better accuracy was obtained from the CASI PCA transformed data; (2) with the same inputs of 12 PCs, the ANN outperforms the LDA due to the ANN's non-linear property and ability of handling data without a prerequisite of a certain distribution of the analysis data.

Hyperspectral characteristics of canopy components and structure for phenological assessment of an invasive weed.

Spectral reflectance values of four canopy components (stems, buds, opening flowers, and postflowers of yellow starthistle (Centaurea solstitialis)) were measured to describe their spectral characteristics. We then physically combined these canopy components to simulate the flowering stage indicated by accumulated flower ratios (AFR) 10%, 40%, 70%, and 90%, respectively. Spectral dissimilarity and spectral angles were calculated to quantitatively identify spectral differences among canopy components and characteristic patterns of these flowering stages. This study demonstrated the ability of hyperspectral data to characterize canopy components, and identify different flowering stages. Stems had a typical spectral profile of green vegetation, which produced a spectral dissimilarity with three reproduction organs (buds, opening flowers, and postflowers). Quantitative differences between simulated flower stages depended on spectral regions and phenological stages examined. Using full-range canopy spectra, the initial flowering stage could be separated from the early peak, peak, and late flowering stages by three spectral regions, i.e. the blue absorption (around 480 nm) and red absorption (around 650 nm) regions and NIR plateau from 730 nm to 950 nm. For airborne CASI data, only the red absorption region and NIR plateau could be used to identify the flowering stages in the field. This study also revealed that the peak flowering stage was more easily recognized than any of the other three stages.

Texture analysis for mapping Tamarix parviflora using aerial photographs along the Cache Creek, California.

Natural color photographs were used to detect the coverage of saltcedar, Tamarix parviflora, along a 40 km portion of Cache Creek near Woodland, California. Historical aerial photographs from 2001 were retrospectively evaluated and compared with actual ground-based information to assess accuracy of the assessment process. The color aerial photos were sequentially digitized, georeferenced, classified using color and texture methods, and mosaiced into maps for field use. Eight types of ground cover (Tamarix, agricultural crops, roads, rocks, water bodies, evergreen trees, non-evergreen trees and shrubs (excluding Tamarix)) were selected from the digitized photos for separability analysis and supervised classification. Due to color similarities among the eight cover types, the average separability, based originally only on color, was very low. The separability was improved significantly through the inclusion of texture analysis. Six types of texture measures with various window sizes were evaluated. The best texture was used as an additional feature along with the color, for identifying Tamarix. A total of 29 color photographs were processed to detect Tamarix infestations using a combination of the original digital images and optimal texture features. It was found that the saltcedar covered a total of 3.96 km(2) (396 hectares) within the study area. For the accuracy assessment, 95 classified samples from the resulting map were checked in the field with a global position system (GPS) unit to verify Tamarix presence. The producer's accuracy was 77.89%. In addition, 157 independently located ground sites containing saltcedar were compared with the classified maps, producing a user's accuracy of 71.33%.