Flowering leafy spurge (Euphorbia esula) detection using unmanned aerial vehicle imagery in biological control sites: Impacts of flight height, flight time and detection method

Leafy spurge, a noxious perennial weed, is a major threat to the prairie ecosystem in North America. Strategic planning to control leafy spurge requires monitoring its spatial distribution and spread. The ability to detect flowering leafy spurge at two biological control sites in southern Saskatchewan, Canada, was investigated using an unmanned aerial vehicle (UAV) system. Three flight missions were conducted on June 30, 2016, during the leafy spurge flowering period. Imagery was acquired at four flight heights and one or two acquisition times, depending on the site. The sites were reflown on June 28, 2017, to evaluate the change in flowering leafy spurge over time. Mixture tuned matched filtering (MTMF) and hue, intensity, and saturation (HIS) threshold analyses were used to determine flowering leafy spurge cover. Flight height of 30 m was optimal; the strongest relationships between UAV and ground estimates of leafy spurge cover (r2 = 0.76 to 0.90; normalized root mean square error [NRMSE] = 0.10 to 0.13) and stem density (r2 = 0.72 to 0.75) were observed. Detection was not significantly affected by the image analysis method (P > 0.05). Flowering leafy spurge cover estimates were similar using HIS (1.9% to 14.8%) and MTMF (2.1% to 10.3%) and agreed with the ground estimates (using HIS: r2 = 0.64 to 0.93, NRMSE = 0.08 to 0.25; using MTMF: r2 = 0.64 to 0.90, NRMSE = 0.10 to 0.27). The reduction in flowering leafy spurge cover between 2016 and 2017 detected using UAV images and HIS (8.1% at site 1 and 2.7% at site 2) was consistent with that based on ground digital photographs (10% at site 1 and 1.8% at site 2). UAV imagery is a useful tool for accurately detecting flowering leafy spurge and could be used for routine monitoring purposes in a biological control program.

Discrimination of leafy spurge (Euphorbia esula) and purple loosestrife (Lythrum salicaria) based on field spectral data

Leafy spurge (Euphorbia esula L.) and purple loosestrife (Lythrum salicaria L.) are invasive weeds that displace native vegetation. Herbicides are often applied to these weeds during flowering, so it would be ideal to identify them early in the season, possibly by the leaves. This paper evaluates the spectral separability of the inflorescences and leaves of these plants from surrounding vegetation. Leafy spurge, purple loosestrife, and surrounding vegetation were collected from sites in southeastern North Dakota and subjected to spectral analysis. Partial least-squares discriminant analysis (PLS-DA) was used to separate the spectral signatures of these weeds in the visible and near-infrared wavelengths. Using PLS-DA, the weeds were discriminated from their surroundings with R2 values of 0.86 to 0.92. Analysis of the data indicated that the bands contributing the most to each model were in the red and red-edge spectral regions. Identifying these weeds by the leaves allows them to be mapped earlier in the season, providing more time for herbicide application planning. The spectral signatures identified in this proof of concept study are the first step before using ultra–high resolution aerial imagery to classify and identify leafy spurge and purple loosestrife.

Gene Space and Transcriptome Assemblies of Leafy Spurge (Euphorbia esula) Identify Promoter Sequences, Repetitive Elements, High-Quality Markers, and a Full-Length Chloroplast Genome

Leafy spurge (Euphorbia esulaL.) is an invasive perennial weed infesting range and recreational lands of North America. Previous research and omics projects withE. esulahave helped develop it as a model for studying many aspects of perennial plant development and response to abiotic stress. However, the lack of an assembled genome forE. esulahas limited the power of previous transcriptomics studies to identify functional promoter elements and transcription factor binding sites. An assembled genome forE. esulawould enhance our understanding of signaling processes controlling plant development and responses to environmental stress and provide a better understanding of genetic factors impacting weediness traits, evolution, and herbicide resistance. A comprehensive transcriptome database would also assist in analyzing future RNA-seq studies and is needed to annotate and assess genomic sequence assemblies. Here, we assembled and annotated 56,234 unigenes from an assembly of 589,235 RNA-seq-derived contigs and a previously published Sanger-sequenced expressed sequence tag collection. The resulting data indicate that we now have sequence for >90% of the expressedE. esulaprotein-coding genes. We also assembled the gene space ofE. esulaby using a limited coverage (18X) genomic sequence database. In this study, the programs Velvet and Trinity produced the best gene-space assemblies based on representation of expressed and conserved eukaryotic genes. The results indicate thatE. esulacontains as much as 23% repetitive sequences, of which 11% are unique. Our sequence data were also sufficient for assembling a full chloroplast and partial mitochondrial genome. Further, marker analysis identified more than 150,000 high-quality variants in ourE. esulaL-RNA–scaffolded, whole-genome, Trinity-assembled genome. Based on these results,E. esulaappears to have limited heterozygosity. This study provides a blueprint for low-cost genomic assemblies in weed species and new resources for identifying conserved and novel promoter regions among coordinately expressed genes ofE. esula.

Leafy spurge (Euphorbia esula) Control and Soil Seedbank Composition Fifteen Years after Release of Aphthona Biological Control Agents

Aphthona spp. flea beetles were released in two ecological sites of the Little Missouri National Grasslands in southwestern North Dakota in 1999 to control leafy spurge. The change in leafy spurge density and soil seedbank composition was monitored to evaluate the effectiveness of the biological weed control agent and the associated change in plant communities 5, 10, and 15 yr after release in loamy overflow (valleys) and loamy sites (ridges). In 2014, 15 yr after release, leafy spurge stem density had decreased 94% from 110 to 7 stems m−2 in the loamy overflow sites and 88% from 78 to 9 stems m−2 in the loamy sites. Leafy spurge represented only 2% and 6% of the loamy overflow and loamy seedbanks in 2004, respectively, compared with nearly 67% and 70%, respectively, in 1999. There was a slow shift to reintroduction of native species into the seedbank over the last 15 yr. The number of desirable species increased to 21 by 2014 (more than three times the number of species in 1999) in the loamy overflow sites, and doubled to 14 species in the loamy sites, while less desirable forb species doubled in both sites. Desirable grass species doubled in the loamy overflow sites by 2014 but remained unchanged in loamy sites. Aphthona spp. successfully controlled leafy spurge for more than 15 yr without any additional control methods or costs to land managers and resulted in the slow return of a subset of native species.