A novel diagnostic gene region for distinguishing between two pest fruit flies: Bactrocera tryoni (Froggatt) and Bactrocera neohumeralis (Hardy) (Diptera: Tephritidae).

Bactrocera tryoni and Bactrocera neohumeralis are morphologically similar sibling pest fruit fly species that possess different biological attributes, geographic distributions, and host ranges. The need to differentiate between the two species is critical for accurate pest status assessment, management, biosecurity, and maintenance of reference colonies. While morphologically similar, adults may be separated based on subtle characters; however, some characters exhibit intraspecific variability, creating overlap between the two species. Additionally, there is currently no single molecular marker or rapid diagnostic assay that can reliably distinguish between B. neohumeralis and B. tryoni; therefore, ambiguous samples remain undiagnosed. Here we report the first molecular marker that can consistently distinguish between B. tryoni and B. neohumeralis. Our diagnostic region consists of two adjacent single nucleotide polymorphisms (SNPs) within the pangolin (pan) gene region. We confirmed the genotypes of each species are consistent across their distributional range, then developed a tetra-primer amplification refractory mutation system (ARMS) PCR assay for rapid diagnosis of the species. The assay utilizes four primers in multiplex, with two outer universal primers, and two internal primers: one designed to target two adjacent SNPs (AA) present in B. tryoni and the other targeting adjacent SNPs present in B. neohumeralis (GG). The assay accurately discriminates between the two species, but their SNP genotypes are shared with other nontarget tephritid fruit fly species. Therefore, this assay is most suited to adult diagnostics where species confirmation is necessary in determining ambiguous surveillance trap catches; maintaining pure colony lines; and in Sterile Insect Technique management responses.

Evaluation of identification methods for cryptic Bactrocera dorsalis (Diptera: Tephritidae) specimens: combining morphological and molecular techniques.

The potential for population genomics to elucidate invasion pathways of a species is limited by taxonomic identification issues. The Oriental fruit fly pest, Bactrocera dorsalis (Hendel) belongs to a complex in which several sympatric species are attracted to the same lure used in trapping and are morphologically cryptic and/or reported to hybridize. In this study, we evaluated the taxonomic ambiguity between B. dorsalis and 2 major cryptic species, based on morphological expertise and 289 target specimens sampled across the whole distribution range. Specimens were then subjected to DNA sequence analyses of the COI mitochondrial barcode and the EIF3L nuclear marker to evaluate the potential for molecular identification, in particular for specimens for which morphological identification was inconclusive. To this aim, we produced reference datasets with DNA sequences from target specimens whose morphological identification was unambiguous, which we complemented with 56 new DNA sequences from closest relatives and 76 published and curated DNA sequences of different species in the complex. After the necessary morphological observation, about 3.5% of the target dataset and 47.6% of the specimens from Southeast Asian islands displayed ambiguous character states shared with B. carambolae and/or B. occipitalis. Critical interpretation of DNA sequence data solved morphological ambiguities only when combining both mitochondrial and nuclear markers. COI discriminated B. dorsalis from 5 species; EIF3L and ITS from another species. We recommend this procedure to ensure correct identification of B. dorsalis specimens in population genetics studies and surveillance programs.

Development of a cost-effective, morphology-preserving method for DNA isolation from bulk invertebrate trap catches: Tephritid fruit flies as an exemplar.

Insect identification and preservation of voucher specimens is integral to pest diagnostic and surveillance activities; yet bulk-trapped insects are a diagnostic challenge due to high catch numbers and the susceptibility of samples to environmental damage. Many insect trap catches rely on examination of morphological characters for species identifications, which is a time consuming and highly skilled task, hence there is a need for more efficient molecular approaches. Many bulk DNA extraction methods require destructive sampling of specimens, resulting in damaged, or fully destroyed, voucher specimens. We developed an inexpensive, rapid, bulk DNA isolation method that preserves specimens as pinned vouchers to a standard that allows for post-extraction morphological examination and inclusion in insect reference collections. Our protocol was validated using a group of insects that are time-consuming to identify when trapped in large numbers-the dacine fruit flies (Diptera: Tephritidae: Dacinae). In developing our method, we evaluated existing protocols against the following criteria: effect on morphology; suitability for large trap catches; cost; ease of handling; and application to downstream molecular diagnostic analyses such as real-time PCR and metabarcoding. We found that the optimum method for rapid isolation of DNA extraction was immersing flies in a NaOH:TE buffer at 75°C for 10 minutes, without the need for proteinase K or detergents. This HotSOAK method produced sufficient high-quality DNA whilst preserving morphological characters suitable for species-level identification with up to 20,000 flies in a sample. The lysates performed well in down-stream analyses such as loop-mediated isothermal amplification (LAMP) and real-time PCR applications, while for metabarcoding PCR the lysate required an additional column purification step. Development of this method is a key step required for upscaling our capacity to accurately detect insects captured in bulk traps, whether for biodiversity, biosecurity, or pest management objectives.

Loop-mediated isothermal amplification (LAMP) assays for detection of the New Guinea fruit fly Bactrocera trivialis (Drew) (Diptera: Tephritidae).

The cue-lure-responding New Guinea fruit fly, Bactrocera trivialis, poses a biosecurity risk to neighbouring countries, e.g., Australia. In trapping programs, lure caught flies are usually morphologically discriminated from non-target species; however, DNA barcoding can be used to confirm similar species where morphology is inconclusive, e.g., Bactrocera breviaculeus and B. rufofuscula. This can take days-and a laboratory-to resolve. A quicker, simpler, molecular diagnostic assay would facilitate a more rapid detection and potential incursion response. We developed LAMP assays targeting cytochrome c oxidase subunit I (COI) and Eukaryotic Translation Initiation Factor 3 Subunit L (EIF3L); both assays detected B. trivialis within 25 min. The BtrivCOI and BtrivEIF3L assay anneal derivatives were 82.7 ± 0.8 °C and 83.3 ± 1.3 °C, respectively, detecting down to 1 × 101 copies/µL and 1 × 103 copies/µL, respectively. Each assay amplified some non-targets from our test panel; however notably, BtrivCOI eliminated all morphologically similar non-targets, and combined, the assays eliminated all non-targets. Double-stranded DNA gBlocks were developed as positive controls; anneal derivatives for the COI and EIF3L gBlocks were 84.1 ± 0.7 °C and 85.8 ± 0.2 °C, respectively. We recommend the BtrivCOI assay for confirmation of suspect cue-lure-trapped B. trivialis, with BtrivEIF3L used for secondary confirmation when required.

A diagnostic LAMP assay for rapid identification of an invasive plant pest, fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae).

Fall armyworm (FAW), Spodoptera frugiperda (Lepidoptera: Noctuidae), is a highly polyphagous invasive plant pest that has expanded its global geographic distribution, including recently into much of Australia. Rapid diagnostic tests are required for identification of FAW to assist subsequent management and control. We developed a new loop-mediated isothermal amplification (LAMP) assay based on the mitochondrial cytochrome c oxidase subunit I (COI) gene for accurate and timely diagnosis of FAW in the field. The specificity of the new assay was tested against a broad panel of twenty non-target noctuids, including eight other Spodoptera species. Only S. frugiperda samples produced amplification within 20 min, with an anneal derivative temperature of 78.3 ± 0.3 °C. A gBlock dsDNA fragment was developed and trialled as a synthetic positive control, with a different anneal derivative of 81 °C. The new FAW LAMP assay was able to detect FAW DNA down to 2.4 pg, similar to an existing laboratory-based real-time PCR assay. We also trialled the new FAW assay with a colorimetric master mix and found it could successfully amplify positive FAW samples in half the time compared to an existing FAW colorimetric LAMP assay. Given the high sensitivity and rapid amplification time, we recommend the use of this newly developed FAW LAMP assay in a portable real-time fluorometer for in-field diagnosis of FAW.