Evaluation of reflective mulch and insect exclusion coverings for allium leafminer (Diptera: Agromyzidae) management in allium crops.

Allium leafminer (Phytomyza gymnostoma Loew) is a recent invasive pest in the United States causing serious economic loss in organic allium crops. Organic management of P. gymnostoma is currently limited to foliar applications of spinosad, but this strategy is not always sufficient under high infestations. Nonchemical management tools used either alone or in combination with spinosad are needed to improve P. gymnostoma management. Reflective mulch alone or combined with spinosad as well as insect exclusion coverings were evaluated for managing P. gymnostoma in allium crops in New York from 2018 to 2021. Reflective mulch alone reduced the numbers of oviposition marks by 16% and densities of larvae plus pupae by 40% compared with those in standard plastic mulch. Reflective mulch combined with 1 spinosad application reduced P. gymnostoma densities to levels lower than those in reflective mulch alone, but 2 spinosad applications were required to provide an acceptable control level. Combining with reflective mulch, row covers, and insect netting reduced P. gymnostoma densities by 76% compared with those without physical barriers, and the level of control was comparable to that provided by 2 spinosad applications. Phytomyza gymnostoma densities in allium crops grown under row covers deployed throughout the entire period when flies were active tended to be lower than those protected during shorter periods (80% reduction). Future management of P. gymnostoma in allium crops should consider either combining reflective mulch with 2 foliar spinosad applications or deploying insect exclusion coverings. The advantages and challenges of using these management strategies are discussed.

Allium leafminer (Diptera: Agromyzidae) host preference: implications for developing a trap cropping strategy.

Allium leafminer (Phytomyza gymnostoma Loew) is an emerging invasive pest of Allium crops and has been threatening Allium crop production in the eastern United States since its introduction in 2015. Phytomyza gymnostoma can cause substantial economic loss in leek crops when late instars tunnel into the lower portion of the plant, which often renders the crop unmarketable. With limited management tools that are cost-effective and practical, especially for leeks produced in organic systems, we examined the attractiveness of other Allium crop species that might be considered in a trap cropping strategy. In 2021 and 2022, controlled environment choice tests and field trials were conducted to evaluate host preference of P. gymnostoma among Allium crop species including chives, scallion, an onion and scallion hybrid, and leek. We also assessed preference of P. gymnostoma for scallions that varied in size/age. Results from field trials indicated that only chives had more oviposition marks, cumulative numbers of eggs, and a higher density of P. gymnostoma larvae and pupae than leeks. Larger/older scallions had more oviposition marks and higher P. gymnostoma densities than smaller ones in controlled environment choice tests, but this size/age preference was not evident in field trials. Based on our findings, chives could be considered as a potential trap crop for minimizing P. gymnostoma damage in leek crops.

Impact of Host Resistance to Tomato Spotted Wilt Orthotospovirus in Peanut Cultivars on Virus Population Genetics and Thrips Fitness.

Thrips-transmitted tomato spotted wilt orthotospovirus (TSWV) is a major constraint to peanut production in the southeastern United States. Peanut cultivars with resistance to TSWV have been widely used for over twenty years. Intensive usage of resistant cultivars has raised concerns about possible selection pressure against TSWV and a likelihood of resistance breakdown. Population genetics of TSWV isolates collected from cultivars with varying levels of TSWV resistance was investigated using five TSWV genes. Phylogenetic trees of genes did not indicate host resistance-based clustering of TSWV isolates. Genetic variation in TSWV isolates and neutrality tests suggested recent population expansion. Mutation and purifying selection seem to be the major forces driving TSWV evolution. Positive selection was found in N and RdRp genes but was not influenced by TSWV resistance. Population differentiation occurred between isolates collected from 1998 and 2010 and from 2016 to 2019 but not between isolates from susceptible and resistant cultivars. Evaluated TSWV-resistant cultivars differed, albeit not substantially, in their susceptibility to thrips. Thrips oviposition was reduced, and development was delayed in some cultivars. Overall, no evidence was found to support exertion of selection pressure on TSWV by host resistance in peanut cultivars, and some cultivars differentially affected thrips fitness than others.

Discrepancies in Serology-Based and Nucleic Acid-Based Detection and Quantitation of Tomato Spotted Wilt Orthotospovirus in Leaf and Root Tissues from Symptomatic and Asymptomatic Peanut Plants.

Thrips-transmitted tomato spotted wilt orthotospovirus (TSWV) causes spotted wilt disease in peanuts. A serological test (DAS-ELISA) is often used to detect TSWV in peanut leaf samples. However, in a few studies, DAS-ELISA detected more TSWV infection in root than leaf samples. It was not clear if the increased detection was due to increased TSWV accumulation in root tissue or merely an overestimation. Additionally, it was unclear if TSWV detection in asymptomatic plants would be affected by the detection technique. TSWV infection in leaf and root tissue from symptomatic and asymptomatic plants was compared via DAS-ELISA, RT-PCR, and RT-qPCR. TSWV incidence did not vary by DAS-ELISA, RT-PCR, and RT-qPCR in leaf and root samples of symptomatic plants or in leaf samples of asymptomatic plants. In contrast, significantly more TSWV infection and virus load were detected in root samples of asymptomatic plants via DAS-ELISA than other techniques suggesting that DAS-ELISA overestimated TSWV incidence and load. TSWV loads from symptomatic plants via RT-qPCR were higher in leaf than root samples, while TSWV loads in leaf and root samples from asymptomatic plants were not different but were lower than those in symptomatic plants. These findings suggested that peanut tissue type and detection technique could affect accurate TSWV detection and/or quantitation.

Resistance to Thrips in Peanut and Implications for Management of Thrips and Thrips-Transmitted Orthotospoviruses in Peanut.

Thrips are major pests of peanut (Arachis hypogaea L.) worldwide, and they serve as vectors of devastating orthotospoviruses such as Tomato spotted wilt virus (TSWV) and Groundnut bud necrosis virus (GBNV). A tremendous effort has been devoted to developing peanut cultivars with resistance to orthotospoviruses. Consequently, cultivars with moderate field resistance to viruses exist, but not much is known about host resistance to thrips. Integrating host plant resistance to thrips in peanut could suppress thrips feeding damage and reduce virus transmission, will decrease insecticide usage, and enhance sustainability in the production system. This review focuses on details of thrips resistance in peanut and identifies future directions for incorporating thrips resistance in peanut cultivars. Research on thrips-host interactions in peanut is predominantly limited to field evaluations of feeding damage, though, laboratory studies have revealed that peanut cultivars could differentially affect thrips feeding and thrips biology. Many runner type cultivars, field resistant to TSWV, representing diverse pedigrees evaluated against thrips in the greenhouse revealed that thrips preferred some cultivars over others, suggesting that antixenosis "non-preference" could contribute to thrips resistance in peanut. In other crops, morphological traits such as leaf architecture and waxiness and spectral reflectance have been associated with thrips non-preference. It is not clear if foliar morphological traits in peanut are associated with reduced preference or non-preference of thrips and need to be evaluated. Besides thrips non-preference, thrips larval survival to adulthood and median developmental time were negatively affected in some peanut cultivars and in a diploid peanut species Arachis diogoi (Hoehne) and its hybrids with a Virginia type cultivar, indicating that antibiosis (negative effects on biology) could also be a factor influencing thrips resistance in peanut. Available field resistance to orthotospoviruses in peanut is not complete, and cultivars can suffer substantial yield loss under high thrips and virus pressure. Integrating thrips resistance with available virus resistance would be ideal to limit losses. A discussion of modern technologies such as transgenic resistance, marker assisted selection and RNA interference, and future directions that could be undertaken to integrate resistance to thrips and to orthotospoviruses in peanut cultivars is included in this article.