Pest categorisation of Spodoptera litura

Off-season survival and life history of beet armyworm, Spodoptera exigua (Hubner) on various host plants

The beet armyworm, Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae), has become a significant pest of chickpea in recent years. The polyphagous nature allows it to survive on various hosts during the off-season, creating a great menace to the crop in the following season. To assess the incidence and document the alternate hosts of S. exigua, a rapid roving survey was conducted in 11 chickpea-growing areas of Prakasam district, Andhra Pradesh, India. Additionally, the life history traits of S. exigua were studied on major alternate host plants under laboratory conditions (27 ± 1 °C and 70 ± 2% RH) to understand the survival, life expectancy and potential contribution to future populations. The results show that, among the different crops surveyed, the maximum larval incidence was noticed in maize (1.93 larvae/plant), cowpea (1.73 larvae/plant), and sunflower (1.68 larvae/plant) during the off-season. Life history studies of S. exigua showed that highest larval survival percentage was observed on chickpea (83.6%), while the lowest was on maize (44.5%). The mean developmental time for larvae was longest on maize (27.1 days) and shortest on chickpea (14.9 days). Larvae did not develop beyond the third instar when fed with chilli. The growth index statistics showed chickpea (9.2) was the most suitable host plant, whereas maize (0.9) was the least suitable host. The age-stage-specific survival rate (Sxj) varied across developmental stages, and the survival curves overlapped, indicating different growth rates among individuals. The life expectancy (exj) at age zero was highest on groundnut (37.06 days). The intrinsic rate of increase (r) of S. exigua was lowest on maize (0.10 ± 0.0013) and highest on chickpea (0.22 ± 0.0010). Similarly, the net reproductive rate (R0) was highest on chickpea (846.39 ± 18.22) and lowest on maize (59.50 ± 2.06). The population doubled every 3.08 ± 0.011 days on chickpea compared to 7.22 ± 0.80 days on maize. The study conclusively indicates that chickpea and sunflower, primarily cultivated during the rabi season in India, are the most preferred hosts for S. exigua. In contrast, maize and cotton, mainly grown during the kharif season, are less preferred and merely support the pest's survival. Consequently, S. exigua switches hosts between different crops growing seasons, so effective management of S. exigua during the kharif season can help prevent pest outbreaks during the rabi season.

Measuring the Flight Trajectory of a Free-Flying Moth on the Basis of Noise-Reduced 3D Point Cloud Time Series Data

Pest control is crucial in crop production; however, the use of chemical pesticides, the primary method of pest control, poses environmental issues and leads to insecticide resistance in pests. To overcome these issues, laser zapping has been studied as a clean pest control technology against the nocturnal cotton leafworm, Spodoptera litura, which has high fecundity and causes severe damage to various crops. For better sighting during laser zapping, it is important to measure the coordinates and speed of moths under low-light conditions. To achieve this, we developed an automatic detection pipeline based on point cloud time series data from stereoscopic images. We obtained 3D point cloud data from disparity images recorded under infrared and low-light conditions. To identify S. litura, we removed noise from the data using multiple filters and a support vector machine. We then computed the size of the outline box and directional angle of the 3D point cloud time series to determine the noisy point clouds. We visually inspected the flight trajectories and found that the size of the outline box and the movement direction were good indicators of noisy data. After removing noisy data, we obtained 68 flight trajectories, and the average flight speed of free-flying S. litura was 1.81 m/s.

Biodegradable electrospun fibers as sustained-release carriers of insect pheromones for field trapping of Spodoptera litura (Lepidoptera: Noctuidae)

BACKGROUND

Insect pheromones are highly effective and environmentally friendly, and are widely used in the monitoring and trapping of pests. However, many researchers have found that various factors such as ultraviolet light and temperature in the field environment can accelerate the volatilization of pheromones, thus affecting the actual control effect. In recent years, electrospinning technology has demonstrated remarkable potential in the preparation of sustained carriers. Moreover, the utilization of biodegradable materials in electrospinning presents a promising avenue for the advancement of eco-friendly carriers.

RESULTS

In this study, homogeneous and defect-free pheromone carriers were obtained by electrospinning using fully biodegradable polyhydroxybutyrate materials and pheromones of Spodoptera litura. The electrospun fibers with porous structure could continuously release pheromone (the longest can be ≤80 days). They also had low light transmission, hydrophobic protection. More importantly, the pheromone-loaded electrospun fiber carriers showed stable release and good trapping effect in the field. They could trap pests for at least 7 weeks in the field environment without other light stabilizers added.

CONCLUSION

Sustained-release carriers constructed by electrospinning and green materials could improve the efficacy of pheromones and ensure environmental friendliness, and provided a tool for the management of S. litura and other pests and sustainable development of agricultural. © 2023 Society of Chemical Industry.

Identification and Expression Profiles of Candidate Sex Pheromone Biosynthesis Genes by the Transcriptome Analysis of Sex Pheromone Glands in Spodoptera litura and Spodoptera exigua

Like many insects, females of the Noctuid moth Spodoptera litura and Spodoptera exigua release chemical signals to attract males from a long distance for successful mating. In this study, 98 and 86 genes related to the sex pheromone biosynthesis of S. litura and S. exigua were identified. The tissue expression profiles of highly expressed genes in sex pheromone glands (PGs) were further examined by real-time quantitative polymerase chain reaction. The results displayed that only SlitDes5 and SexiDes5 gene were specifically and significantly overexpressed in the PGs of S. litura and S. exigua. The functional study of SlitDes5 gene showed that RNA interference reduced its expression level by 49.42%. In addition, the content of the sex pheromones of S. litura, Z9E11-14:OAc, Z9E12-14:OAc, E11-14:OAc, and Z9-14:OAc, decreased by 41.98% on average. Our findings provide a basis for better understanding the key genes that affect the biosynthesis of sex pheromones and for determining potential gene targets for pest control strategies.

RNAi-based knockdown of candidate gut receptor genes altered the susceptibility of Spodoptera frugiperda and S. litura larvae to a chimeric toxin Cry1AcF

Background

A multitude of Cry toxins (secreted by Bacillus thuringiensis or Bt) has been deployed globally either via transgenic mean or bio-pesticidal formulations in order to manage insect pests. However, Bt resistance development in insects is emerging as a major concern. To avoid this problem, multiple gene pyramiding or protein-engineered chimeric toxin-based strategy has been analyzed.

Methods

In the present study, one such chimeric toxin Cry1AcF (contain the swapped domains of Cry1Ac and Cry1F) was used to investigate its in vivo pathogenesis process in lepidopteran pests Spodoptera frugiperda and S. litura. A number of biochemical and molecular analysis were performed.

Results

Oral ingestion of Cry1AcF caused greater toxicity in S. frugiperda than S. litura with larvae displaying increased hemolymph melanization. Histopathology of the midgut transverse sections exhibited Cry1AcF-induced extensive gut damage in both the test insects followed by cytotoxicity in terms of reduced hemocyte numbers and viability. Elevated hemolymph phenoloxidase activity indicated the immune-stimulatory nature of Cry1AcF. In order to analyze the role of gut receptor proteins in Cry1AcF intoxication in test insects, we performed RNAi-mediated silencing using bacterially-expressed dsRNAs of individual receptor-encoding genes including CAD, ABCC2, ALP1 and APN. Target-specific induced downregulation of receptor mRNAs differentially altered the insect susceptibility to Cry1AcF toxin in our study. The susceptibility of ALP1 and APN dsRNA pre-treated S. frugiperda was considerably decreased when treated with Cry1AcF in LD₅₀ and LD₉₀ doses, whereas susceptibility of CAD and ABCC2 dsRNA pre-treated S. litura was significantly reduced when ingested with Cry1AcF in different doses. CAD/ABCC2-silenced S. frugiperda and ALP1/APN-silenced S. litura were vulnerable to Cry1AcF alike of control larvae. In conclusion, our results indicate ALP1/APN and CAD/ABCC2 as the functional receptor for Cry1AcF toxicity in S. frugiperda and S. litura, respectively.

Radiation Hormesis to Improve the Quality of Adult Spodoptera litura (Fabr.)

Mass rearing of insects of high biological quality is a crucial attribute for the successful implementation of sterile insect release programs. Various ontogenetic stages of Spodoptera litura (Fabr.) were treated with a range of low doses of ionizing radiation (0.25-1.25 Gy) to assess whether these gamma doses could elicit a stimulating effect on the growth and viability of developing moths. Doses in the range of 0.75 Gy to 1.0 Gy administered to eggs positively influenced pupal weight, adult emergence, and growth index, with a faster developmental period. The enhanced longevity of adults derived from eggs treated with 0.75 Gy and 1.0 Gy, and for larvae and pupae treated with 1.0 Gy, indicated a hormetic effect on these life stages. Furthermore, the use of these hormetic doses upregulated the relative mRNA expression of genes associated with longevity (foxo, sirtuin 2 like/sirt1, atg8) and viability/antioxidative function (cat and sod), suggesting a positive hormetic effect at the transcriptional level. These results indicated the potential use of low dose irradiation (0.75-1 Gy) on preimaginal stages as hormetic doses to improve the quality of the reared moths. This might increase the efficiency of the inherited sterility technique for the management of these lepidopteran pests.

Effect of Irradiation on Reproduction of Female Spodoptera litura (Fabr.) (Lepidoptera: Noctuidae) in Relation to the Inherited Sterility Technique

Radiobiological investigations on the reproductive behavior of female Spodoptera litura (Fabr.) were conducted with the aim of determining the suitable radio-sterilizing dose for females in order to release them along with sub-sterile males for effective implementation of the Inherited Sterility technique against this pest. Calling and copulation duration significantly increased, while mating success, oviposition, fertility and longevity significantly decreased with increasing radiation dose (100-200 Gy) compared to control. In view of the effect of irradiation on mating behavior and reproductive viability of female S. litura, 130 Gy was identified as a suitable radio-sterilization dose. Further molecular studies were conducted to corroborate this dose for female sterilization, along with a higher dose of 200 Gy in order to validate the gradational response of ionizing radiation. GC-MS analysis indicated decreased sex pheromone titer at 130 Gy, which was more pronounced at 200 Gy. Pheromone-associated genes, PBAN and PBAN-R showed decreased expression at 130 Gy, and were drastically reduced at 200 Gy. The fertility-related Vg gene also showed a negative correlation with radiation exposure. Based on these radiation responses of female S. litura, 130 Gy might be considered a suitable dose for complete female sterility and its inclusion in sterile insect programs against S. litura.

Transcriptomics and metagenomics of common cutworm (Spodoptera litura) and fall armyworm (Spodoptera frugiperda) demonstrate differences in detoxification and development

BACKGROUND

Spodoptera litura is an important polyphagous pest that causes significant damage to the agricultural sector. We performed RNA-seq of 15 S. litura individuals from larval (fifth and sixth instar larvae), chrysalis, and adult developmental stages. We also compared the S. litura transcriptome data with Spodoptera frugiperda across the same developmental stages, which was sequenced in our previous study.

RESULTS

A total of 101,885 differentially expressed transcripts (DETs) were identified in S. litura. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses indicated that S. litura may undergo active xenobiotic and detoxifying metabolism during its larval and adult stages, which may explain difficulties with current population control measures. We also found that DETs of single-copy orthologous genes between S. litura and S. frugiperda were involved in basic metabolism and development. However, energy and metabolic processes genes had a higher expression in S. litura, whereas nervous and olfactory function genes had a higher expression in S. frugiperda. Metagenomics analysis in larval S. litura and S. frugiperda revealed that microbiota participate in the detoxification and metabolism processes, but the relative abundance of detoxification-related microbiota was more abundant in S. frugiperda. Transcriptome results also confirmed the detoxification-related pathway of S. frugiperda was more abundant than in S. litura.

CONCLUSIONS

Significant changes at transcriptional level were identified during the different development stages of S. litura. Importantly, we also identified detoxification associated genes and gut microbiota between S. litura and S. frugiperda at different developmental stages, which will be valuable in revealing possible mechanisms of detoxification and development in these two lepidopterans.

Worldwide border interceptions provide a window into human-mediated global insect movement

As part of national biosecurity programs, cargo imports, passenger baggage, and international mail are inspected at ports of entry to verify compliance with phytosanitary regulations and to intercept potentially damaging nonnative species to prevent their introduction. Detection of organisms during inspections may also provide crucial information about the species composition and relative arrival rates in invasion pathways that can inform the implementation of other biosecurity practices such as quarantines and surveillance. In most regions, insects are the main taxonomic group encountered during inspections. We gathered insect interception data from nine world regions collected from 1995 to 2019 to compare the composition of species arriving at ports in these regions. Collectively, 8,716 insect species were intercepted in these regions over the last 25 yr, with the combined international data set comprising 1,899,573 interception events, of which 863,972 were identified to species level. Rarefaction analysis indicated that interceptions comprise only a small fraction of species present in invasion pathways. Despite differences in inspection methodologies, as well as differences in the composition of import source regions and imported commodities, we found strong positive correlations in species interception frequencies between regions, particularly within the Hemiptera and Thysanoptera. There were also significant differences in species frequencies among insects intercepted in different regions. Nevertheless, integrating interception data among multiple regions would be valuable for estimating invasion risks for insect species with high likelihoods of introduction as well as for identifying rare but potentially damaging species.

The First Report on the Transovarial Transmission of Microsporidian Nosema bombycis in Lepidopteran Crop Pests Spodoptera litura and Helicoverpa armigera

Microsporidia are ubiquitous fungi-related parasites infecting nearly all vertebrates and invertebrates. Microsporidian Nosema bombycis is a natural pathogen of multiple insects, including the silkworm and many agricultural and forest pests. N. bombycis can transovarially transmit in silkworm and cause huge economic losses to the sericulture. However, it remains unclear whether N. bombycis vertically transmits in the crop pests Spodoptera litura and Helicoverpa armigera. Here, we investigated the infection of N. bombycis in S. litura and H. armigera to illuminate its infectivity and transovarial transmission. In result, tissue examination with light microscopy revealed that the fat body, midgut, malpighian tubules, hemolymph, testis, and ovary were all infected in both pest pupae. Immunohistochemical analysis (IHA) of the ovariole showed that a large number of parasites in maturation and proliferation presented in follicle cell, nurse cell, and oocyte, suggesting that N. bombycis can infect and multiply in these cells and probably transovarially transmit to the next generations in both pests. Microscopic examination on the egg infection rate demonstrated that 50% and 38% of the S. litura and H. armigera eggs were congenitally infected, respectively. IHA of both eggs manifested numerous spores and proliferative pathogens in the oocyte, confirming that N. bombycis can invade into the female germ cell from the parent body. After hatching of the infected eggs, we detected the infection in offspring larvae and found large quantities of proliferative pathogens, confirming that N. bombycis can transovarially transmit in S. litura and H. armigera, and probably persists in both pest populations via congenital infection. In summary, our work, for the first time, proved that N. bombycis is able to vertically transmit in S. litura and H. armigera via infecting the oocyte in the parent, suggesting that N. bombycis could be a biological insecticide for controlling the population of crop pests.