Light intensity differentially mediates the life cycle of lepidopteran leaf feeders and stem borers.

BACKGROUND: Leaf feeders, such as Spodoptera frugiperda and Spodoptera litura, and stem borers Ostrinia furnacalis and Chilo suppressalis, occupy two different niches and are well adapted to their particular environments. Borer larvae burrow and inhabit the interior of stems, which are relatively dark. By contrast, the larvae of leaf feeders are exposed to sunlight during feeding. We therefore designed series of experiments to evaluate the effect of light intensity (0, 2000, and 10 000 lx) on these pests with different feeding modes. RESULTS: The development of all four pests was significantly delayed at 0 lx. Importantly, light intensity affected the development of both male and female larvae of borers, but only significantly affected male larvae of leaf feeders. Furthermore, the proportion of female offspring of leaf feeders increased with increasing light intensity (S. frugiperda: 33.89%, 42.26%, 57.41%; S. litura: 38.90%, 51.75%, 65.08%), but no significant differences were found in stem borers. This research also revealed that the survival rate of female leaf feeders did not vary across light intensities, but that of males decreased with increasing light intensity (S. frugiperda: 97.78%, 85.86%, 61.21%; S. litura: 95.83%, 73.54%, 58.99%). CONCLUSION: These results improve our understanding of how light intensity affects sex differences in important lepidopteran pests occupying different feeding niches and their ecological interactions with abiotic factors in agroecosystems. © 2024 Society of Chemical Industry.

Parthenogenesis affects interspecific competition between Megalurothrips usitatus and Frankliniella intonsa (Thysanoptera: Thripidae) in changing environment: evidence from life table study.

The thrips Megalurothrips usitatus Bagnall and Frankliniella intonsa Trybom (Thysanoptera: Thripidae) are important pests in cowpea-growing areas of south China. Parthenogenesis is an important reproductive form of thysanopterans, and plays an important role in maintaining population growth. To understand the developmental and reproductive characteristics of these 2 thrips species during parthenogenesis, we compared the age-stage life tables of M. usitatus and F. intonsa on cowpea pods under natural regimes during the summer and winter. The results showed that the total preadult period and total preoviposition period of M. usitatus were significantly longer than those of F. intonsa in both seasons. Moreover, longevity of adult M. usitatus (29.53 days) was shorter compared with adult F. intonsa (34.00 days) in summer, whereas higher fecundity (220.8 eggs/female) and more oviposition days (37.83 days) were observed in M. usitatus compared with F. intonsa in winter (fecundity = 179.83 eggs/female, oviposition days = 33.03 days). The net and gross reproductive rates of M. usitatus were significantly greater than those of F. intonsa during winter. In addition, the intrinsic and finite rates of increase of M. usitatus were significantly lower than those of F. intonsa, and the mean generation time of M. usitatus was significantly longer than that of F. intonsa both in summer and winter. These results indicated that parthenogenesis has species specificity among thrips, which in turn affects population development, especially under changing environments.

Assessing the effectiveness of imidacloprid and thiamethoxam via root irrigation against Megalurothrips usitatus (Thysanoptera: Thripidae) and its residual effects on cowpea.

Systemic neonicotinoid insecticides (NEOs) applied by seed-treatment or root application have emerged as a prevalent strategy for early-season insect pest management. This research investigated the effectiveness of imidacloprid and thiamethoxam, administered through root irrigation, in managing thrips in cowpea [Vigna unguiculata (Linn.) Walp.], and the residual properties of both insecticides in cowpea and soil. The results show that thrips density depends on the application rate of insecticides. At the maximum application rate (1,500 µg/ml, active ingredient), imidacloprid and thiamethoxam controlled thrips densities below the economic injury level (EIL, the EIL of thrips on cowpea was 7/flower) for 20 days and 25 days with the density of 6.90 and 6.93/flower at the end of the periods, respectively. Imidacloprid and thiamethoxam residues decreased gradually over time and decreased sharply after 15 days after treatment (DAT), the 2 insecticides were not detected (<0.001 mg/kg) at 45 DAT. According to our findings, the application of imidacloprid and thiamethoxam via root irrigation proved residual control lasting up to 20-25 days for controlling thrips damage at experimental rates, with a strong association to their residual presence in cowpea (0.6223 < R2 < 0.9545). Considering the persistence of the imidacloprid and thiamethoxam, the maximum tested application rate (1,500 µg/ml) was recommended. As the residues of imidacloprid and thiamethoxam were undetectable in cowpea pods at all tested rates, it may be suggested that the use of each insecticide is safe for consumers and effective against thrips, and could be considered for integrated thrips management in the cowpea ecosystem.

Comparison of the biology of Frankliniella intonsa and Megalurothrips usitatus on cowpea pods under natural regimes through an age-stage, two-sex life table approach.

Two thrips, Megalurothrips usitatus (Bagnall) and Frankliniella intonsa (Trybom) are major pests of cowpea in South China. To realistically compare the growth, development and reproductive characteristics of these two thrips species, we compared their age-stage, two-sex life tables on cowpea pods under summer and winter natural environmental regimes. The results showed that the total preadult period of M. usitatus was 8.09 days, which was significantly longer than that of F. intonsa (7.06 days), while the adult female longevity of M. usitatus (21.14 days) was significantly shorter than that of F. intonsa (25.77 days). Significant differences were showed in male adult longevity (10.68 days for F. intonsa and 16.95 days for M. usitatus) and the female ratio of offspring (0.67 for F. intonsa and 0.51 for M. usitatus), and the total preadult period of M. usitatus (16.20 days) was significantly longer than that of F. intonsa (13.66 days) in the winter regime. The net reproductive rate (summer: R0 = 85.62, winter: R0 = 105.22), intrinsic rate of increase (summer: r = 0.3020 day-1, winter: r = 0.2115 day-1), finite rate of increase (summer: λ = 1.3526 day-1, winter: λ = 1.2356 day-1) and gross reproduction rate (summer: GRR = 139.34, winter: GRR = 159.88) of F. intonsa were higher than those of M. usitatus (summer: R0 = 82.91, r = 0.2741, λ = 1.3155, GRR = 135.71; winter: R0 = 80.62, r = 0.1672, λ = 1.1820, GRR = 131.26), and the mean generation times (summer: T = 14.73 days, winter: T = 22.01 days) of F. intonsa were significantly shorter than those of M. usitatus (summer: T = 16.11 days, winter: T = 26.25 days). These results may contribute to a better understanding of the bioecology of different thrips species, especially the interspecific competition between two economically important cowpea thrips with the same ecological niche in a changing environment.

Synergism of Adjuvants Mixed With Spinetoram for the Management of Bean Flower Thrips, Megalurothrips usitatus (Thysanoptera: Thripidae) in Cowpeas.

The bean flower thrips, Megalurothrips usitatus (Bagnall) is an economically important insect pest of cowpea, Vigna unguiculata (L.) Walp in south China. Spinetoram is a newly available commercial active ingredient in the spinosyn mode of action group of insecticides that has been recommended for thrips management in China. In this study, the toxicity and efficacy of spinetoram for controlling M. usitatus were evaluated and compared to six other conventional insecticides. In addition, the synergistic effects of adjuvants (Silwet 806, Silwet 618, AgroSpred 910, and AgroSpred Prime) mixed with spinetoram for thrips control on cowpea were evaluated in both the laboratory and the field. Results of this study showed that spinetoram had higher toxicity and field efficacy of M. usitatus than other tested insecticides. A significant increase in efficacy was observed when spinetoram was applied at a recommended rate of 0.67 ml/L, mixed with Silwets (806 and 618) at the rate of 0.5 ml/L. Reductions of 50% and 40% in thrips infestation in the field over treatments without adjuvants were observed 3- and 7-days posttreatments, respectively. However, no significant reduction of M. usitatus was recorded when spinetoram was mixed with AgroSpreds (910 and Prime). Furthermore, no significant differences were found in thrips infestation between spinetoram sprayed alone at the rate of 0.67 ml/L and a reduced rate of spinetoram (0.45 ml/L) mixed with Silwets (806 and 618). The current research shows that Silwets mixed with spinetoram has a synergistic effect in the management of thrips.

Evaluation of a cuticle protein gene as a potential RNAi target in aphids.

BACKGROUND: RNA interference (RNAi) has potential as a pest insect control technique. One possible RNAi target is the cuticle protein, which is important in insect molting and development. As an example, here we evaluate the possibility of designing double-stranded RNA (RNA) that is effective for silencing the cuticle protein 19 gene (CP19) in aphids but is harmless to non-target predator insects. RESULTS: The sequences of CP19s were similar (86.6-94.4%) among the tested aphid species (Aphis citricidus, Acyrthosiphon pisum, and Myzus persicae) but different in the predator Propylaea japonica. Ingestion of species-specific dsRNAs of CP19 by the three aphids produced 39.3-64.2% gene silencing and 45.8-55.8% mortality. Ingestion of non-species-specific dsRNA (dsAcCP19) by Ac. pisum and M. persicae gave gene silencing levels ranging from 40.4% to 50.3% and 43.3-50.8% mortality. The dsApCP19 did not affect PjCP19 expression or developmental duration in P. japonica. CONCLUSION: The results demonstrate that CP19 is a promising RNAi target for aphid control via one dsRNA design. The targeting of genes that are conserved in insect pests but not present in beneficial insects is a useful RNAi-based pest control strategy. © 2019 Society of Chemical Industry.

Morphology and Distribution of the Antennal Sensilla of Two Species, Megalurothrips usitatus and Thrips palmi (Thysanoptera: Thripidae).

The morphology and distribution of the antennal sensilla of Megalurothrips usitatus Bagnall and Thrips palmi Karny were examined using scanning electron microscopy (SEM). These are serious pests of various economically important crops, and their antennae are important in chemical communication. The antennae of both species consist of a scape, pedicel, and flagellum, but the flagellum of M. usitatus is made up of six sub-segments, whereas that of T. palmi consists of five sub-segments. Seven morphological sensilla types, including Böhm bristle (BB), sensilla campaniformia (Sca), three types of sensilla basiconica (Sb1, Sb2 and Sb3), two types of sensilla chaetica (Sch1 and Sch2), sensilla styloconica (Sst), sensilla trichodea (St), and sensilla cavity (Scav), were recorded in both species. The scape and pedicels exhibited Sch1, BB and Sca. The flagellum exhibited two types of Sch, three types of Sb, St, Sst and Scav. Based on these results, the putative function of the sensilla of M. usitatus and T. palmi are also discussed.

Status of insecticide resistance and selection for imidacloprid resistance in the ladybird beetle Propylaea japonica (Thunberg).

Field populations or strains of Propylaea japonica collected from four places in southern China (Guangzhou, Nanning, Guilin, and Yuxi) were tested for susceptibility to four insecticides (abamectin, imidacloprid, beta-cypermethrin, and chlorpyrifos) by the Petri-dish Potter tower method and compared with an insecticide-susceptible strain. Concentrations that proved lethal for 50% of the tested individuals (LC50) were estimated by probit analysis, and resistance factors (RF) were calculated at the LC50 level, which ranged from 1.6 to 10.1, depending on the insecticide. In addition, the Guangzhou strain formed the original population for imidacloprid resistance selection. After selection for 20 generations, the resistance had increased 39.3-fold. Fitness analysis in terms of such traits as fecundity, days to maturity, and survival showed that although both resistant and susceptible populations developed at comparable rates, the resistant strain was less fecund (it laid fewer eggs and a smaller proportion of those eggs hatched and resulted in adults), attaining a fitness score of only 0.56 relative to the susceptible strain. These observations suggest that it is possible to detect strains of P. japonica highly resistant to insecticides under laboratory conditions, and that resistance to imidacloprid carries considerable fitness costs to P. japonica. The study served to expand our understanding of the impact of imidacloprid resistance on biological parameters of P. japonica in more detail and to facilitate the deployment of natural enemies resistant to insecticides in integrated pest management.

Suitability of Bactrocera dorsalis (Diptera: Tephritidae) Pupae for Spalangia endius (Hymenoptera: Pteromalidae).

Spalangia endius (Walker) (Hymenoptera: Pteromalidae) is found to be one of the most important natural enemies of Bactrocera dorsalis Hendel (Diptera: Tephritidae) pupae in China. In this study, the influence of host pupal age on the preference for and suitability of the host by the parasitoid S. endius was determined using choice and nonchoice tests. S. endius females accepted the 1-7 d-old B. dorsalis pupae for oviposition, and their offspring developed successfully. However, the S. endius preferentially parasitized the 2-, 3-, and 4-d-old host pupae. The emergence rate of the adult progeny was not affected by the host pupal age, nor was the male body weight, male longevity, and sex ratio of the parasitoid offspring. However, the shortest development time of both male and female progeny and the greatest size and adult longevity of female progeny were observed in hosts that were ≤4 d old. Females emerged later and lived longer than males, and they weighed more than the males. Host mortality decreased as the age of the host increased for 1-7-d-old hosts. Our findings suggest that 2-, 3-, and 4-d-old B. dorsalis pupae would be the best host ages at which to rear S. endius for effective control in field releases.

Parasitism, emergence, and development of Spalangia endius (Hymenoptera: Pteromalidae) in pupae of different ages of Bactrocera cucurbitae (Diptera: Tephritidae).

The wasp Spalangia endius Walker (Hymenoptera: Pteromalidae) is a major parasitoid of the pupae of fruit flies, which are a common agricultural pest. An understanding of this intricate host-parasitoid interaction could provide basic information necessary for the sustainable integrated biological control of fruit flies. In this study, we investigated the effect of S. endius on different-aged pupae of the melon fly Bactrocera cucurbitae Coquillett by using choice and nonchoice tests under laboratory conditions. We showed that S. endius females oviposited, and their progeny successfully developed, in different-aged pupae of B. cucurbitae regardless of the method of exposure. There was an oviposition preference for 3-5-d-old pupa. The highest mean percentage parasitism occurred on 4- and 5-d-old hosts, followed by 2- and 3-d-old hosts. The average development time for both males and females was significantly longer in 6-7-d-old hosts than in the younger host stages. Adult females that developed from younger host pupae (2-5-d old) were significantly heavier than those from older host pupae (6-7-d old), and they also lived longer. The sex ratio (proportion of females) of the parasite progeny decreased with an increase in host age. Host mortality also decreased gradually as the pupal age increased. The differences in development time, body weight, and longevity between females and males were significant. These results suggest that S. endius is a good candidate for the biological control of B. cucurbitae.

De novo sequencing-based transcriptome and digital gene expression analysis reveals insecticide resistance-relevant genes in Propylaea japonica (Thunberg) (Coleoptea: Coccinellidae).

The ladybird Propylaea japonica (Thunberg) is one of most important natural enemies of aphids in China. This species is threatened by the extensive use of insecticides but genomics-based information on the molecular mechanisms underlying insecticide resistance is limited. Hence, we analyzed the transcriptome and expression profile data of P. japonica in order to gain a deeper understanding of insecticide resistance in ladybirds. We performed de novo assembly of a transcriptome using Illumina's Solexa sequencing technology and short reads. A total of 27,243,552 reads were generated. These were assembled into 81,458 contigs and 33,647 unigenes (6,862 clusters and 26,785 singletons). Of the unigenes, 23,965 (71.22%) have putative homologues in the non-redundant (nr) protein database from NCBI, using BLASTX, with a cut-off E-value of 10(-5). We examined COG, GO and KEGG annotations to better understand the functions of these unigenes. Digital gene expression (DGE) libraries showed differences in gene expression profiles between two insecticide resistant strains. When compared with an insecticide susceptible profile, a total of 4,692 genes were significantly up- or down- regulated in a moderately resistant strain. Among these genes, 125 putative insecticide resistance genes were identified. To confirm the DGE results, 16 selected genes were validated using quantitative real time PCR (qRT-PCR). This study is the first to report genetic information on P. japonica and has greatly enriched the sequence data for ladybirds. The large number of gene sequences produced from the transcriptome and DGE sequencing will greatly improve our understanding of this important insect, at the molecular level, and could contribute to the in-depth research into insecticide resistance mechanisms.