Early Instar Larvae Research Articles

Lethal and sub-lethal effects of bio-and chemical insecticides on the tomato leaf miner, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae)

The tomato leaf miner, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae), is a destructive pest affecting the tomato crop causing a loss of 60–100% of the tomato crop production. In the present study, the lethal and sub-lethal effects of Bacillus thuringiensis (Bt), spinosad, and emamectin benzoate as bio-insecticides comparing with 2 synthetic insecticides: chlorpyrifos and indoxacarb against T. absoluta under laboratory conditions were evaluated. Take in consideration, life tables and reproductive parameters of treated larvae with LC30 for each pesticide (bio- and chemical insecticides) by using leaf dipping technique were calculated. The results showed that Lepinox formulation of Bt was the most effective one against the early 2nd instar larvae with 0.01 × 10−2 mg/l (ppm), followed by spinosad with 0.07 × 10−2 ppm comparing to indoxacarb and chlorpyrifos (0.01 and 11.44 μg/ml, respectively). Sub-lethal effect of the Bt formulations significantly increased pupal mortality by 29.77, 33.30, and 26.94% for Lepinox, Dipel 2×, and Protecto, respectively. In addition to their efficiency, Bt formulations and emamectin benzoate increased larval developmental period and reduced adult emergency percentage to 70.28, 66.70, 73.06, and 75.55% for Lepinox, Dipel 2×, Protecto, and emamectin benzoate, respectively. All bio-insecticides including Bt, spinosad, and emamectin benzoate caused abnormalities in different life stages. The results suggest these bio-insecticides are promising alternatives to synthetic insecticides for T. absoluta management.

Effect of three insect-resistant maizes expressing Cry1Ie, Cry1Ab/Cry2Aj and Cry1Ab on the growth and development of armyworm Mythimna separata (Walker)

Three transgenic maize events (IE09S034, Shuangkang 12–5 and C0030.3.5) produced Cry1le, Cry1Ab/Cry2Aj and G10-EPSPS, Cry1Ab and EPSPS, respectively, all of which target the Asian corn borer. The oriental armyworm Mythimna separata (Walker) is the secondary target. In this study, the effects of the three Bt maizes on the development and survival of armyworm were studied. The results showed that IE09S034 had insecticidal activity against 1st instar larvae, and the survival rate of armyworm fed with Bt maize for 10 days was 46.2%, significantly lower than that of the control. The larvae at 3rd–6th instar were more tolerant of the Bt toxin than the early instar larvae. However, Shuangkang 12-5 had good insecticidal activity against 1st–5th instar larvae. The mortality was nearly 100% when the larvae were fed with Shuangkang 12-5 before 3rd instar, and the toxin had quick-acting efficacy. This event significantly inhibited the development of armyworm; that is, the larval duration of the 3rd and 4th instar larvae fed with Shuangkang 12-5 was prolonged by 4.5 and 3.0 days, respectively. The pupal weight and egg number were also significantly lower than those of the control. For C0030.3.5, it could control 1st–5th instar larvae effectively. The mortality rates were all over 50% if 1st–3rd larvae were fed with this event. The pupal weight of 4th–6th instar larvae fed with Bt maize were only 53.9, 56.8 and 54.6%, respectively, compared to that of the control. The number of eggs laid was significantly less than the control. The results indicate that all three transgenic maize events exhibit the potential to provide effective control of early instar larvae of armyworm, which can be commercialized in future to control lepidoptera pests such as Asian corn borer and armyworm.

Effects of entomopathogenic nematodes and/or fungus on the red palm weevil, Rhynchophorus ferrugineus (Curculionidae: Coleoptera)

Summary The invasive red palm weevil (RPW) (Rhynchophorus ferrugineus) (Coleoptera: Curculionidae) is one of the most destructive palm pests in the world. More options are needed for its management, including optimised use of microbial entomopathogens (EP). The sub-lethal effects of an entomopathogenic fungus (EPF) Beauveria bassiana (isolate WG-11) (Ascomycota: Hypocreales) (at 1 × 104 spores ml−1) and two entomopathogenic nematode (EPN) species, Heterorhabditis bacteriophora (at 100 infective juveniles (IJ) ml−1) and Steinernema carpocapsae (at 50 IJ ml−1) were evaluated in various combinations against laboratory-reared 3rd, 5th and 8th instar larvae of RPW in bioassays. Individual and combined effects of both EP were observed after the pre-application of B. bassiana fungus at 0-, 1- and 2-week intervals. Combined treatments were tested for antagonistic, additive and synergistic effects. Additive and synergistic effects were observed in B. bassiana/S. carpocapsae and in B. bassiana/H. bacteriophora combined treatments. Early instar larvae of RPW were more susceptible than older instars. Sole and combined EP treatments decreased pupation, subsequent emergence of RPW adults and hatching. Sub-lethal effects of EP were observed in R. ferrugineus following application at the above-mentioned rates to 3rd, 5th and 8th instar larvae, including reduced larval weights, increased larval, pre-pupal and pupal durations, reduced adult weights and life spans. Simultaneous combined applications of B. bassiana/S. carpocapsae and B. bassiana/H. bacteriophora at the above-mentioned rates against last instar (9th) larvae of R. ferrugineus reduced food consumption, frass production and affected weight gains.

Novel Biogenic Synthesis of Silver Nanoparticles Using Alstonia venenata Leaf Extract: An Enhanced Mosquito Larvicidal Agent with Negligible Impact on Important Eco-biological Fish and Insects

Bio-fabrication of metal nanocrystals can be achieved using eco-friendly and cost-effective routes with plants as reducing and capping mediators. A rapid and simple method for producing silver nanoparticles (AgNPs) using the plant Alstonia venenata (R. Br.) (Family: Apocynaceae) was investigated. The mosquito larvicidal potential and the effect of the A. venenata aqueous leaf extract and AgNPs on non-target fish and insects were evaluated. The AgNPs were studied using UV–Vis spectroscopy, FTIR spectroscopy, SEM, TEM, AFM, and XRD analysis. The larvicidal effectiveness on early third instar larvae was higher for the AgNPs than the plant extract; this was observed by testing on Anopheles stephensi (LC50 = 12.28 µg/mL), Aedes aegypti (LC50 = 13.49 µg/mL), and Culex quinquefasciatus (LC50 = 14.50 µg/mL). Furthermore, the plant extract and AgNPs were found to be safe for the environment-friendly Gambusia affinis fish, and Anisops bouvieri and Diplonychus indicus aquatic insects. This study confirmed that A. venenata is a potential bio-resource for the fabrication of nanocrystals as an effective mosquito control tool with negligible harmful on aquatic fish and insects in the environment.

Feeding effects of dsNPF interference in Ostrinia furnacalis

The corn borer is a world-wide agricultural pest. In this study, a high-efficiency RNAi method was explored to knock down the neuropeptide F (NPF) to determine if NPF regulates larval feeding in the Asian corn borer, Ostrinia furnacalis. Results showed that the expression peaks of npf mRNA in both midgut and fat body are at 28 h of early 5th instar larvae. When NPF was knocked down either by feeding dsNPF-containing artificial diet or direct dsNPF injection into the 5th instar larvae, npf expression was effectively inhibited in the midgut. Larval feeding, body weight and development time were significantly impacted. In contrast, the expression of the npf receptors npfr1 and npfr2 in fat body of 5th instar larvae was significantly increased by dsNPF injection. These results indicate that NPF can be effectively knocked down in O. furnacalis, and dsNPF by injection is an effective and fast way to silence npf expression. This study provides a critical basis for further exploration in mechanism of feeding regulation in O. furnacalis.

Biological control of Aedes mosquito larvae with carnivorous aquatic plant, Utricularia macrorhiza

BackgroundBiological controls with predators of larval mosquito vectors have historically focused almost exclusively on insectivorous animals, with few studies examining predatory plants as potential larvacidal agents. In this study, we experimentally evaluate a generalist plant predator of North America, Utricularia macrorhiza, the common bladderwort, and evaluate its larvacidal efficiency for the mosquito vectors Aedes aegypti and Aedes albopictus in no-choice, laboratory experiments. We sought to determine first, whether U. macrorhiza is a competent predator of container-breeding mosquitoes, and secondly, its predation efficiency for early and late instar larvae of each mosquito species.MethodsNewly hatched, first-instar Ae. albopictus and Ae. aegypti larvae were separately exposed in cohorts of 10 to field-collected U. macrorhiza cuttings. Data on development time and larval survival were collected on a daily basis to ascertain the effectiveness of U. macrorhiza as a larval predator. Survival models were used to assess differences in larval survival between cohorts that were exposed to U. macrorhiza and those that were not. A permutation analysis was used to investigate whether storing U. macrorhiza in laboratory conditions for extended periods of time (1 month vs 6 months) affected its predation efficiency.ResultsOur results indicated a 100% and 95% reduction of survival of Ae. aegypti and Ae. albopictus larvae, respectively, in the presence of U. macrorhiza relative to controls within five days, with peak larvacidal efficiency in plant cuttings from ponds collected in August. Utricularia macrorhiza cuttings, which were prey-deprived, and maintained in laboratory conditions for 6 months were more effective larval predators than cuttings, which were maintained prey-free for 1 month.ConclusionsDue to the combination of high predation efficiency and the unique biological feature of facultative predation, we suggest that U. macrorhiza warrants further development as a method for larval mosquito control.

Chemical composition and larvicidal activity of essential oils from Peganum harmala, Nepeta cataria and Phellodendron amurense against Aedes aegypti (Diptera: Culicidae)

Essential oils from aerial parts of the herbs Peganum harmala and Nepeta cataria, and leaves of the tree Phellodendron amurense were analyzed by GC-FID and GC-MS, and their larvicidal activities were assayed on the early fourth instar larvae of Aedes aegypti. The major constituents of the oils were limonene (14.5%) and thymol (11.5%) in P. harmala, thymol (46.5%), 4aα,7α,7aβ-nepetalactone (18.3%) and 4aα,7β,7aα-neptalactone (19.7%) in N. cataria, eugenol (14.5%) andγ-eudesmol (9.5%) in P. amurense.The oil of N. cataria had a strong larvicidal activity (LC50 < 50 µg/mL; LC90 < 86.8 µg/mL) on A. aegypti while the remaining oils showed a moderated killing effect. The larvicidal activity of N. cataria oil was associated to the contents of 1,8-cineol, camphor, 4aα,7α,7aβ-Nepetalactone, 4aα,7β,7aα-Nepetalactone and thymol. Our results indicate that the oil of N. catariadeserves to be used as a source of larvicidal agents against A. aegypti.

Identification of Genes That Result in High Mortality of Oryctes rhinoceros (Scarabaeidae: Coleoptera) When Targeted Using an RNA Interference Approach: Implications for Large Invasive Insects

Abstract Coconut rhinoceros beetle, Oryctes rhinoceros (L., 1758), is a large scarab beetle native to Southeast Asia and a major pest of coconut (Cocos nucifera) and oil (Elaeis guineensis) palms in its invaded range. Few tools are available for coconut rhinoceros beetle management, particularly for an emerging haplotype with resistance to known strains of Oryctes rhinoceros nudivirus, the traditional biological control agent used in coconut rhinoceros beetle management programs. RNA interference (RNAi) represents an emerging tool for insect pest control that exploits an existing pathway for gene regulation in the target organism. In this study, we evaluated RNAi as a potential tool for coconut rhinoceros beetle management. Using transcriptome data generated from gut tissue of early instar larvae, we identified 24 RNAi target sequences that were either highly expressed or had demonstrated efficacy in other insect systems. Double-stranded (ds)RNAs ranging from 249 to 297 bp in length were generated for 23 of these target sequences and 150 ng were microinjected into coconut rhinoceros beetle 1st, 2nd, and 3rd instar larvae and adults. Five of these dsRNAs that targeted genes putatively encoding V-type ATPase, polyadenylate binding protein, and three forms of actin induced 30.8–100% mortality within 14 days post injection (dpi). Microinjection of 2nd instars with 10 and 100 ng of these same five dsRNAs induced 20–100% and 80–100% mortality at 7 and 14 dpi, respectively. These results indicate RNAi should be explored as a possible management option for coconut rhinoceros beetle. Coconut rhinoceros beetle may also represent a model species for using RNAi in the management of large invasive insect species.

&lt;p&gt;&lt;strong&gt;New rearing method, life cycle, tunneling behavior and ecological notes on the splendid ghost moth &lt;em&gt;Aenetus djernaesae&lt;/em&gt; Simonsen, 2018 from Western Australia (Lepidoptera: Hepialidae)&lt;/strong&gt;&lt;/p&gt;

This study successfully documents, for the first time, the entire life cycle of Aenetus djernaesae Simonsen, 2018 and confirms the efficacy of using supplemental sources of fungi to feed the early instar larvae. Fresh cut pieces of the commercial mushroom Agaricus bisporus (J.E. Lange) and sections of Eucalyptus L’Her. bark were placed around the base of potted host plants –Myoporum insulare R.Br. (Scrophulariaceae) and the potential host plant Dodonea hackettiana W.Fitz. (Sapindaceae). First instar larvae were added to this matrix where they fed on the mushroom and bark. The life cycle comprised egg development of 20 days, fungal feeding of ~36 days, and host plant development (including pupal) of ~300 days. Adult emergence of reared and field collected samples occurred within a 22 day period. Larvae transferring from fungi to host plants transitioned during the night by constructing a web of silk and plant tissues within two hours and proceeding to excavate a tunnel from within. The mature larval tunnel is relatively short, up to 220 mm in length and usually extending below the entrance around which the larvae grazes on callus tissue forming after bark removal. Most adults emerged within an hour of dusk with the pupa protruding from the top of the vestibule. The rearing method described here demonstrates the feasibility of laboratory based studies of larval development in Aenetus Herrrich-Schäffer and other callus-feeding stem boring Hepialidae. Key words: Hepialidae, Aenetus, life cycle, artificial diet, Myoporum, Dodonea, larval foodplant

NPC1b as a novel target in controlling the cotton bollworm, Helicoverpa armigera.

Insects cannot synthesize sterols and must acquire them from food. The mechanisms underlying how insects uptake dietary sterols are largely unknown except that NPC1b, an integral membrane protein, has been shown to be responsible for dietary cholesterol uptake in Drosophila melanogaster. However, whether NPC1b orthologs in other insect species, particularly the economically important pests, function similarly remains to be determined. In this study, we characterized the function of NPC1b in Helicoverpa armigera, a global pest that causes severe yield losses to many important crops. Limiting dietary cholesterol uptake to insects significantly inhibited food ingestion and weight gain. Compared to the wild-type H. armigera, the CRISPR/Cas9-edited NPC1b mutant larvae were incapable of getting adequate cholesterol and died in their early life stage. Gene expression profile and in situ hybridization analyses indicated that NPC1b was mainly expressed in the midgut where dietary cholesterol was absorbed. Expression of NPC1b was also correlated with the feeding life stages and was especially upregulated during early larval instars. Protein-ligand docking and sequence similarity analyses further demonstrated that NPC1b proteins of lepidopteran insects shared a relatively conserved cholesterol binding region, NPC1b_NTD, which, however, was highly divergent from bees-derived sequences. NPC1b was crucial for dietary cholesterol uptake and growth of H. armigera, and therefore could serve as an insecticide target for the development of a novel pest-management approach to control this economically significant insect pest with little off-target effect on bees and sterol-autotrophic animals. © 2020 Society of Chemical Industry.

Hiding among traps? Mortality of early instar odonate larvae in the presence of bladderwort plants

We investigated the effects of the presence of bladderwort plants on survival of early instar larvae of one coenagrionid and two libellulids in laboratory experiments. In all three species survival...

Larvicidal Activity of Neolamarckia cadamba Against the Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus

The excessive use of conventional insecticides to Culicidae leads to the emergence of mosquitoe resistant, adverse environmental and human effects. Therefore, the present study was carried out to explore the larvicidal efficacy of petroleum ether, chloroform, ethyl acetate and methanol extracts of roots, leaves, and bark of Neolamarckia cadamba against the early 4th instar larvae of Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus at concentrations ranging from 12.5 to 200 ppm under laboratory conditions. The root extracts showed the highest larvicidal potential than the leaf and bark extracts. After 24 h of exposure period, the highest larvicidal activities were observed in the methanol extract of roots with LC50 value of 43.29 and LC90 value of 202.85 ppm against An. stephensi; LC50 value of 70.82 and LC90 value of 253.73 ppm against Ae. Aegypti and LC50 value of 99.51 and LC90 value of 311.02 ppm against the Cx. Quinquefasciatus, respectively. Thus, the plant extracts of N. cadamba showed considerable mosquito larvicidal activities and can be considered for further investigation.

Host stage preference and performance of Dolichogenidea gelechiidivoris (Hymenoptera: Braconidae), a candidate for classical biological control of Tuta absoluta in Africa

Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) is native to South America but has invaded the Afro-Eurasian supercontinent where it is currently the most devastating invasive arthropod pest of tomato. As a part of the first classical biological control programme against T. absoluta in Africa, a larval parasitoid, Dolichogenidea gelechiidivoris Marsh. (Syn.: Apanteles gelechiidivoris Marsh) (Hymenoptera: Braconidae), of T. absoluta was imported from Peru into the quarantine facility of the International Centre of Insect Physiology and Ecology, Kenya. We report on the host larval preference of D. gelechiidivoris and the host suitability, and the parasitoid’s reproductive strategy, including lifetime fecundity and egg maturation dynamics. Dolichogenidea gelechiidivoris females preferentially oviposited in early (1st and 2nd) larval instars of T. absoluta but parasitized and completed development in all four instars of the host. Host instar did not affect D. gelechiidivoris sex-ratio but females reared on the first instar had significantly fewer eggs than when reared in late larval instars (3rd and 4th). Females of the parasitoid emerged with a high mature egg load which peaked 2 d post eclosion. The females of D. gelechiidivoris survived 8.51 ± 0.65 d and produced 103 ± 8 offspring per female at 26 ± 4 °C (range: 24 to 29 °C) and 50–70% relative humidity (RH) with males present and fed honey-water (80% honey). Increasing maternal age decreased the proportion of female offspring. Under the aforementioned laboratory conditions, the Gross and Net reproductive rates were 72 and 39.5 respectively, while the mean generation time was 20 d. The potential intrinsic rate of natural increase was 0.18. This study shows that D. gelechiidivoris is a potential biological control agent of T. absoluta and should be considered for release in Kenya and across Africa following host specificity testing and risk assessments.

Occurrence of Green Semilooper Chrysodeixis acuta Walker (Lepidoptera: Noctuidae) in Onion (Allium cepa L.) (Amaryllidaceae)

The occurrence of green semilooper, Chrysodeixis acuta Walker (Lepidoptera: Noctuidae) in onion was recorded for the first time from the Pune district of Maharashtra State of India. Weekly observations of the pest and its damage were made to assess the intensity of the caterpillar population and its impact to the onion crop. Early instar larvae of C. acuta feed by scraping the leaves, whereas the later instars make circular cuts in the leaves. The presence of large cuts and defoliation is a sign of infestation in onion. Since C. acuta is polyphagous, it may spread to onion crops in the region and cause significant damage to the crop. Therefore, further research is required to monitor its spread and ability to become a pest in onion.

TWO REMARKABLE FOSSIL INSECT LARVAE FROM BURMESE AMBER SUGGEST THE PRESENCE OF A TERMINAL FILUM IN THE DIRECT STEM LINEAGE OF DRAGONFLIES AND DAMSELFLIES (ODONATA)

The fossil record of dragonfly relatives (Odonatoptera) dates back to the Carboniferous, yet knowledge about these extinct animals is meagre. For most of the species little is known except for the characteristics of the wing venation. As a result, it is difficult to include fossil larvae in a (wing character based) phylogenetic tree as the wing venation is not visible in most of the larval instars. Two larval specimens from Cretaceous Burmese amber are in the focus of this study. The two specimens likely represent two subsequent early stage larval instars of the same individual. Not only is this an exceptional case to study ontogenetic processes in fossils – the larval instars are morphologically completely different from all known larvae of Odonata with respect to the posterior abdominal region. Therefore, besides the difficulties regarding the phylogenetic interpretation and though all Burmese amber odonatans are known from adults only, a new species – Arcanodraco filicauda n. sp. – is formally described. Aside from likely representing a new species, the morphology of the posterior abdominal region is highly informative for reconstructing the character evolution within the lineage towards modern dragonflies and damselflies. A long median process in both of the fossils meets all criteria to be interpreted as a terminal filum (structure or derivative of tergite of abdominal segment 11, annulated in one of the specimens). Although the exact phylogenetic affinity of Arcanodraco filicauda n. sp. remains enigmatic, the presence of a larval terminal filum can be reconstructed for the ground pattern of Odonatoptera (including its direct stem lineage).

Evaluation of the larvicidal potential of root and leaf extracts of Saussurea costus (Falc.) Lipsch. against three mosquito vectors:Anopheles stephensi, Aedes aegypti, and Culex quinquefasciatus.

INTRODUCTION: The larvicidal potential of Saussurea costus (Falc.) Lipsch. was studied against the early 4th instar larvae of Anopheles stephensi Liston., Aedes aegypti Linn.,and Culex quinquefasciatus Say. because of the emergence of mosquito resistance to conventional synthetic insecticides. METHODS: At concentrations of 12.5-200 ppm, larvicidal activities were studied under laboratory conditions. RESULTS: After 24 h of exposure, the methanol extract of the roots recorded the highest larvicidal activity against An. stephensi, with LC50 and LC90values of 7.96 and 34.39 ppm, respectively. CONCLUSIONS: We are developing potent larvicidal compound(s) from S. costus for controlling the mosquito larval population.

Influence of Host Plant Species and Fertilization Regime on Larval Performance and Feeding Preference of the Redbacked Cutworm and the Pale Western Cutworm (Lepidoptera: Noctuidae).

The redbacked cutworm, Euxoa ochrogaster (Guenée), and the pale western cutworm, Agrotis orthogonia (Morrison), are generalist pests that cause sporadic economic damage to several annual crops in the Canadian Prairies. Early larval instars feed on foliage, whereas mature larvae eat into the stem and sever crop seedlings. Here, we evaluate the influence of annual crop species and host fertilization on the larval performance and feeding preference of both cutworm species. Performance is the ability of an insect to reach its maximum growth potential. The first set of experiments evaluated larval development and preference on canola (Brassica napus L. [Capparales: Brassicaeae]), field peas (Pisum sativa L. [Fabales: Fabaceae]), and spring wheat (Triticum aestivum L. [Cyperales: Poaceae]). The redbacked cutworm had higher performance on canola and peas, while the pale western cutworm had higher performance on wheat. In multiple-choice feeding experiments, the redbacked cutworm consumed more canola, whereas the pale western cutworm consumed more spring wheat. The third set of experiments evaluated larval development on fertilized and unfertilized seedlings of canola and spring wheat seedlings. When fed unfertilized seedlings, the redbacked cutworm had better performance on canola than spring wheat, whereas pale western cutworm had better performance on spring wheat than canola. Fertilizer application enhanced the performance of both cutworms regardless of the crop species. Despite their generalist feeding behavior, both cutworm species have a larval feeding preference for the host plant on which they achieve high performance. Canola-cereal cropping is a common crop rotation schedule in the region; however, this tactic will not negatively impact cutworm performance.

Control of western corn rootworm via RNAi traits in maize: lethal and sublethal effects of Sec23 dsRNA.

RNA interference (RNAi) triggered by maize plants expressing RNA hairpins against specific western corn rootworm (WCR) transcripts have proven to be effective at controlling this pest. To provide robust crop protection, mRNA transcripts targeted by double-stranded RNA must be sensitive to knockdown and encode essential proteins. Using WCR adult feeding assays, we identified Sec23 as a highly lethal RNAi target. Sec23 encodes a coatomer protein, a component of the coat protein (COPII) complex that mediates ER-Golgi transport. The lethality detected in WCR adults was also observed in early instar larvae, the life stage causing most of the crop damage, suggesting that WCR adults can serve as an alternative to larvae for dsRNA screening. Surprisingly, over 85% transcript inhibition resulted in less than 40% protein knockdown, suggesting that complete protein knockdown is not necessary for Sec23 RNAi-mediated mortality. The efficacy of Sec23 dsRNA for rootworm control was confirmed in planta; T0 maize events carrying rootworm Sec23 hairpin transgenes showed high levels of root protection in greenhouse assays. A reduction in larval survival and weight were observed in the offspring of WCR females exposed to Sec23 dsRNA LC25 in diet bioassays. We describe Sec23 as RNAi target for in planta rootworm control. High mortality in exposed adult and larvae and moderate sublethal effects in the offspring of females exposed to Sec23 dsRNA LC25 , suggest the potential for field application of this RNAi trait and the need to factor in responses to sublethal exposure into insect resistance management programs. © 2019 Society of Chemical Industry.

Behavior manipulation of mosquitoes by a mermithid nematode

We examined manipulation of mosquito behavior by the parasitic mermithid nematode, Strelkovimermis spiculatus. This nematode species typically infects early instar host larvae and emerges after parasitic development to kill last-instar larvae. Parasitized adults, however, have occasionally been reported from field collections. We obtained low rates (1.7–11.5%) of parasitized adults in laboratory exposures only when Culex pipiens pipiens fourth-instar larvae nearing pupation were exposed to infective nematodes. This did not allow an adequate interval for parasitic development in immature host stages. Parasitized adult females in a multiple-choice assay were three times more likely to seek water than a blood source (63.1 vs. 20.5%), whereas uninfected females were twice as likely to seek blood than water (64%3.9 vs. 32.6%). This altered host behavior benefits the parasite by providing the only mechanism for dispersal and colonization of new host habitats while concurrently avoiding risks from the defensive behaviors associated with blood-feeding. Behavioral alternation in Cx. p. pipiens larval hosts was also examined using larvae infected as second instars to allow for a normal duration of parasitic development. As larvae neared pupation and parasite emergence, parasitized larvae became more spatially aggregated than unparasitized larvae. This altered host behavior benefits the parasite by providing a corresponding increase in post-parasite aggregation, which facilitates formation of large mating clusters and concomitantly reproductive success. Parasites derive fitness gains by overriding host autonomy, whereas hosts have zero fitness once parasitism is established, suggesting a coevolutionary response is inoperative and that the behavioral modifications may be adaptive.

Individual and combined impact of nuclear polyhedrosis virus and spinosad to control the tropical armyworm, Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae), in cotton in Pakistan

The tropical armyworm, Spodoptera litura Fabricius (Noctuidae; Lepidoptera), is among the most harmful pests causing economic loss in the quality and production of a variety of crops, particularly cotton. Entomopathogens play an important role in insect pest management. The nuclear polyhedrosis virus (NPV) isolate of S. litura (V-SpltNPV) was isolated from infected larvae in a cotton crop, and viral occlusion bodies were confirmed, using an inverted microscope. The pathogenicity of V-SpltNPV against 2nd, 3rd, and 4th larval instars of S. litura was evaluated at various concentrations (1 × 104 to 1 × 108 OBs/ml). Mortality rate was high (37.65–96.82%) in early instar larvae against tested concentrations. LC50 and LT50 values increased with increasing larval age. There was 689,865 times increase in LC50 value (1.35 × 102 OBs/ml) for 2nd instar larvae to LC50 value (6.90 × 105 OBs/ml) for 4th instar larvae. LT50 values enhanced from 4.99 days for 2nd instar larvae to 7.49 days for 4th instar larvae, due to a decrease in efficacy of NPVs with the increasing age of larvae. In a greenhouse experiment, a combined application of spinosad with V-SpltNPV (1 × 108 OBs/ml) caused (100%) mortality of 2nd instar larvae. A single application of V-SpltNPV (1 × 108 OBs/ml) resulted to mean mortality (52.63%) of tested larvae. The native isolate V-SpltNPV seems to have a potential to be used in integrated manner with other IPM tactics to significantly reduce the use of toxic chemical pesticides.