Methoprene

A review of the potential impacts of coastal mosquito control programs on Australian Stingless Bees (Apidae, Meliponini)-likely exposure pathways and lessons learned from studies on honey bees

The impact of the programmatic use of larvicides for mosquito control on native stingless bees (e.g., Apidae, Meliponini) is a growing concern in Australia due to heightened conservation awareness and the growth of hobbyist stingless bee keeping. In Australia, the two most widely used mosquito larvicides are the bacterium Bacillus thuringiensis var. israelensis (Bti) and the insect hormone mimic methoprene (as S-methoprene). Each has a unique mode of action that could present a risk to stingless bees and other pollinators. Herein, we review the potential impacts of these larvicides on native Australian bees and conclude that their influence is mitigated by their low recommended field rates, poor environmental persistence, and the seasonal and intermittent nature of mosquito control applications. Moreover, evidence suggests that stingless bees may display a high physiological tolerance to Bti similar to that observed in honey bees (Apis mellifera), whose interactions with B. thuringiensis-based biopesticides are widely reported. In summary, neither Bti or methoprene is likely to pose a significant risk to the health of stingless bees or their nests. However, current knowledge is limited by regulatory testing requirements that only require the use of honey bees as toxicological models. To bridge this gap, we suggest that regulatory testing is expanded to include stingless bees and other nontarget insects. This is imperative for improving our understanding of the potential risks that these and other pesticides may pose to native pollinator conservation.

COMPARATIVE LABORATORY ACTIVITY AND SEMI-FIELD EFFICACY OF OMNIPRENE® G AND ALTOSID® PELLETS AGAINST AEDES AEGYPTI

Mosquito larval control by biorational larvicides plays a crucial role in mosquito and mosquito-borne disease management. However, the availability of larvicides that meet the criteria of efficacy, safety, and quality is limited and conventional pesticides are no longer preferred for larval control. Although efforts are made to research new active ingredients (AIs), it is equally important to innovate new formulations based on currently available AIs such as microbial agents and insect growth regulators. Studies were therefore conducted to compare the laboratory activity and semi-field efficacy of OmniPrene® G and Altosid® Pellets with DR-tech, both containing 4.25% S-methoprene, at 2.8 kg/ha and 11.2 kg/ha against the yellow fever mosquito Aedes aegypti (L.) in outdoor microcosms. Both products performed equally in bioassays against the test species with comparable inhibition of emergence activities. In the semi-field study, the lower dose of Altosid Pellets at 2.8 kg/ha, showed lower efficacy than OmniPrene G during the initial 6 weeks; this difference became negligible on week 7, followed by higher efficacy in Altosid Pellets on weeks 8 and 9. More uniform efficacy was observed at the higher dose of 11.2 kg/ha. Equal performance was revealed during weeks 2 to 6, with the OmniPrene G outperforming the Altosid Pellets in week 1, but the opposite during weeks 7 to 9. Mortality patterns were similar in both products, i.e., majority of mortality occurred before emergence, although more incomplete emergence was noted in lower doses, particularly in Altosid Pellets. Overall, newly available OmniPrene G provided comparable activity and efficacy with Altosid Pellets against the test species, with the advantages of fast initial AI release and even coverage, particularly when applied at low doses.

Laboratory and Semi-Field Evaluation on S-Methoprene Formulations Against Anopheles sinensis (Diptera: Culicidae) - Yuxi City, Yunnan Province, China

What is already known about this topic?

Anopheles sinensis (An. sinensis) is the predominant malaria vector in China. The impact of S-methoprene on the emergence process of mosquito larvae suggests its potential as a control method for vector mosquitoes. However, the efficacy of S-methoprene in controlling An. sinensis has not yet been demonstrated.

What is added by this report?

The effectiveness of S-methoprene against An. sinensis was assessed in laboratory and semi-field conditions in Yunnan Province.

What are the implications for public health practice?

These results offer valuable options and guidance for utilizing S-methoprene products in malaria reimportation prevention areas within Yunnan Province.

Combination Insecticide Treatments with Methoprene and Pyrethrin for Control of Khapra Beetle Larvae on Different Commodities

Trogoderma granarium Everts, the khapra beetle, is a serious pest of stored products throughout the world. Larvae pose a significant threat to stored products because they feed on >100 different commodities, possess the ability to enter facultative diapause, and are difficult to detect. Control methods for T. granarium include fumigation, contact insecticides, trapping, and insecticide-incorporated packaging. The objective of this study was to determine the residual efficacy of two insecticide formulations (methoprene + deltamethrin + piperonyl butoxide synergist Gravista® and methoprene + deltamethrin, DiaconIGR®Plus). These insecticides were evaluated on three stored product commodities, corn, wheat, and brown rice, by exposing T. granarium larvae during a 12-month testing period. Both formulations significantly reduced adult emergence on corn and wheat for 12 months and on brown rice for up to 6 months. Adult emergence was highest at month 12 for corn (8.41%), and brown rice (85.88%), and month 9 for wheat (39.52%), treated with DiaconIGR®Plus or Gravista®, respectively. A biological index used to measure the development of exposed larvae on the treated grain from the larval stage (low values) to adult emergence (high values) was lower (fewer adults) on corn and wheat compared to controls. Despite differences in formulations, each of these grain protectants could be utilized by stored commodity managers to protect commodities during storage and transportation for T. granarium when and if this pest is detected at USA ports of entry.

Comparative Activity And Efficacy of Sumilarv 0.5G and Altosid Xr Briquet Against Culex Quinquefasciatus and Aedes Aegypti in Simulated Catch Basins

Mosquito control plays a crucial role in the mitigation of mosquito-borne diseases. Larviciding that targets one of the aquatic stages is among the routine practices in mosquito control operations. One of the most extensive and challenging mosquito production sources in urban environments is underground storm drain systems. Along with the research and development of biorational larvicides in recent decades, numerous products based on microbial and insect growth regulators have become available. However, the performance of these products often varies because of product design and challenges associated with urban storm drain systems. This paper validates the comparative bioactivity and semifield efficacy of 2 control release products based on pyriproxyfen and S-methoprene. In laboratory bioassays, pyriproxyfen was significantly more active than S-methoprene against the test species, Culex quinquefasciatus Say and Aedes aegypti (L.). Culex quinquefasciatus was less susceptible than Ae. aegypti to both test materials. During a 26-wk-long semifield evaluation using the cast concrete simulated catch basins, the inhibition of emergence pretreatment and posttreatment in untreated control was negligible. The Sumilarv 0.5G applied at 75 g per catch basin provided 100% IE, whereas the Altosid XR briquet applied at 1 per catch basin yielded only partial control fluctuating from 12.7% to 82.7% (average 40.7%) of Cx. quinquefasciatus and 8.0% to 78.8% (average 37.4%) of Ae. aegypti. The Altosid XR briquet had an average residual weight of 59.9% at the end of semifield evaluation. Results are discussed in relation to field mosquito control operations in urban storm drain systems.

Juvenile hormone receptor Methoprene tolerant: Functions and applications

During the past 15years, after confirming Methoprene tolerant (Met) as a juvenile hormone (JH) receptor, tremendous progress has been made in understanding the function of Met in supporting JH signal transduction. Met role in JH regulation of development, including metamorphosis, reproduction, diapause, cast differentiation, behavior, im`munity, sleep and epigenetic modifications, have been elucidated. Met's Heterodimeric partners involved in performing some of these functions were discovered. The availability of JH response elements (JHRE) and JH receptor allowed the development of screening assays in cell lines and yeast. These screening assays facilitated the identification of new chemicals that function as JH agonists and antagonists. These new chemicals and others that will likely be discovered in the near future by using JH receptor and JHRE will lead to highly effective species-specific environmentally friendly insecticides for controlling pests and disease vectors.

Qualitative structure-activity relationships of aryl isoprenoid derivatives as biorational juvenoids - reweighing

Juvenoids are juvenile hormone (JH) mimetics, with specific structural features and defined molecular size that disrupt the target insect development. Juvenoid activity (= JH-type activity) of various isoprenoid-based derivatives as insecticidal candidates of the insect growth disruptors (IGDs) type were rated against the house fly, Musca domestica L. The epoxidized decenyl and nonenyl phenyl ether derivatives have more active compounds than those of both parent alkoxidized or olefinic structures. The highest juvenoid potency was shown by 3,4-methylenedioxyphenyl ethers of 8,9-epoxy-5,9-dimethy1-3,8-decadiene. Qualitative structure-activity relationships are offered to relate the chemical structure criteria to observed juvenoid-related activity. Differences in activity among the reported isoprenoid-based derivatives were qualitatively rationalized. This study advances understanding of the structural qualifications and activity determinants of isoprenoid juvenoids, which is important for the development of new filth flies eco-friendly insecticides.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42690-023-01025-3.

Assessment of Truck-Mounted Area-Wide S-methoprene Applications to Manage West Nile Virus Vector Species in the Suburbs of Chicago, IL, USA

West Nile virus remains the leading cause of arboviral neuroinvasive disease in the United States, despite extensive efforts to control the mosquito vectors involved in transmission. In this study, we evaluated the effectiveness of Altosid SR-20 (active ingredient, S-methoprene 20%) larvicide applications using truck-mounted ultra-low volume (ULV) dispersal equipment to target Culex pipiens Linnaeus (Diptera: Culicidae) and Cx. restuans (Theobald)larvae. A combination of emergence bioassays, open-field measurements of deposited S-methoprene and spray distribution using gas chromatography-mass spectrometry, and assessments of adult Culex spp. populations in response to applications were conducted over the summer of 2020 within the North Shore Mosquito Abatement District (IL, USA). Open-field applications revealed that dispersed Altosid SR-20 using ULV equipment was effective (75% emergence inhibition in susceptible lab strain Cx. pipiens larvae) up to 53 m. In suburban neighborhood applications, we found that S-methoprene deposition and larval emergence inhibition (EI) in front yards did not differ significantly from backyards. An overall EI of 46% and 28% were observed for laboratory strain Cx. pipiens and wild Cx. restuans larvae respectively, and both had an EI significantly higher than the untreated control group. The EI of exposed wild Cx. pipiens larvae did not differ from the untreated controls, suggesting an increased tolerance to S-methoprene. No difference in abundance of gravid or host-seeking adult Culex spp. post-application was detected between treated and untreated sites. These results document the ability of area-wide application to distribute S-methoprene, but this strategy will need further modifications and evaluation for Culex spp. management.

Laboratory and semi-field evaluation on OmniPrene™ G against Aedes, Anopheles and Culex mosquitoes

With the current existing, emerging, and resurging mosquito species and mosquito-borne diseases, and very low availability of mosquito control products, exploration of novel active ingredients and innovative formulations based on available active ingredients are crucial to ensure the sustainable mosquito control interventions. The larviciding by applying biorational mosquito larvicides with microbial and insect growth regulator origins is a routine practice in most areas. The current paper evaluated an innovated granular S-methoprene formulation, namely OmniPrene™ in the laboratory bioassays, as well as outdoor microcosms at 2.8, 5.6 and 11.2 kg/ha with 30.5 cm water depth, and at 2.8 kg/ha with shallow water (15.25 cm) against Aedes aegypti, Anopheles hermsi and Culex quinquefasciatus. Over 90% efficacy was achieved against Aedes and Anopheles for at least 49 days, and against Culex for up to 42 days at 2.8, 5.6 and 11.2 kg/ha when water depth was 30.5 cm. The control levels were further elevated when water depth was 15.25 cm, where over 90% control was observed for 63 days against Aedes and Anopheles, but 56 days against Culex. The high initial and residual efficacy were attributable to the unique diatomaceous earth carrier granules and proprietary binding process. With well documented bioactivity and safety of S-methoprene, demonstrated performance of the OmniPrene G, plus the unique feature of the formulation to render vegetation penetration, even coverage, and other advantages of dry granules, it would be reasonable to recommend this product as one of the routine larvicidal tools in a wide variety of habitats against mosquito species of public health importance while complying with the local regulations.

The impact of insect growth regulators on adult emergence inhibition and the fitness of Aedes aegypti field populations in Thailand

BACKGROUND

The frequent use of insecticides in vector control causes the development of insecticide resistance. Insect growth regulators (IGRs), which effect insect development, are used as a promising alternative to control resistant insect vectors. This study aimed to develop novel effective tools for Aedes aegypti control by evaluating the efficacy of different IGRs on larval development, blood feeding capacity, fecundity, and fertility in females and sperm productivity in males across geographical regions of Thailand.

METHODS

The efficacy of 16 technical grade IGRs were evaluated against laboratory strain Ae. aegypti larvae in order to determine their emergence inhibition (EI) at 50% and 95% under laboratory conditions. Six IGRs were selected for fecundity, fertility, and sperm productivity studies using feed-through treatments at EI95 concentration levels against adult Ae. aegypti field strains.

RESULTS

The results from larval bioassay tests indicate that juvenile hormone mimics (EI50 = 0.010-0.229 ppb; EI95 = 0.066-1.118 ppb) and chitin synthesis inhibitors affecting CHS1 (EI50 = 0.240-2.412 ppb; EI95 = 0.444-4.040 ppb) groups effectively inhibited adult Ae. aegypti emergence. Methoprene and fenoxycarb significantly reduced blood feeding capacity. Egg production was comparable among strains while methoprene, pyriproxyfen and diflubenzuron induced egg production. Egg retention was detected in females fed on diflubenzuron. Methoprene, fenoxycarb, diflubenzuron, and teflubenzuron reduced egg hatching rates in mosquito field strains compared to laboratory strain. Male mosquitoes fed on fenoxycarb showed significantly lower sperm production compared to other treatments.

CONCLUSION

Juvenile hormone analogues and chitin synthesis inhibitors affecting CHS1 groups showed excellent results in adult emergence inhibition in this study. They also disrupted reproductive systems in both adult males and females. This study suggested that they can be used as an alternative larvicide in mosquito control programs.

Effects of itol A on the larval growth and development of Spodoptera frugiperda (Lepidoptera: Noctuidae)

BACKGROUND

Itol A, extracted from Itoa orientalis Hemsl. (Flacourtiaceae), possesses bioactivity on Spodoptera litura (Lepidoptera: Noctuidae) and Nilaparvata lugens (Stål) (Hemiptera: Delphacidae). Our previous study showed that the effects on Spodoptera frugiperda, a destructive pest found worldwide, were similar to those of fenoxycarb (FC), a juvenile hormone analog. Thus, we speculate that itol A could have growth-regulating effects. The current work explored juvenile hormone (JH) levels and mRNA levels of crucial JH signaling pathway enzyme genes in S. frugiperda larvae treated with itol A and FC.

RESULTS

Itol A caused severe growth obstacles in S. frugiperda, extended the larval duration and reduced the mean worm weight and body length rates. Three and 7 days after exposure to a sublethal concentration of itol A (500 mg L^-1 ), the JH level of the larvae significantly decreased by 36.59% and 22.70%, respectively. qPCR inferred that the mRNA expression levels of crucial JH metabolism enzymes (SfJHE and SfJHEH) significantly increased by 6.58-fold and 2.12-fold, respectively, relative to the control group 3 days after treatment.

CONCLUSIONS

Itol A adversely affects the development of S. frugiperda. We propose that this effect was caused by decreasing JH levels and disrupting the JH signaling pathway via mediating its synthetic and metabolic crucial enzymes. © 2021 Society of Chemical Industry.

Reporter gene assays for screening and identification of novel molting hormone- and juvenile hormone-like chemicals

A reporter gene assay (RGA) is used to investigate the activity of synthetic chemicals mimicking the molting hormones (MHs) and juvenile hormones (JHs) of insects, so-called insect growth regulators (IGRs). The MH receptor, a heterodimer of the ecdysone receptor (EcR) and ultraspiracle (USP), and the JH receptor Methoprene-tolerant (Met) are ligand-dependent transcription factors. Ligand-bound EcR-USP and Met bind to specific cis-acting DNA elements, referred to as the ecdysone-responsive element (EcRE) and the JH-responsive element (JHRE), respectively, in order to transactivate target genes. Insect hormone-induced transactivation systems have been reconstituted by the introduction of reporter genes under the control of EcRE and JHRE, or two-hybrid reporter genes, into insect, mammalian, and yeast cells expressing receptor proteins. RGA is easy to use and convenient for examining the MH- and JH-like activities of synthetic chemicals and is suitable for the high-throughput screening of novel structural classes of chemicals targeting EcR-USP and Met.

Lethal and Sublethal Effects of Ingested Hydroprene and Methoprene on Development and Fecundity of the Common Bed Bug (Hemiptera: Cimicidae)

In the last two decades, bed bugs (Cimex lectularius L. and Cimex hemipterus F.) have become perennial and difficult to control indoor pests. Current pest control options are severely constrained by high prevalence of insecticide resistance and availability and relatively high costs of alternative interventions. Among various measures to counter the drawbacks of insecticide resistance include efforts to diversify the modes of action of insecticides with residual applications of combinations of insecticides, which include a juvenile hormone analog (JHA). JHAs, such as hydroprene and methoprene, have a desirable safety profile and are effective against a variety of indoor pests. We evaluated the potential of hydroprene and methoprene to be incorporated into an ingestible bait, with dose-response studies on fifth-instar male and female bed bugs. Females were more susceptible than males to both JHAs, and methoprene was more effective by ingestion than hydroprene at inducing both lethal and sublethal effects. Ingestion of ≥10 µg/ml blood of either JHA by last instar nymphs reduced oviposition; untreated females that mated with males exposed to high concentrations of either JHA also exhibited lower oviposition. We suggest that methoprene could be incorporated into integrated pest management programs in liquid baits and residual sprays in combination with other active ingredients.

The juvenile hormone receptor as a target of juvenoid "insect growth regulators"

Synthetic compounds that mimic the action of juvenile hormones (JHs) are founding members of a class of insecticides called insect growth regulators (IGRs). Like JHs, these juvenoids block metamorphosis of insect larvae to reproductive adults. Many biologically active juvenoids deviate in their chemical structure considerably from the sesquiterpenoid JHs, raising questions about the mode of action of such JH mimics. Despite the early deployment of juvenoid IGRs in the mid-1970s, their molecular effect could not be understood until recent discoveries of JH signaling through an intracellular JH receptor, namely the ligand-binding transcription factor Methoprene-tolerant (Met). Here, we briefly overview evidence defining three widely employed and chemically distinct juvenoid IGRs (methoprene, pyriproxyfen, and fenoxycarb), as agonist ligands of the JH receptor. We stress that knowledge of the target molecule is critical for using these compounds both as insecticides and as research tools.

From Incriminating Stegomyia fasciata to Releasing Wolbachia pipientis: Australian Research on the Dengue Virus Vector, Aedes aegypti, and Development of Novel Strategies for Its Surveillance and Control

Globally, the dengue viruses (DENVs) infect approximately 300 million people annually. Australia has a history of epidemic dengue, with outbreaks in the early decades of the twentieth century responsible for tens of thousands of cases. Seminal experiments conducted by Australian scientists during these outbreaks were the first to incriminate Aedes aegypti as a major vector of dengue viruses. One hundred years later, Australian scientists are playing a lead role in the development of surveillance and suppression strategies that target this mosquito species. Surveillance of Ae. aegypti populations and their associated dengue risk was greatly improved by understanding the contribution of key premises, key containers, and cryptic larval habitats to mosquito productivity, and, more recently, the development of novel adult traps. In terms of mosquito control, targeted indoor residual pyrethroid spraying and community-based biological control utilizing predatory copepods can significantly reduce Ae. aegypti populations. The release of Ae. aegypti transinfected with the virus-blocking bacterium, Wolbachia, provides a promising strategy for limiting DENV transmission. These diverse strategies developed by Australian scientists have the potential to alleviate the burden of dengue in the future, whether it is at the local level or as part of a country-wide program.

Vectors, Hosts, and Control Measures for Zika Virus in the Americas

We examine Zika virus (ZIKV) from an ecological perspective and with a focus on the Americas. We assess (1) the role of wildlife in ZIKV disease ecology, (2) how mosquito behavior and biology influence disease dynamics, and (3) how nontarget species and ecosystems may be impacted by vector control programs. Our review suggests that free-ranging, non-human primates may be involved in ZIKV transmission in the Old World; however, other wildlife species likely play a limited role in maintaining or transmitting ZIKV. In the Americas, a zoonotic cycle has not yet been definitively established. Understanding behaviors and habitat tolerances of Aedes aegypti and Aedes albopictus, two ZIKV competent vectors in the Americas, will allow more accurate modeling of disease spread and facilitate targeted and effective control efforts. Vector control efforts may have direct and indirect impacts to wildlife, particularly invertebrate feeding species; however, strategies could be implemented to limit detrimental ecological effects.

Genetic tools to study juvenile hormone action in Drosophila

The insect juvenile hormone receptor is a basic helix-loop-helix (bHLH), Per-Arnt-Sim (PAS) domain protein, a novel type of hormone receptor. In higher flies like Drosophila, the ancestral receptor germ cell-expressed (gce) gene has duplicated to yield the paralog Methoprene-tolerant (Met). These paralogous receptors share redundant function during development but play unique roles in adults. Some aspects of JH function apparently require one receptor or the other. To provide a foundation for studying JH receptor function, we have recapitulated endogenous JH receptor expression with single cell resolution. Using Bacteria Artificial Chromosome (BAC) recombineering and a transgenic knock-in, we have generated a spatiotemporal expressional atlas of Met and gce throughout development. We demonstrate JH receptor expression in known JH target tissues, in which temporal expression corresponds with periods of hormone sensitivity. Larval expression largely supports the notion of functional redundancy. Furthermore, we provide the neuroanatomical distribution of JH receptors in both the larval and adult central nervous system, which will serve as a platform for future studies regarding JH action on insect behavior.

Environmental safety review of methoprene and bacterially-derived pesticides commonly used for sustained mosquito control

Some pesticides are applied directly to aquatic systems to reduce numbers of mosquito larvae (larvicides) and thereby reduce transmission of pathogens that mosquitoes vector to humans and wildlife. Sustained, environmentally-safe control of larval mosquitoes is particularly needed for highly productive waters (e.g., catchment basins, water treatment facilities, septic systems), but also for other habitats to maintain control and reduce inspection costs. Common biorational pesticides include the insect juvenile hormone mimic methoprene and pesticides derived from the bacteria Bacillus thuringiensis israelensis, Lysinibacillus sphaericus and Saccharopolyspora spinosa (spinosad). Health agencies, the public and environmental groups have especially debated the use of methoprene because some studies have shown toxic effects on non-target organisms. However, many studies have demonstrated its apparent environmental safety. This review critically evaluates studies pertinent to the environmental safety of using methoprene to control mosquito larvae, and provides concise assessments of the bacterial larvicides that provide sustained control of mosquitoes. The review first outlines the ecological and health effects of mosquitoes, and distinguishes between laboratory toxicity and environmental effects. The article then interprets non-target toxicity findings in light of measured environmental concentrations of methoprene (as used in mosquito control) and field studies of its non-target effects. The final section evaluates information on newer formulations of bacterially-derived pesticides for sustained mosquito control. Results show that realized environmental concentrations of methoprene were usually 2-5µg/kg (range 2-45µg/kg) and that its motility is limited. These levels were not toxic to the vast majority of vertebrates and invertebrates tested in laboratories, except for a few species of zooplankton, larval stages of some other crustaceans, and small Diptera. Studies in natural habitats have not documented population reductions except in small Diptera. Bacterial larvicides showed good results for sustained control with similarly limited environmental effects, except for spinosad, which had broader effects on insects in mesocosms and temporary pools. These findings should be useful to a variety of stakeholders in informing decisions on larvicide use to protect public and environmental health in a 'One Health' framework.

Discovery of methylfarnesoate as the annelid brain hormone reveals an ancient role of sesquiterpenoids in reproduction

Animals require molecular signals to determine when to divert resources from somatic functions to reproduction. This decision is vital in animals that reproduce in an all-or-nothing mode, such as bristle worms: females committed to reproduction spend roughly half their body mass for yolk and egg production; following mass spawning, the parents die. An enigmatic brain hormone activity suppresses reproduction. We now identify this hormone as the sesquiterpenoid methylfarnesoate. Methylfarnesoate suppresses transcript levels of the yolk precursor Vitellogenin both in cell culture and in vivo, directly inhibiting a central energy–costly step of reproductive maturation. We reveal that contrary to common assumptions, sesquiterpenoids are ancient animal hormones present in marine and terrestrial lophotrochozoans. In turn, insecticides targeting this pathway suppress vitellogenesis in cultured worm cells. These findings challenge current views of animal hormone evolution, and indicate that non-target species and marine ecosystems are susceptible to commonly used insect larvicides.

DOI:http://dx.doi.org/10.7554/eLife.17126.001

All organisms need energy to survive and grow. However, sources of energy are limited and so organisms need to decide how to spend the resources they have available. For instance, animals must choose whether they should continue to grow or if they should invest energy into reproduction instead. This decision becomes even more important for animals that reproduce in an “all-or-nothing” manner and invest all their available energy into reproduction and die soon after.

Bristle worms live in coastal areas around world. In mass spawning events, thousands of individuals raise from the sea floor to the surface, to release sperm and eggs. While the fertilized eggs start to develop in the water, the parents invariably die. The female worms spend roughly half their body mass in producing eggs and supplying them with yolk as a source of energy. It has been known for decades that the brains of bristle worms produce a master hormone that promotes growth and suppresses reproduction. Yet the identity of this hormone that controls the life-or-death decision was not clear.

Schenk et al. took advantage of new molecular tools to solve this puzzle. The experiments show that this hormone directly regulates how much yolk the female animals produce. This allowed Schenk et al. to design a new molecular assay that helped to identify the hormone itself. Unexpectedly, the hormone – called methylfarnesoate – belongs to a family of small molecules called sesquiterpenoids, which researchers previously thought were only found in insects and related groups. Hence, many insecticides have been developed to target sesquiterpenoid signaling and they are used in massive amounts to fight pests like the tiger mosquito (which transmits the Zika virus). Schenk et al. also found that these insecticides also cause severe problems in bristle-worms.

These findings challenge current views of how animal hormones have evolved and indicate that common insecticides may be harming bristle worms and other animals in marine environments. The next steps are to find out whether methylfarnesoate is found in other closely related animals, such as snails and mussels, and whether the insecticides are harmful to these animals too. Another future challenge will be to investigate how this hormone actually promotes animal growth.

DOI:http://dx.doi.org/10.7554/eLife.17126.002

Genomic and functional characterization of a methoprene-tolerant gene in the kissing-bug Rhodnius prolixus

Metamorphosis, which depends upon a fine balance between two groups of lipid-soluble hormones such as juvenile hormones (JHs) and ecdysteroids, is an important feature in insect evolution. While it is clear that the onset of metamorphosis depends on the decrease of JH levels, the way in which these hormones exert their activities is not fully understood in Triatominae species. The discovery of a Drosophila melanogaster mutant resistant to the treatment with the JH analog methoprene, led finally to the description of the methoprene-tolerant gene in Tribolium castaneum (TcMet) as a putative JH receptor. Here we present the genomic and functional characterization of an ortholog of the methoprene-tolerant gene in the hemimetabolous insect Rhodnius prolixus (RpMet). The analysis of the R. prolixus gene showed that the exonic structure is different from that described for holometabolous species, although all the critical protein motifs are well conserved. Expression analysis showed the presence of RpMet mRNA in all the tested tissues: ovary, testis, rectum, Malpighian tubules and salivary glands. When juvenile individuals were treated with RpMet specific double strand RNA (dsRNA), we observed abnormal molting events that resulted in individuals with morphological alterations (adultoids). Similarly, treatment of newly emerged fed females with dsRNA resulted in an abnormal development of the ovaries, with eggs revealing anomalies in size and accumulation of yolk, as well as a decrease in the amount of heme-binding protein. Altogether, our results validate that RpMet is involved in the transduction of JH signaling, controlling metamorphosis and reproduction in R. prolixus.

Acute larvicidal activity against mosquitoes and oxygen consumption inhibitory activity of dihydroguaiaretic acid derivatives

(-)-Dihydroguaiaretic acid (DGA) and its derivatives having 3-hydroxyphenyl (3-OH-DGA) and variously substituted phenyl groups instead of 3-hydroxy-4-methoxyphenyl groups were synthesized to measure their larvicidal activity against the mosquito Culex pipiens Linnaeus, 1758 (Diptera: Culicidae). Compared with DGA and 3-OH-DGA (LC50 (M), 3.52 × 10(-5) and 4.57 × 10(-5), respectively), (8R,8'R)-lignan-3-ol (3) and its 3-Me (10), 2-OH (12), 3-OH (13), and 2-OMe (15) derivatives showed low potency (ca. 6-8 × 10(-5) M). The 4-Me derivative (11) showed the lowest potency (12.1 × 10(-5) M), and the 2-F derivative (4) showed the highest (2.01 × 10(-5) M). All of the synthesized compounds induced an acute toxic symptom against mosquito larvae, with potency varying with the type and position of the substituents. The 4-F derivative (6), which killed larvae almost completely within 45 min, suppressed the O2 consumption of the mitochondrial fraction, demonstrating that this compound inhibited mitochondrial O2 consumption contributing to a respiratory inhibitory activity.

QTL mapping of genome regions controlling temephos resistance in larvae of the mosquito Aedes aegypti

INTRODUCTION

The mosquito Aedes aegypti is the principal vector of dengue and yellow fever flaviviruses. Temephos is an organophosphate insecticide used globally to suppress Ae. aegypti larval populations but resistance has evolved in many locations.

METHODOLOGY/PRINCIPAL FINDINGS

Quantitative Trait Loci (QTL) controlling temephos survival in Ae. aegypti larvae were mapped in a pair of F3 advanced intercross lines arising from temephos resistant parents from Solidaridad, México and temephos susceptible parents from Iquitos, Peru. Two sets of 200 F3 larvae were exposed to a discriminating dose of temephos and then dead larvae were collected and preserved for DNA isolation every two hours up to 16 hours. Larvae surviving longer than 16 hours were considered resistant. For QTL mapping, single nucleotide polymorphisms (SNPs) were identified at 23 single copy genes and 26 microsatellite loci of known physical positions in the Ae. aegypti genome. In both reciprocal crosses, Multiple Interval Mapping identified eleven QTL associated with time until death. In the Solidaridad×Iquitos (SLD×Iq) cross twelve were associated with survival but in the reciprocal IqxSLD cross, only six QTL were survival associated. Polymorphisms at acetylcholine esterase (AchE) loci 1 and 2 were not associated with either resistance phenotype suggesting that target site insensitivity is not an organophosphate resistance mechanism in this region of México.

CONCLUSIONS/SIGNIFICANCE

Temephos resistance is under the control of many metabolic genes of small effect and dispersed throughout the Ae. aegypti genome.

Interactive effects of mosquito control insecticide toxicity, hypoxia, and increased carbon dioxide on larval and juvenile eastern oysters and hard clams

Mosquito control insecticide use in the coastal zone coincides with the habitat and mariculture operations of commercially and ecologically important shellfish species. Few data are available regarding insecticide toxicity to shellfish early life stages, and potential interactions with abiotic stressors, such as low oxygen and increased CO2 (low pH), are less understood. Toxicity was assessed at 4 and 21 days for larval and juvenile stages of the Eastern oyster, Crassostrea virginica, and the hard clam, Mercenaria mercenaria, using two pyrethroids (resmethrin and permethrin), an organophosphate (naled), and a juvenile growth hormone mimic (methoprene). Acute toxicity (4-day LC50) values ranged from 1.59 to >10 mg/L. Overall, clams were more susceptible to mosquito control insecticides than oysters. Naled was the most toxic compound in oyster larvae, whereas resmethrin was the most toxic compound in clam larvae. Mortality for both species generally increased with chronic insecticide exposure (21-day LC50 values ranged from 0.60 to 9.49 mg/L). Insecticide exposure also caused sublethal effects, including decreased swimming activity after 4 days in larval oysters (4-day EC50 values of 0.60 to 2.33 mg/L) and decreased growth (shell area and weight) in juvenile clams and oysters after 21 days (detected at concentrations ranging from 0.625 to 10 mg/L). Hypoxia, hypercapnia, and a combination of hypoxia and hypercapnia caused mortality in larval clams and increased resmethrin toxicity. These data will benefit both shellfish mariculture operations and environmental resource agencies as they manage the use of mosquito control insecticides near coastal ecosystems.

An assessment of the importance of subsurface catch basins for Aedes aegypti adult production during the dry season in a neighborhood of Merida, Mexico

We compared the number of adult Aedes aegypti emerging from subsurface catch basins located in the streets against the number of pupae (as a proxy of adults emerging) from the entire container larval habitats found at residential premises within 1 ha of a neighborhood in the Mexican city of Merida during 8 days in the dry season of 2012. Aedes aegypti adults were collected from 60% of the subsurface catch basins. They produced 12 adults/day/ha (95% confidence interval [CI], 6.4 to 17.9), 5 females (95% CI, 2.1 to 7.7), and 7 males (95% CI, 3.8 to 10.7). In contrast, only 7 containers holding water were identified in 30 premises inspected, 1 bucket was positive for Ae. aegypti larvae, but no pupae-positive containers were found. No other mosquito species were found. This study revealed the importance of this type of nonresidential and subterranean aquatic habitat for Ae. aegypti adult production in this neighborhood of Merida during the dry season.

Crystal structure of silkworm Bombyx mori JHBP in complex with 2-methyl-2,4-pentanediol: plasticity of JH-binding pocket and ligand-induced conformational change of the second cavity in JHBP

Juvenile hormones (JHs) control a diversity of crucial life events in insects. In Lepidoptera which major agricultural pests belong to, JH signaling is critically controlled by a species-specific high-affinity, low molecular weight JH-binding protein (JHBP) in hemolymph, which transports JH from the site of its synthesis to target tissues. Hence, JHBP is expected to be an excellent target for the development of novel specific insect growth regulators (IGRs) and insecticides. A better understanding of the structural biology of JHBP should pave the way for the structure-based drug design of such compounds. Here, we report the crystal structure of the silkworm Bombyx mori JHBP in complex with two molecules of 2-methyl-2,4-pentanediol (MPD), one molecule (MPD1) bound in the JH-binding pocket while the other (MPD2) in a second cavity. Detailed comparison with the apo-JHBP and JHBP-JH II complex structures previously reported by us led to a number of intriguing findings. First, the JH-binding pocket changes its size in a ligand-dependent manner due to flexibility of the gate α1 helix. Second, MPD1 mimics interactions of the epoxide moiety of JH previously observed in the JHBP-JH complex, and MPD can compete with JH in binding to the JH-binding pocket. We also confirmed that methoprene, which has an MPD-like structure, inhibits the complex formation between JHBP and JH while the unepoxydated JH III (methyl farnesoate) does not. These findings may open the door to the development of novel IGRs targeted against JHBP. Third, binding of MPD to the second cavity of JHBP induces significant conformational changes accompanied with a cavity expansion. This finding, together with MPD2-JHBP interaction mechanism identified in the JHBP-MPD complex, should provide important guidance in the search for the natural ligand of the second cavity.

Effects of juvenile hormone analog formulations on development and reproduction in the bed bug Cimex lectularius (Hemiptera: Cimicidae)

BACKGROUND

Bed bugs (Cimex lectularius L.) have become a common insect pest in urban areas and are often difficult to manage. Eradication is made more problematic by widespread insecticide resistance, raising interest in alternative control products. Juvenile hormone analogs (JHAs) such as methoprene and hydroprene are relatively harmless to non-arthropods and have proved to be effective against other urban insect pests. Two JHA products (Gentrol(®) and Precor(®), Central Life Sciences, Schaumburg, IL) were tested for efficacy against various bed bug stages as direct spray and as dry residue using three bed bug strains.

RESULTS

At 1× and 2× the label rate, Precor(®) [active ingredient (S)-methoprene] had no significant effect on the development or fecundity of bed bugs. At 2× the label rate, confinement to residues of Gentrol(®) [active ingredient (S)-hydroprene] had no significant effect, but residues at 3× and 10× the label rate caused a reduction in fecundity and impaired development. Field strains were more susceptible to the reproductive effects of (S)-hydroprene than a long-maintained laboratory strain.

CONCLUSIONS

While JHAs are attractive alternatives for pest management because of their inherent safety and distinct mode of action, these JHA formulations would have little impact on bed bug populations without relabeling to allow for higher application rates.

Insecticides that Interfere with Insect Growth and Development

The insecticides discussed in this chapter target structures and physiological systems unique to insects and similar creatures and are consequently of low mammalian toxicity. This is despite one group, the ecdysone agonists, targeting an insect steroid hormonal system. Curiously, a remarkably large number of these compounds, notwithstanding their structural diversity, have adverse effects on the mammalian haematological system. As these compounds target insect development, they are not ‘knock-down’ insecticides and are not very effective against established adult insect infestation.

Juvenile hormone (JH) esterase of the mosquito Culex quinquefasciatus is not a target of the JH analog insecticide methoprene

Juvenile hormones (JHs) are essential sesquiterpenes that control insect development and reproduction. JH analog (JHA) insecticides such as methoprene are compounds that mimic the structure and/or biological activity of JH. In this study we obtained a full-length cDNA, cqjhe, from the southern house mosquito Culex quinquefasciatus that encodes CqJHE, an esterase that selectively metabolizes JH. Unlike other recombinant esterases that have been identified from dipteran insects, CqJHE hydrolyzed JH with specificity constant (k_cat/K_M ratio) and V_max values that are common among JH esterases (JHEs). CqJHE showed picomolar sensitivity to OTFP, a JHE-selective inhibitor, but more than 1000-fold lower sensitivity to DFP, a general esterase inhibitor. To our surprise, CqJHE did not metabolize the isopropyl ester of methoprene even when 25 pmol of methoprene was incubated with an amount of CqJHE that was sufficient to hydrolyze 7,200 pmol of JH to JH acid under the same assay conditions. In competition assays in which both JH and methoprene were available to CqJHE, methoprene did not show any inhibitory effects on the JH hydrolysis rate even when methoprene was present in the assay at a 10-fold higher concentration relative to JH. Our findings indicated that JHE is not a molecular target of methoprene. Our findings also do not support the hypothesis that methoprene functions in part by inhibiting the action of JHE.

The acute toxicity of fipronil to two non-target invertebrates associated with mosquito breeding sites in Australia

Mosquito-borne arboviruses are a significant health issue in the irrigation areas of south-eastern Australia. Fipronil, a pyrazole insecticide with strong activity against larval Culex species, was tested for its acute effects on Simocephalus elizabethae (Daphniidae) and Polypedilum nubiferum (Chironomidae), two non-target invertebrates associated with Australian rice field mosquito habitats. Technical and formulated fipronil were assessed in the presence or absence of particulate artificial diets in 48 h static bioassays. LC(50) values for neonate S. elizabethae ranged from 11.13 to 19.12 μgl(-1) whilst those for final instar P. nubiferum ranged from 0.89 to 2.18 μgl(-1). Feeding during exposure significantly reduced the susceptibility of P. nubiferum to both technical and formulated fipronil. The effect of feeding was less consistent in Simocephalus bioassays, where much less food was present. We investigated whether adsorption to unconsumed food particles may have limited fipronil bioavailability by using solid-phase microextraction and gas chromatography to measure the available fipronil from fed and unfed systems 24h after establishment. Differences between the systems were not significant (P>0.05). The significantly higher LC(50) values in the fed Polypedilum bioassays do not appear to be a consequence of reduced fipronil bioavailability. Observed differences in toxicity probably reflect increased stresses associated with food deprivation in the unfed bioassays. Our results support published data on the toxicity of fipronil to aquatic invertebrates which suggest that the use of this material as a mosquito larvicide may cause disruption to aquatic ecosystems.

Analysis of molecular markers for metamorphic competency and their response to starvation or feeding in the mosquito, Aedes aegypti (Diptera: Culicidae)

The nutritional condition of fourth instar larvae of the yellow fever mosquito, Aedes aegypti, governs female longevity and egg production, both are key determinants of pathogen transmission. As well, nutrition provisions larval growth and development and attains its greatest pace in the last larval instar in preparation for metamorphosis to an adult. These developmental processes are regulated by a complex endocrine interplay of juvenile hormone, neuropeptides, and ecdysteroids that is nutrition sensitive. We previously determined that feeding for only 24h post-ecdysis was sufficient for fourth instar Ae. aegypti larvae to reach critical weight and accumulate sufficient nutritional stores to commit to metamorphosis. To understand the genetic basis of metamorphic commitment in Ae. aegypti, we profiled the expression of 16 genes known to be involved in the endocrine and nutritional regulation of insect metamorphosis in two ways. The first set is a developmental profile from the beginning of the fourth instar to early pupae, and the second set is for fourth instars starved or fed for up to 36 h. By comparing the two sets, we found that seven of the genes (AaegCYP302, AaegJHE43357, AaegBrCZ4, AaegCPF1-2, AaegCPR-7, AaegPpl, and AaegSlif) were expressed during metamorphic commitment in fourth instars and in fed but not starved larvae. Based on these results, the seven genes alone or in combination may serve as molecular indicators of nutritional and metamorphic status of fourth instar Ae. aegypti larvae and possibly other mosquito species in field and laboratory studies to gauge sub-lethal effects of novel and traditional cultural or chemical controls.

Juvenile hormone esterase: biochemistry and structure

Normal insect development requires a precisely timed, precipitous drop in hemolymph juvenile hormone (JH) titer. This drop occurs through a coordinated halt in JH biosynthesis and increase in JH metabolism. In many species, JH esterase (JHE) is critical for metabolism of the resonance-stabilized methyl ester of JH. JHE metabolizes JH with a high kcat/KM ratio that results primarily from an exceptionally low KM. Here we review the biochemistry and structure of authentic and recombinant JHEs from six insect orders, and present updated diagnostic criteria that help to distinguish JHEs from other carboxylesterases. The use of a JHE-encoding gene to improve the insecticidal efficacy of biopesticides is also discussed.

Accumulated organic debris in catch basins improves the efficacy of S-methoprene against mosquitoes in Toronto, Ontario, Canada

To control West Nile Virus in the greater Toronto area of Ontario, Canada, S-methoprene (Altosid XRbriquets 2.1% AI) is applied each year to storm water catch basins. Because the efficacy of the XRbriquets to reduce adult mosquito populations had not been evaluated locally and was influenced by organic debris in a pilot study, we compared the efficacy of the briquets in 17 sediment and debris-filled catch basins versus 20 catch basins that were vacuumed free of debris. Emergence rates approached 100% in the 5 untreated control catch basins. Emergence rates were significantly lower, and S-methoprene was detected more often and at higher levels, in debris-filled basins versus cleaned catch basins. Overall, 20% of pupae emerged from clean catch basins versus only 3% from debris-filled ones, the difference between treatments becoming significant after 26 days. S-methoprene and total organic carbon concentrations in the catch basins were positively correlated (P < 0.001). We hypothesize that S-methoprene is binding to the organic fraction in the water and sediment in the debris-filled basins, prolonging S-methoprene doses, which are reflected in lower mosquito emergence rates.