Effect of plumbagin on gonadotrophic cycle of the housefly, Musca domestica L

Topical treatment of the phytochemical plumbagin in doses ranging 0.005-5 micrograms prevented oocyte development and affected fecundity and fertility in M. domestica. The treatment to wandering larvae was less effective as the compound could only effect the fertility to a significant level whereas the fecundity was not significantly reduced. The effect of the compound was more pronounced in adult treatments where both fecundity and fertility reduced drastically. The compound also effected the oocyte maturation as it arrested the development of vitellogenic oocyte at stage six. As the juvenile hormone analogue methoprene and moulting hormone 20-hydroxyecdysone or the mixture of these hormones could not restore the development of the oocyte in ovaries of plumbagin treated flies, it is concluded that the compound does not effect the female houseflies through hormonal pathways, instead in all probability it acts like a cytotoxic compound.

Effect of inactivation by sunlight on the larvicidal activities of mosquitocidal Bacillus thuringiensis H-14 isolates from Nigerian soils

Bacillus thuringiensis H-14 is a well recognized bioinsecticide against mosquitoes. This study investigates the inactivation effect of sunlight on the larvicidal activities of four Nigerian isolates of B. thuringiensis H-14, code-named OBG1, OBG8, GSC3 and GNA13 as compared with a standard mosquito larvicide, B. thuringiensis var. israelensis (BTI). A 0.125 mg/1 suspension of spore-delta-endotoxin powder of each isolate was exposed to direct sunlight for 7 h on a concrete roof (ca. 13 m high). After exposure, the toxicity of each bacterial suspension to Aedes aegypti larvae was determined by bioassay. Three of the isolates, OBG8, OBG1 and GSC3 exhibited higher larvicidal activities than B.T.I. against A. aegypti larvae after exposure to sunlight. The resistance of Nigerian isolates to inactivation effects of sunlight advocates their potential in biocontrol of mosquitoes.

Adverse assessments of Gambusia affinis: an alternate view for mosquito control practitioners

Adverse opinions on the introduction of Gambusia affinis for the control of larval mosquitoes are reviewed. The sources span a period of some 59 years and come from a variety of sources. The principal opposition to the introduction of G. affinis comes from ichthyologists, although some mosquito researchers have expressed concerns about the environmental impact of placing the fish in habitats to which it is not native. Questions concerning the appropriateness of using the fish are presented.

New insights into the mechanisms of fungal pathogenesis in insects

Entomopathogenic fungi are attracting attention as potential biological control agents of insect pests. The mechanisms of pathogenesis have parallels with those of some plant-pathogenic fungi, particularly in the areas of formation of an infection structure, entry into the host and toxin-mediated host death. Understanding these processes will provide a rational basis for strain selection and improvement.

Optimization of blends of synthetic pheromone components for trapping male limabean pod borers (Etiella zinckenella Tr.) (Lepidoptera: Phycitidae): preliminary evidence on geographical differences

From the five previously identified pheromone components of the limabean pod borer (Etiella zinckenella Tr.) (Lepidoptera: Phycitidae), the 100:3 mixture of (Z)-11-tetradecenyl acetate and (Z)-9-tetradecenyl acetate was necessary for maximal attraction of males into traps in tests performed in Hungary. The addition of (E)-11-tetradecenyl acetate in percentages higher than 10-30% generally had an adverse effect on catches, while the addition of the other two compounds present in pheromone extracts had no influence on catches. In contrast to the results in Hungary, none of the traps baited with combinations of the above compounds captured any moths in tests performed in Taiwan, suggesting possible geographical differences in pheromonal response of European and Eastern Asian populations of E. zinckenella.

Efficient synthesis of mosquitocidal toxins in Asticcacaulis excentricus demonstrates potential of gram-negative bacteria in mosquito control

The control of mosquitoes with chemical insecticides pollutes the environment and leads to resistance in mosquito populations. Bacterial control of mosquito larvae with Bacillus sphaericus and Bacillus thuringiensis subsp. israelensis, which produce protein toxins, has proved useful, safe, and nonpolluting. These bacteria do, however, suffer from disadvantages, including rapid setting, UV sensitivity, and lack of persistance of spores, proteolysis of toxins, narrow host range, and high production costs. Here we show that the Gram-negative bacterium Asticcacaulis excentricus is a promising host for delivering toxins to mosquito larvae. Plasmid-transformed A. excentricus cells expressing the binary toxin of B. sphaericus exhibited toxicity to Culex and Anopheles mosquito larvae similar to that of the high-toxicity strains of B. sphaericus which produce several toxins. A. excentricus has potential advantages as a larvicide compared with the bacilli, especially persistance in the larval feeding zone, resistance to UV light, lack of toxin-degrading proteases, and low production costs.

Improved delivery of biocontrol Pseudomonas and their antifungal metabolites using alginate polymers

Alginate polymer was evaluated as a carrier for seed inoculation with a genetically modified strain Pseudomonas fluorescens F113LacZY, which protects sugar-beet against Pythium-mediated damping-off. F113LacZY survived in alginate beads at 5 log10 CFU/bead or higher counts for 8 weeks of storage, regardless of the conditions of incubation. In plant inoculation experiments, colonisation of the growing area of the root by F113LacZY, derived from alginate beads placed in the soil next to the seed or from an alginate coating around the seeds, was improved compared with application of just free cells of the strain. F113LacZY trapped in alginate beads was an effective producer of antifungal phloroglucinols as indicated by direct HPLC quantification of phloroglucinols and in vitro inhibition of both the indicator bacterium Bacillus subtilis A1 and the pathogenic fungus Pythium ultimum. Alginate polymer represents a promising carrier for the delivery of biocontrol inoculants for root colonisation and production of antifungal metabolites.

Transgenic Indica rice breeding line IR58 expressing a synthetic cryIA(b) gene from Bacillus thuringiensis provides effective insect pest control

The Indica rice breeding line IR58 was transformed by particle bombardment with a truncated version of a synthetic cryIA(b) gene from Bacillus thuringiensis. This gene is expressed under control of the CaMV 35S promoter and allows efficient production of the lepidopteran specific delta-endotoxin. R0, R1 and R2 generation plants displayed a significant insecticidal effect on several lepidopterous insect pests. Feeding studies showed mortality rates of up to 100% for two of the most destructive insect pests of rice in Asia, the yellow stem borer (Scirpophaga incertulas) and the striped stem borer (Chilo suppressalis), and feeding inhibition of the two leaffolder species Cnaphalocrocis medinalis and Marasmia patnalis. Introduction of stem borer resistance into the germplasm of an Indica rice breeding line now makes this agronomically important trait available for conventional rice breeding programs.

Biotechnology and new integrated pest management approaches

Area-wide pest management technologies will take on new appearances as the drive to eliminate and/or greatly reduce the use of chemical pesticides increases. The use of genetically altered insects has the most potential for successfully displacing certain pesticides, although the development of genetic engineering technologies for agricultural pest species is still in its infancy. Transformation vectors need to be developed as do transformation methodologies. Here we report the possibility of developing an interspecies vector and discuss ways in which such a vector could be used successfully in an integrated pest management system. If such an approach were developed, it could be utilized with other alternative methods, thereby providing a safe, ecologically sound means of controlling insect pests without damaging the agricultural economy.

Chaos: a potential problem in the biological control of insect pests

Erratic variations are normally observed in the populations of insect pests that destroy crop plants. To establish a scientific basis for developing effective control procedures, we have developed a model system for the European Corn Borer (ECB) (Ostrinia nubilalis) for which extensive field data, as well as laboratory results, have been accumulated during the past four decades. The model includes both a natural ECB pathogen and a genetically engineered toxin-producing agent as possible means of biological control. Our aim was to determine the conditions that could cause the population to vary erratically, as observed in the field. The erratic behavior in our simulations was analyzed to determine whether it is chaotic; chaos is a distinct type of erratic behavior which shows extreme sensitivity to initial conditions, i.e., the starting size of the population. Our simulations show that an increase in the death rate of the infected ECB, or a decrease in the birth rate of uninfected ECBs from infected ones, variables that are known to be affected by weather conditions, can induce a chaotic regime in which ECB population peaks reach values far higher than before chaos set in. Population peaks are even greater in the presence of both biological control agents. The results show that a biological control regime cannot be effective under conditions that induce chaotic population dynamics. Microcosm studies could be used to determine whether this situation would occur in the field.

Some factors affecting the activity and pathogenicity of Heterorhabditis heliothidis and Steinernema carpocapsae nematodes

The activity of Heterorhabditis heliothidis (Khan etal) and Steinernema carpocapsae (Weiser) and pathogenicity of those nematodes in relation to cotton leafworm Spodoptera littoralis (Boisd) has been studied under certain factors (soil moisture, temperature, acidity and depth of soil). The results have indicated that the activity and pathogenicity of H. heliothidis and S. carpocapsae infective juveniles of entomopathogenic nematodes placed below the soil surface depend on factors mentioned before. It has been demonstrated that the free living stages of infective larvae of H. heliothidis and S. carpocasae are more active and quicker in killing of insects in the soil of the PH. value above 6.9 and 8.0 than 5.6. In 15 degrees C only H. heliothidis caused the host mortality. In 20 degrees C and 25 degrees C both nematodes showed high activity. There was no significant difference in the effect of 20 degrees C on the activity of both H. heliothidis and S. carpocapsae but in the temperature of 25 degrees C the activity of H. heliothidis was higher than that of S. carpocapsae. When cotton leafworm larvae are placed on the soil surface at different distances from the nematodes the shortening of those distances resulted in the shortening of the insect longevity and increasing the percentage of insect mortality.

[Biological control in the preservation of pome fruits]

Fresh fruits are susceptible of be attacked by several pathogenic fungi after harvest due to both their high water and nutrients content and their loss of most of the intrinsic resistance that protected them over their development while attached to the plant. Most rot pathogens can be controlled by various methods such as refrigeration, controlled atmospheres and fungicides. Biological control strategies are emerging as promising alternatives to the use of synthetic fungicides. Several factors must be considered for the selection of biocontrol agents to be used against postharvest fruits diseases. Survivability of the antagonist is a major factor to determine its usefulness. Antagonists must survive and be effective after their exposure to both postharvest treatments and storage conditions. Several antagonistic microorganisms have been found that can effectively inhibit postharvest diseases. Just as there is a diversity among microorganisms, there is also a diversity of mechanisms by which they operate. Although in most cases these mechanisms have not been satisfactorily elucidated, they are likely to involve antibiosis, nutrient competition, stimulation of host defense, predation and parasitism. In many cases, probably more than one mechanism operate. The marketing of some of these antagonists may be feasible and they could be an alternative to synthetic pesticides.

Bacillus thuringiensis: fermentation process and risk assessment. A short review

Several factors make the local production of Bacillus thuringiensis (Bt) highly appropriate for pest control in developing nations. Bt can be cheaply produced on a wide variety of low cost, organic substrates. Local production results in considerable savings in hard currency which otherwise would be spent on importation of chemical and biological insecticides. The use of Bt in Brazil has been limited in comparison with chemical insecticides. Although Bt is imported, some Brazilian researchers have been working on its development and production. Fermentation processes (submerged and semi-solid) were applied, using by-products from agro-industries. As the semi-solid fermentation process demonstrated to be interesting for Bt endotoxins production, it could be adopted for small scale local production. Although promising results had been achieved, national products have not been registered due to the absence of a specific legislation for biological products. Effective actions are being developed in order to solve this gap. Regardless of the biocontrol agents being considered atoxic and harmless to the environment, information related to direct and indirect effects of microbials are still insufficient in many cases. The risk analysis of the use of microbial control agents is of upmost importance nowadays, and is also discussed.

Biological control program against simuliids in the State of São Paulo, Brazil

In Brazil, the use of biological vector-control methods has been largely confined to experimental research, with little or no application of such techniques by public institutions responsible for implementing control programs. The notable exceptions have been the black fly control program carried out by the Health Secretariat in the State of São Paulo. Since the 1980s, São Paulo's "Superintendência de Controle de Endemias" has been conducting studies on the viability of using Bacillus thuringiensis (H-14) for simuliid control, and the results have been so encouraging that the agency has now incorporated this method into its Simuliid Control Program.

RAPD-PCR for identification of Zoophthora radicans isolates in biological control of the potato leafhopper

Biological control studies require the ability to distinguish released pathogens from locally occurring isolates of the same species. We have developed a technique that differentiates genotypes using random amplified polymorphic DNA (RAPD) for the apomictic species Zoophthora radicans (Zygomycota: Entomophthorales), a pathogen of the potato leafhopper, Empoasca fabae (Homoptera: Cicadellidae). RAPD analysis was performed on Z. radicans isolates released in test plots in 1990 and 1991 for leafhopper control; isolates later recovered from the same plots and diverse other isolates were included in the analysis. RAPD fragment profiles of five recovered isolates proved very similar to those of the released isolates and different from all other isolates tested; they are probable descendants of the released isolates. One of the recovered isolates had RAPD profiles similar to isolates derived from aphids and probably represents a population endemic at the release site. In addition to verifying the successful establishment of our experimental releases, RAPD analysis revealed clear relationships among isolates derived from the same host taxon. We propose that this simple and relatively inexpensive method will be valuable in determining the establishment and spread of organisms released in biological control studies.

Protocol for the introduction of new Bacillus thuringiensis Israelensis products into the routine mosquito control program in Germany

The efficacy of new and frequently used formulations of Bacillus thuringiensis israelensis (B.t.i.) has been evaluated in the laboratory and in the field conditions under which they will be used in Germany. The principles governing the successful introduction of new formulations of microbial control agents into routine programs have been highlighted. The potency of the formulations in use (Teknar, Bactimos, and Vectobac) and their efficacy against the indigenous mosquito species Aedes vexans, Aedes cantans, and Culex pipiens have been assessed to determine the minimum effective dosage in the laboratory and the optimum effective dosage in small field trials. These results should ensure the proper selection of the most appropriate formulation and dosage for the particular climatic and ecological conditions.

Evaluation of granular corncob formulations of Bacillus thuringiensis serovar israelensis against mosquito larvae using a semi-field bioassay method

A semi-field bioassay method using commonly available laundry tubs evaluated efficacy and residual activity of 6 granular corncob formulations of Bacillus thuringiensis serovar israelensis against laboratory-reared late 3rd- and early 4th-instar larvae of Aedes taeniorhynchus introduced to the tubs. These formulations produced appreciable larval mortalities (up to 75%) for 9 days posttreatment, with indications of having more prolonged activity. The consistent posttreatment larval mortality trends and the elucidated efficacy differences between formulations suggest that this bioassay method for the granular formulations is reliable and reproducible.

[Ectoparasites of animals: methods of ecological, biological, genetic and mechanical control]

The use of insecticides is still the basic procedure for controlling most ectoparasites, but various methods are being developed to act in addition to, or in synergy with these products, so as to enhance the efficacy and reduce the adverse effects of insecticides, by contributing to ecologically acceptable strategies. These methods are classified as ecological control (modification of the environment of the parasite), biological control (predation, parasitism, action of pathogens, etc.), genetic control (release of sterile males, hybridisation, genetic manipulations) and mechanical control (insect traps, use of repellents). The application of such methods depends on the biological and ecological characteristics of the ectoparasite, and they may act directly or indirectly, affecting mortality and/or reproduction. The authors review the principal methods applicable to major groups of ectoparasites of veterinary interest. Non-chemical methods are the subject of wide-ranging and promising research, particularly in view of recent developments in biotechnology.

Larvicidal activity of the entomopathogenic fungus Tolypocladium cylindrosporum (Deuteromycotina: Hyphomycetes) on the mosquito Aedes triseriatus and the black fly Simulium vittatum (Diptera: Simuliidae)

Laboratory bioassays evaluated the pathogenicity of the entomopathogenic fungus Tolypocladium cylindrosporum against Aedes triseriatus. All larval instars were found to be susceptible at temperatures from 18 to 25 degrees C. Blastoconidia were more virulent than conidia. Mortality was proportional to exposure time in assays using blastoconidia. Blastoconidia and exposure time appear to be a more adequate method for bioassay of T. cylindrosporum virulence than the standard procedure of using conidia and continuous exposure. Tolypocladium cylindrosporum was also active against Simulium vittatum black fly larvae, but the mortality recorded was much lower than with mosquito larvae.

Aquatic safety of Lagenidium giganteum: effects on freshwater fish and invertebrates

Eleven freshwater species were exposed to a zoosporeproducing fungus, Lagenidium giganteum, with the goal of determining species sensitivity with standard and new test procedures. The tests included standard, 4-day acute exposures of cladocerans (Ceriodaphnia dubia, Daphnia pulex, and D. magna) and the fathead minnow (Pimephales promelas). Standard 7-day chronic exposures of C. dubia and a 7-day embryo-larval exposure of P. promelas were also conducted. New, 4-day acute, methods were developed for mosquitos (Aedes aegypti), chironomids (Chironomus sp.), oligochaetes (Lumbriculus sp.), cyclopoid copepods, snails (Physa sp.), hydrozoans (Hydra sp.), and ostracods. To assess L. giganteum zoospore (z) infectivity, each test included daily bioassays with the mosquito (A. aegypti), a target organism. Four-day A. aegypti LC50s ranged from 81 to 516 z/ml. Ceriodaphnia dubia acute test LC50s were as low as 6700 z/ml and the 96-hr LC50 from the chronic test was near 6250 z/ml with reproductive impairment at 12,500 z/ml. Daphnia sp. were also susceptible, with LC50s near 7700 z/ml for D. pulex and 9400 z/ml for D. magna. Chironomus tentans was infected at concentrations of > or = 5000 z/ml, but mortality was low and an LC50 could not be calculated even after exposures to 50,000 z/ml. The 7-day, early life stage test with P. promelas produced reduced larva growth in most treatments. Several species (Hydra sp., L. variegatus, ostracoda, copepoda, Physa sp., and P. promelas) were not affected in acute tests at exposures of 50,000 z/ml. The data show, contrary to many reports, that L. giganteum may affect some nontarget aquatic species. The key to successful laboratory tests is monitoring and maintaining the zoospores infection capacity.

Integrated management of waste tire mosquitoes utilizing Mesocyclops longisetus (Copepoda: Cyclopidae), Bacillus thuringiensis var. israelensis, Bacillus sphaericus, and methoprene

This study evaluated the compatibility and efficacy of using a predatory copepod, Mesocyclops longisetus in concert with 3 "biorational" compounds for mosquito control in waste tires. The toxicity of Bacillus thuringiensis var. israelensis (B.t.i), Bacillus sphaericus, and methoprene to Mesocyclops longisetus was assessed in the laboratory using concentrations 10 times the maximum labeled or suggested rate and based on a water depth of 7.6 cm. Microbials were tested using mature copepods exposed for durations of 24, 48, and 72 h. Methoprene bioassays consisted of individually exposing newly hatched copepods (i.e., nauplius larvae) and monitoring their development to maturity. The toxicity tests indicated B.t.i., B. sphaericus, and methoprene were not deleterious to copepods at concentrations exceeding those expected in the field. Copepods exposed to methoprene matured normally, and when mated, 50% developed egg sacs. A 5-month field test, integrating the copepod and B.t.i., B. sphaericus, and methoprene provided better mosquito reduction together than either copepods or control agents alone. When copepods were combined with B.t.i. or methoprene, overall reduction of 3rd- and 4th-instar larvae during the 5-month interval was equal to or greater than 90%. Bacillus thuringiensis var. israelensis alone temporarily produced a high degree of larval reduction (up to 100%), however reapplications were necessary to maintain that level of control. Of all the treatments, B. sphaericus alone produced the lowest degree of mosquito suppression due to lack of toxicity to Aedes albopictus, the predominant species during the study. It is recommended that mosquito control managers consider integrating M. longisetus and B.t.i. or methoprene against mosquitoes in waste tires.

Recombinant Bacillus thuringiensis crystal proteins with new properties: possibilities for resistance management

To obtain Bacillus thuringiensis crystal proteins with new properties and to identify the regions involved in insecticidal activity, we generated hybrid genes composed of cryIC and cryIE by in vivo recombination. Analysis of the hybrid proteins showed that domain III of CryIC is involved in the toxicity towards Spodoptera exigua and Mamestra brassicae. Transfer of this domain to CryIE, which is not active against these insects, resulted in a new protein with a broader activity. This hybrid protein binds to different receptors than CryIC, suggesting its use as an alternative for CryIC in resistance management programs.

Emergency control of Aedes aegypti in the Dominican Republic using the Scorpion 20 ULV forced-air generator

In an effort to develop a more effective measure for use in emergency control of the dengue vector Aedes aegypti. applications of a combination of a larvicide (Bacillus thuringiensis israelensis [B.t.i.]) and an adulticide (permethrin) were made using a truck-mounted forced-air generator (Scorpion 20) and evaluated in the Dominican Republic. This method has the potential to simultaneously control adults and larvae. In bioassay cages placed in household water containers at the time of application, larval mortalities were 95.1 and 100% for 2 application rates of permethrin mixed with B.t.i. Adult mortalities were not as impressive, probably because of resistance to permethrin. Higher adult mortality in caged specimens (78.5%) and a substantial reduction in the natural population (68.4%) of Ae. aegypti were obtained following a 2.1-g AI/ha application of deltamethrin alone.

Pilot scale production and application in wildlife ponds of Lagenidium giganteum (Oomycetes: Lagenidiales)

Lagenidium giganteum, a facultative parasite of mosquito larvae, has recently been registered by the U.S. Environmental Protection Agency for operational mosquito control. We report here the first pilot scale production of the mycelium formulation. Scale-up from 10 to 650 liters was accomplished by a proportionate increase of medium components and volume of water. Foaming of the culture medium had not been encountered previously, but was a serious problem in pilot scale production due to the very rapid growth of a large volume of L. giganteum. Addition of an antifoaming agent did not adversely affect growth, but reduced the ability of the fungus to sporulate. Despite what was effectively a 100-fold reduction in the desired application rate due to reduced sporulation, L. giganteum infected sentinel mosquito larvae and reduced field populations for more than 2 months following application.

Laboratory and field evaluation of Bacillus thuringiensis and B. sphaericus against mosquito larvae

Laboratory and field trials were carried out with two formulations of Bacillus thuringiensis and four strains of Bacillus sphaericus (B 42, B 64, B 87 and B 33) against mosquito larvae in different breeding habitats of Tezpur, Assam. LC90 of B. thuringiensis var israelensis (formulation Teknar) against Ae. albopictus, Cx. quinquefasciatus and Cx. gelidus were recorded as 0.443, 0.453 and 2.15 ppm respectively and LC90 of B. thuringiensis (Deltox: VCRC B-17) against Ae. albopictus, Cx. quinquefasciatus, Cx. gelidus and Cx. malayi were 8.414, 11.22, 5.24 and 6.761 ppm respectively. LC90 of B. sphaericus strains B 42, B 64, B 87 and B 33 against Cx. quinquefasciatus were 0.055, 0.115, 0.046 and 0.257 ppm respectively. At the dosage of 1 l/ha 87 per cent mortality was achieved after 24 hrs with Bti and it increased to 90-95 per cent at 1.5 l/ha. In polluted cemented drains 93-97 per cent kill of Cx. quinquefasciatus was observed at 2.5 l/ha. Out of four strains of B. sphaericus evaluated, strain B 87 was found to be the most effective as 87-96 per cent kill was achieved with only 0.1 kg/ha for Cx. quinquefasciatus, Cx. vishnui and A. vagus. For others 0.2 kg/ha dosage, eliminated 96-100 per cent Cx. vishnui gr. for B 42, 92-93 per cent for B 64 and 90-93 per cent for B 33 strain.

Selfish DNA as a method of pest control

The inheritance of most genes is tightly controlled, governed by the rules of mendelian inheritance if nuclear or uniparental inheritance if cytoplasmic. A few notable genes and cytoplasmic genomes have escaped this regulation. Such genes may spread by increasing their own rate of transmission despite reducing host fitness and may be regarded as 'selfish'. Their population genetics are described and it appears they may impose a significant genetic load on the host population. Modern molecular techniques may enable similar loads to be imposed on pest species either by transferring selfish genes between species, or by linking deleterious genes to a selfish locus. Alternatively, 'modifier' genes that eliminate the virulent, or disease vectorial capacity, of the pest population may be introduced by linkage to a selfish locus. Selfish elements present in multiple copies may be preferable to single-copy elements as the former are capable of a larger reduction in host fitness. The practical application of these agents depends on five factors: (i) the rate of 'reversion' to a non-selfish form; (ii) the evolution of host repressor systems; (iii) their effect on host fitness, which determines their rate of invasion; (iv) the mechanism regulating host population size in the field; and (v) their ease of manipulation in the laboratory. The first two factors are the most uncertain in most systems, but should be amenable to experimental analysis. It is proposed that the development of such techniques may result in powerful new methods of population control which may be applied to both agricultural pests and disease vectors.

Protozoan-enhanced toxicity of Bacillus thuringiensis var. israelensis delta-endotoxin against Aedes aegypti larvae

The toxicity of Bacillus thuringiensis var. israelensis (Bti) in mosquito larvae was enhanced by encapsulation in the protozoan Tetrahymena pyriformis. Aedes aegypti larvae which fed on T. pyriformis loaded with Bti died about three times faster than when fed on the same concentrations of Bti alone due to ingestion of higher toxin concentrations, reflected by shorter death times of exposed populations. The best larvicidal activities were achieved at ratios of cell/spore numbers in the range of 1:200 to 1:500. This enhancement of mortality by preincubation with T. pyriformis was higher at low Bti concentrations or in late third-instar larvae. Ninety minutes of preincubation yielded the best enhancement effect. Toxicity enhancement is very likely a consequence of concentrating large quantities of Bti spores and crystals (containing delta-endotoxin) by T. pyriformis cells and delivering them to the larvae. Shortening larval mortality time by encapsulation in T. pyriformis should reduce the exposure time of Bti to unfavorable field conditions that inactivate its larvicidal activity. Whether this method will indeed improve Bti efficacy is still to be determined.

Fixed-wing, aerial application of liquid Bacillus thuringiensis H-14 (Acrobe) for control of spring Aedes mosquitoes in Michigan

Liquid Bacillus thuringiensis H-14 (Acrobe) was applied from fixed-wing aircraft at a rate of 4.68 liters of water-insecticide mixture (1.17 liter concentrate) per hectare to woodland pools in Michigan. A post-treatment larval survey indicated an 88.5% reduction in Aedes species larvae. A volume median diameter of 208 microns was determined.

Incorporation of body components of diverse microorganisms by larval mosquitoes

A pulse-purge schedule of exposure to labeled microorganisms was used to compare their digestibility by larval mosquitoes. Larvae were placed for an hour in suspensions of diverse axenically grown microorganisms that had been labeled with radioactive carbon (in the form of glucose or glycine). The guts of these mosquitoes were then purged with nonlabeled Sephadex particles for 30 min, and retained radioactivity was measured. Larvae imbibed no dissolved material. Larval mosquitoes differ in their capacity to derive label from algae (sensu lato), and certain algae contribute more label to these mosquitoes than do others. The nature of any algal food, as well as the feeding habits and developmental stage of the larva, influence its capacity to derive label from algae. This pulse-purge method of analysis can assist in the selection of algal "vectors" suitable as vehicles for transgenic larvicide. Although larval mosquitoes fail to assimilate the contents of Palmellacoccus cells with which they are confined, as much as 1/3 of the body contents of a Euglena gracilis cells become incorporated into their bodies. Because larval mosquitoes internalize more material from Euglena than they do from various other algae, these microorganisms provide a promising candidate vehicle for transgenic Bacillus thuringiensis israelensis.

Efficacy of Clostridium bifermentans serovar Malaysia on target and nontarget organisms

Clostridium bifermentans serovar malaysia (C.b.m.) is highly toxic to mosquito larvae. In this study, the following aquatic nontarget invertebrates were treated with high C.b.m. concentrations (up to 1,600-fold the toxic concentration for Anopheles stephensi) to study their susceptibility towards the bacterial toxin: Planorbis planorbis (Pulmonata); Asellus aquaticus (Isopoda); Daphnia pulex (Cladocera); Cloeon dipterum (Ephemeroptera); Plea leachi (Heteroptera); and Eristalis sp., Chaoborus crystallinus, Chironomus thummi, and Psychoda alternata (Diptera). In addition, bioassays were performed with mosquito larvae (Aedes aegypti, Anopheles stephensi, and Culex pipiens). Psychoda alternata larvae were very susceptible, with LC50/LC90 values comparable to those of mosquito larvae (about 10(3)-10(5) spores/ml). The tests with Chaoborus crystallinus larvae showed significant mortality rates at high concentrations, but generally not before 4 or 5 days after treatment. The remaining nontarget organisms did not show any susceptibility. The investigation confirms the specificity of C.b.m. to nematocerous Diptera.

Genetic manipulation of insect vectors as a strategy for the control of vector-borne disease

A variety of very effective methods have been employed for suppressing insect vector populations, including the application of biological control agents and the elimination of breeding sites, with a continuing and heavy reliance on the use of chemical insecticides. However, the development of insecticide resistance by vector insects, the cost of developing and registering new insecticidal compounds, and the increase in legislation to combat the detrimental effects of insecticidal residues on the environment, have emphasized the need to assess alternative strategies for vector control. What is required is a completely novel approach to either suppress vector populations, or to alter their ability to transmit disease-causing organisms in such a way as to have a profound and long-lasting effect on disease transmission. Genetic manipulation of insect vectors may provide just such an approach. The major requirements for genome manipulation in insects and the progress which has been made to create transgenic vector insects are reviewed. The potential applications of this methodology are then explored in the context of its future use for the control of vector-borne diseases.

Factors influencing predation of the waterbugs Sphaerodema annulatum (Fab). and S. rusticum (Fab.) on the disease transmitting snail Lymnaea (Radix) luteola (Lamarck)

Variations in the rate of predation of the waterbugs Sphaerodema annulatum and S. rusticum on the snails Lymnaea (Radix) luteola have been noted in respect to the morphs of the waterbugs, size of the prey individuals, densities of prey and predators, temperature and surface area of the waterbody concerned and the seasons. Consumption rate was highest (7.2 and 2.2 individuals per day per individual of S. annulatum and S. rusticum, respectively) in prereproductive ages of the waterbugs. This was followed by a gradual decline with the increase in age of the predators. The consumption rate was gradually higher with the increase of temperature from 20 degrees C to 35 degrees C. The bugs failed to survive beyond 35 days at 35 degrees C. Though the bugs prey upon the snails of all sizes preference for 6.5 x 4.5 mm to 8 x 5 mm individuals by S. annulatum and for 5 x 3 mm to 6.5 x 4.5 mm individuals by S. rusticum is established. The waterbugs, irrespective of species, consumed the snail individuals belonged to 3 x 2 - 4 x 3 mm size group maximum when supplied separately. The rate of predation gradually declined with the rise of predator's density irrespective of waterbug species. Predation rate increased with increasing prey density. This was level off when the prey snails were 1100 and 700 in number for S. annulatum and S. rusticum respectively. An adult S. annulatum and S. rusticum consumed 5.04, 3.7, 1.43 and 3.36, 2.49, 1.04 snails per day respectively in summer, monsoon and winter.

Pathogenicity and limited transoothecal transmission of Steinernema carpocapsae (Nematoda: Steinernematidae) in adult female German cockroaches (Dictyoptera: Blattellidae)

Nongravid female German cockroaches, Blattella germanica (L.), and females with young or old oothecae were exposed to infective stage juvenile (L3) Steinernema carpocapsae (Weiser) All strain by topical application and continuous exposure. Topically applied nematodes resulted in < or = 40% mortality 7 d after inoculation, whereas 100% of females continuously exposed to nematodes were dead within 7 d. Females carrying old oothecae had significantly lower LT50s (0.07 h) than females with young oothecae (42.71 h) and females without oothecae (38.49 h). Fully formed oothecae were not penetrated by the nematodes, even though infective juveniles were found in the vestibulum between the female genitalia and on the proximal end of the ootheca. Partially formed oothecae were infested (100%) by nematodes derived from the female. Topical application of nematodes to the proximal end, side, or keel of mature oothecae that were detached from females did not result in nematode infestation, and the nymphs emerged normally. Oothecae continuously exposed to nematodes were similarly not infested but exhibited significant mortality, probably caused by bacterial infection.

Control of larval Aedes aegypti (Diptera: Culicidae) by cyclopoid copepods in peridomestic breeding containers

Mesocyclops longisetus (Thiébaud), Mesocyclops thermocyclopoides Harada, Mesocyclops venezolanus Dussart, and Macrocyclops albidus (Jurine) were tested for their effectiveness in controlling Aedes aegypti (L.) larvae in a variety of containers around homes in El Progreso, Honduras. All four cyclopoid species killed > 20 larvae per cyclopoid per d under container conditions. M. longisetus was most effective, not only because it was the most voracious predator, but also because it survived best in the containers. M. longisetus maintained long-term populations in 200-liter drums, tires, vases, and cement tanks (without drains), providing the cyclopoids were not dried or poured out. M. longisetus reduced third- and fourth-instar Ae. aegypti larvae by > 98% compared with control containers without cyclopoids. M. longisetus should be of practical value for community-based Ae. aegypti control if appropriate attention is directed to maintaining it in containers after introduction.

Influence of soil moisture on survival and oviposition of Romanomermis iyengari a mermithid nematode parasite of mosquitoes

The oviposition pattern of Romanomermis iyengari (Nematoda: Mermithidae) in relation to the moisture level in the soil was studied by seeding postparasites into two sets of 12 cm soil columns. While one set of the soil column was allowed to dry over a period of 60 days, the other set was maintained at a constant level of 15-20% moisture throughout the soil column. In the soil column maintained at constant moisture level, the postparasites oviposited primarily in the upper 3 cm layer (80-97%). In the soil column which was allowed to dry the percentages of eggs laid in the upper 3 cm layer were 96, 24 and 6 respectively on D20, D30 and D40 after seeding and the moisture levels on respective days were 12, 5 and 3%. On corresponding days the postparasites laid 1, 56 and 42% of the eggs in the lower most layer (9-12 cm) and the moisture levels were respectively 15, 10 and 9%. The results indicate that the adult nematodes migrated towards the bottom layer of the soil and laid eggs as the moisture of the upper layers decreased.

Biocontrol efficacy of Gerris (A) spinolae, Laccotrephes griseus and Gambusia affinis on larval mosquitoes

Predation experiments using Gerris (A) spinolae, Laccotrephes griseus and Gambusia affinis were conducted against IV stage culicine larvae with varying prey densities. Ranking of individual predatory efficiency showed the sequence: large Gambusia > medium Gambusia > small Gambusia > female Laccotrephes > male Laccotrephes > Gerris. Predation under coexistence reveals the significance of predatory efficiency of different predator combinations with reference to prey density and exposure period.

Extracellular serine protease synthesis by mosquito-pathogenic strains of Bacillus sphaericus

Nine strains of Bacillus sphaericus toxic to mosquito larvae produced haloes of hydrolysis when cultured on casein-nutrient agar. In batch culture, protease synthesis by B. sphaericus BSE 18 occurred during exponential growth and was repressed by high concentrations of peptone. Extracellular protease from this bacterium showed optimal activity at about pH 10.2, was inhibited by phenylmethylsulphonyl chloride and chymostatin but not soybean trypsin inhibitor or EDTA. Hydrolysis of N-CBZ-glycine-nitrophenyl ester was consistent with the major enzyme being a serine protease.

New high-toxicity mosquitocidal strains of Bacillus sphaericus lacking a 100-kilodalton-toxin gene

Five new high-toxicity mosquitocidal strains of Bacillus sphaericus were isolated in Singapore. They all belong to phage group 8 and have binary toxin (51.4- plus 41.9-kDa) genes located on the chromosome but lack a 100-kDa-toxin gene. These strains of B. sphaericus constitute a new subgroup, as only two weakly toxic strains in phage group 8 have previously been described and all the known high-toxicity strains have both binary toxin and 100-kDa-toxin genes.

Juvenile turtles for mosquito control in water storage tanks

Juvenile turtles, Trachemys scripta, provided highly effective control of mosquito larvae in cement tanks (pilas) where water was stored for household cleaning. When single turtles were introduced to tanks with histories of high mosquito production, nearly all turtles remained in good health and no mosquito larvae survived to the pupal stage. Families welcome turtles in their water storage containers in Honduras. Humane conditions for turtles can be assured by providing small quantities of table scraps to supplement their diet and by placing a small floating platform in the tank for basking. Although turtles can serve as alternate hosts for Salmonella, available evidence suggests that turtles in tanks should not be a source of human infection. Further confirmation that there is no Salmonella hazard should precede routine use of turtles for mosquito control.

Protection from ultraviolet irradiation by melanin of mosquitocidal activity of Bacillus thuringiensis var. israelensis

A process for production, isolation, and purification of melanin produced by the fermentation of Streptomyces lividans 66 harboring a recombinant plasmid pIJ702-bearing tyrosinase gene has been developed. The efficacy of melanin in the protection of mosquito larvacidal activity of Bacillus thuringiensis var. israelensis against uv light has been studied. Results obtained by the live cell counts and the bioassay of residual mosquitocidal activity of B. thuringiensis var. israelensis after exposure to uv radiation showed that melanin is an excellent photoprotective agent.

The introduction of Melanoides tuberculata (Mollusca: Thiaridae) to the island of Saint Lucia (West Indies) and its role in the decline of Biomphalaria glabrata, the snail intermediate host of Schistosoma mansoni

A malacological survey was carried out in May 1992 in the whole hydrographic system of Saint Lucia 11 years after the end of a biological control programme to eliminate Biomphalaria glabrata, the snail intermediate host of Schistosoma mansoni. A competitor snail, Melanoides tuberculata, was introduced to Saint Lucia in 1978 and field experiments in several habitats were conducted by Prentice between 1978 and 1986. At the present time M. tuberculata is the most common freshwater snail in Saint Lucia. The results of the survey, undertaken in sites where B. glabrata occurred in large populations in the past showed (i) the absence of the snail hosts from seven sites now extensively colonized by the competitor (ii) the presence of B. glabrata in low or very low densities in 17 sites together with the competitor and (iii) the presence of the intermediate hosts in large populations in only two sites where M. tuberculata was absent. These results confirm the positive results observed by Prentice. The presence of another planorbid snail, B. straminea, is reported for the first time in Saint Lucia.