An injection method for spraying biological control agents and a monomolecular surface film for control of immature mosquitoes.
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The pathogenicity of Bacillus sphaericus strain 1593 was tested against laboratory-reared larvae of four local species of mosquitoes of public health importance in Malaysia; Aedes aegypti, Anopheles balabacensis, Mansonia uniformis and Culex quinquefasciatus. The bacteria was shake-cultured at 28 +/- 1 degrees C for three days, using Glucose-Yeast Extract Salts medium. After which, the spores and vegetative cells were harvested and stored at 4 degrees C before use. Conditions for bioassays were mean temperature of 25 +/- 1 degrees C and relative humidity 65 +/- 5.0. Twenty third-instar larvae of each species were assayed in 90 ml of diluted spore solution. Each concentration and a control were replicated three times for each bioassay. Larval mortalities at 24 hours and 48 hours were taken and analyzed through Probit Analysis using a computer (IBM 370). LC50 values after 48 hours of exposure showed an increasing order of larval susceptibility as follows: Ae. aegypti (417.70 x 10(4)), An. balabacensis (45.84 x 10(4)), Ma. uniformis (18.23 x 10(4)) and Cx. quinquefasciatus (4.14 x 10(4) spores/ml). With the ability to kill 90% of the Cx. quinquefasciatus larvae tested with just a concentration of 10(5) spores/ml, B. sphaericus (strain 1593) has shown good potential as a biocontrol agent for this species of mosquito.
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Toxorhynchites brevipalpis, a predator on larvae of Aedes aegypti and other mosquitos, was successfully colonized in the laboratory. At 25 degrees C, embryonic development was completed within 50 hours of oviposition, while larval and pupal development together took a further 27-41 days. The adult mosquitos mated in cages as small as 15x15x15 cm, and the embryonated eggs were obtained 6-31 days after the adults emerged.
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Chemical control of C. p. fatigans frequently fails because the mosquito rapidly develops resistance to insecticides. A possible alternative or complementary method is biological control, including the introduction of pathogens. The microsporidian Plistophora culicis was known to infect readily and have an adverse effect on C. p. fatigans populations in the laboratory, so an attempt was made to introduce and establish this pathogen in a wild population of the mosquito on the Pacific island of Nauru. Two years after introduction the pathogen was still present in the wild population. However, the infection rate was similar to that found in naturally occurring infections in other mosquitos and is almost certainly not high enough to affect a natural population of C. p. fatigans adversely.
Studies with Aedes aegypti were undertaken to determine if an alien genotype can be (1) incorporated into a natural population and (2) maintained for several generations on its own without any subsequent introductions. Such information is an essential prerequisite for successful application of any genetic control method. Data from a walk-in, field population cage and from field releases of a genetic marker and a chromosomal translocation have demonstrated both genetic incorporation and persistence for at least three successive generations. This is the first demonstration of its type with any vector species.
Comparative field studies were made on the dispersal and survival of chemosterilized, irradiated, and cytoplasmically incompatible (D3 strain) male Culex pipiens fatigans. There was no significant difference in dispersal patterns and daily survival rates between laboratory colony males and wild males, and between chemosterilized or irradiated males and untreated males. Although there was no difference in dispersal between adults of the D3 strain and the laboratory colony of the Delhi strain, the former had a lower daily survival rate.
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Technology was developed for sterilizing and packaging as many as 1 million male Anopheles albimanus Wiedemann per day for field release. The average sterility produced in the males during a 1.5-year period was 98.9%. Mortality due to transport of the males in specially developed cages averaged 4.5% for 72-hour-old adults and 2.5% for 24-hour-old adults. The average mortality due to transport and field exposure of pupae was 1.2 to 9.8%, depending on the technique used.
A possible method for genetic control of insect vector species involves the use of translocation heterozygotes. The potential of single and double heterozygotes already available in Aedes aegypti has been investigated with computer simulations of release strategies. Such simulations indicate a possible role for translocation heterozygotes of these types in an insect population characterized by a 5-fold population growth per generation, or less.
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We divide genetical control methods in two major groups:--mechanisms operating between different species and an example is the attempt of control of A. gambiae in West Africa,--mechanisms operating in a single species with: gamma rays and chemical treatment for sterilisation, cytoplasmic incompatibility (ex.: C. pipiens). Genetical control also suppose the knowledge of biology and ecology of the target insect. This paper does not want to be an exhaustive review of this problem. Its only ambition is to try and expose the possible applications of genetics in the control of vectors.