[Malaria in the Republic of Djibouti. Strategy for control using a biological antilarval campaign: indigenous larvivorous fishes (Aphanius dispar) and bacterial toxins]

The authors take stock of the present situation of malaria in the Republic of Djibouti which, after several decades of silence, seems to have been reintroduced at the beginning of the seventies. Actually it is hypo-endemic malaria with Plasmodium falciparum of which the only vector seems to be Anopheles arabiensis, gambiae complex. The specificity of the larvae nests allows a control strategy based on the only treatment of larvae sites by biological control: larvivorous fishes (Aphanius Dispar) and in addition the pin-point use of bacterial toxins as a complementary measure. The first results obtained in the rural zones around the capital are encouraging and permit to envisage the extensions of such a strategy to the whole of the territory of the Republic.

The biotechnology of Bacillus thuringiensis

One of the challenges in the application of biotechnology to pest control is the identification of agents found in nature which can be used effectively. Biotechnology offers the potential of developing pesticides based on such agents which will provide environmentally sound and economically feasible insect control alternatives. Such an agent, the insect pathogen Bacillus thuringiensis, is the subject of intense investigations in several laboratories. Insecticides which use the entomocidal properties of B. thuringiensis are currently produced and sold worldwide; new products are currently in the development stage. Herein, the biology and genetics of B. thuringiensis and the problems associated with current products are critically reviewed with respect to biotechnology. Moreover, the economic and regulatory implications of technologically advanced products are evaluated.

Reduction of malaria prevalence after introduction of Romanomermis culicivorax (Mermithidae: Nematoda) in larval Anopheles habitats in Colombia

The worldwide resurgence of malaria has become a major public health problem. New methods of controlling the vectors of the disease are required, and we therefore studied the biological control of Anopheles albimanus by Romanomermis culicivorax in Colombia. The investigation was carried out in El Valle and Nuquí, two towns on the northern Pacific coast of the country. All of the mosquito larval habitats surrounding El Valle were seeded with the eggs and adults of R. culicivorax. The nematode established itself in the new habitat and recycled over 27 months. The larval population of A. albimanus, the only malarial vector detected in the two towns, decreased in El Valle. In contrast, no change in the larval populations of the vector was detected in ponds located near Nuquí, the untreated control town. A rapid and progressive decrease of the prevalence of malaria among schoolchildren in El Valle was observed during the 2-year evaluation period.

[Is chemical pest control coming to an end?]

Problems that arise when chemical control of pests is applied--risks for producer, applier, consumer and the environment as well as development of resistance against pesticides--have led to the conclusion that other forms of pest control have to be searched for to guarantee production of sufficient crops in the future. Out of the many available methods to prevent or reduce pests two methods seem to offer most possibilities for the near future: development of (partial) host-plant resistance against pests and the search for natural enemies of pests (the so called biological control). In this article the development of biological pest control is described and a comparison is made between working methods, efficiency and risks in application of chemical and biological control. Several examples are given of successful biological control projects as applied in greenhouses in the Netherlands.

A field evaluation of Bacillus thuringiensis var. israelensis as a biological control agent for Simulium chutteri (Diptera:Nematocera) in the middle Orange River

Bacillus thuringiensis Berliner var. israelensis de Barjac (Serotype H-14) (B.t.i.) at a concentration of 1.6 ppm/10 min and a toxicity of 1500 AAU/mg was tested against Simulium chutteri Lewis larvae in the Orange River near Prieska, South Africa. Samples of benthic fauna from the stones-in-current biotope were collected before application of the product and at various intervals up to 80 h afterwards at 4 stations from 200 m to 11 km downstream of the application site. Fauna drift increased slightly after the arrival of the Bacillus at 2 stations 1.4 and 6 km respectively downstream of the application site. Large numerical decreases in benthic simuliid larval numbers after the application of B.t.i. in the Orange River were not statistically different (P greater than 0.05). This indicated that the size of replicated samples that showed significant decreases (P less than 0.05) of simuliid numbers in the Vaal River was not adequate to show statistical differences in the Orange River. The quantity of dead larvae on stones collected from rapids after application of the B.t.i., and the numerical decreases found by comparing median values of larval counts on stones indicated that B.t.i. effectively killed simuliid larvae. Three days after application of the Bacillus, recruitment of small simuliid larvae on stones 1.4 km downstream of the application site was discernible again. Tanytarsini were also numerically reduced after B.t.i. application. At a flow rate of 38 m3/s B.t.i. was visibly effective in killing S. chutteri up to 6 km downstream of the application site and statistically significant decreases (P less than 0.05) in numbers of larvae were seen at a site 11 km downstream of the application site.(ABSTRACT TRUNCATED AT 250 WORDS)

[Reevaluation of the biological control of vector mosquitoes using predatory larvae of Toxorhynchites mosquitoes]

Attempts to control mosquito-borne disease using predatory mosquitoes such as Toxorhynchites larvae have led to indefinite results for many years, mainly because of the lack of adequate species or strains of Toxorhynchites. Recent improvements of natural and artificial matings of adults in the laboratory and of mass breeding of larvae, however, have made it possible to establish laboratory colonies of most Toxorhynchites species whenever and wherever necessary. Effects of biological control by releasing large numbers of Toxorhynchites mosquitoes should be reevaluated from a new concept of comparing the usual chemical insecticides with the living and flying "insecticides" which cause no environmental pollution.

Survival, growth and reproduction of the imported ampullarid snail Marisa cornuarietis in Central Sudan

The ampullarid snail Marisa cornuarietis was imported to the Sudan in 1981 for evaluation as a biological control agent against the planorbid snails which transmit human schistosomes. In initial field studies in small protected ponds the generation time of M. cornuarietis was 4 months, as in Puerto Rico. The snails reached 4 cm in diameter after 1 year, compared to 3 cm in Puerto Rico. Their population density varied from 60 to 175 snails per metre of shore-line, compared to a similar pond in Puerto Rico where the stable density was about 115 snails per metre. The proportion surviving after 1 year was 0.03, less than the annual proportion surviving of 0.10 in Puerto Rico. Thus the preliminary results indicated that the ampullarid snails could establish strong populations in permanent habitats in central Sudan where there was adequate food, although it might take longer than it does in Puerto Rico.

Nosema algerae (Nosematidae, Microsporida): laboratory infections of Anopheles larvae and prospects for field application

Different larval instars of Anopheles arabiensis were infected experimentally using different dosages of Nosema algerae spores. The N. algerae infections killed most of the exposed larvae at the instar treated. Chronic infections in adults developing from treated larvae decreased fecundity and shortened the life of the adults. The possibility of applying N. algerae as a control agent against An. arabiensis larvae is discussed.

Estimated risks and benefits from introducing Marisa cornuarietis into the Sudan

As it has been proposed that Marisa cornuarietis, the ampullarid snail from the tropical Americas be used in the Sudan as a biological control agent against schistosomiasis, an estimate was made of the potential ecological hazards which might result from its widespread distribution in the Nile River Valley, and also of the potential benefits. The only foreseeable hazard would result from colonization of rice fields by the ampullarid snail and subsequent crop damage. However calculations on population dynamics indicated that Marisa cornuarietis would not be able to establish significant populations in rice fields in the Nile Valley as the flooded periods are too short. Thus the expected benefits of schistosomiasis control far outweigh any expected risks, and the ampullarid should be used in expanded field trials in the Gezira Irrigated Area as a control method for schistosomiasis.

The effect of Helisoma duryi on the cercarial production of Schistosoma mansoni-infected Biomphalaria alexandrina: evaluation of chemical interferences and direct competition

The effect of Helisoma duryi or uninfected Biomphalaria alexandrina conditioning of water, as well as the effect of direct competition with these, on the total cercarial production, growth and survival of Schistosoma mansoni-infected B. alexandrina was investigated. The total cercarial production varied considerably between experimental groups. No differences were found between groups maintained in water-conditioned by H. duryi or by uninfected B. alexandrina with regard to the total cercarial production, growth and survival. The cercarial production was correlated with snail size, but showed no differences between the two treatments. Direct competition with H. duryi resulted in reduced cercarial production and growth of infected B. alexandrina as compared with snails in competition with uninfected conspecifics.

Oral toxicity of Bacillus thuringiensis subsp. israelensis to adult mosquitoes

The solubilized entomotoxin of Bacillus thuringiensis subsp. israelensis killed adult male and female mosquitoes of several genera and of various physiological states when it was administered orally. Adult mosquito mortality was further influenced when the preparation was contained in sucrose solution. The potential implication for the control of adult mosquitoes is discussed.

Comparative bioassays of Tolypocladium cylindrosporum Gams (Californian strain) against four species of mosquitoes in Malaysia

Comparative laboratory bioassays of Tolypocladium cylindrosporum, California strain (Kal) was conducted against third instar larvae of four species of mosquito, viz. Aedes aegypti, Anopheles balabacensis, Culex quinquefasciatus and Mansonia uniformis in Malaysia. Of the four mosquito species tested, Ma. uniformis was found to be the most susceptible, followed by Cx. quinquefasciatus, An. balabacensis and Ae. aegypti, in a decreasing order. The LC50 values for Ma. uniformis, Cx. quinquefasciatus, An. balabacensis and Ae. aegypti after four days of exposure were 1.18 X 10(4), 2.02 X 10(5), 4.76 X 10(5) and 1.84 X 10(7) spores per ml test media, respectively. The high sensitivity of Ma. uniformis and its longer life cycle seems to indicate that T. cylindrosporum Kal has good potential as a biocontrol agent for this species of mosquito. But, for Ae. aegypti, this fungus appears to be less effective.

Laboratory and field trials with two Bacillus thuringiensis var. israelensis products for Simulium(Diptera: Nematocera) control in a small polluted river in South Africa

The effects on Simulium adersi and S. hargreavesi larvae of 2 Bacillus thuringiensis var. israelensis products, the liquid formulation "Teknar" (Sandoz) and a powder formulation produced by the Ben Gurion University, Israel, were compared in the laboratory and in the Pienaars River. This river was heavily polluted with effluent from a nearby sewage works and contained 77 mg/l chloride. In the laboratory S. adersi and S. hargreavesi larvae showed 26; 48; 95 and 100% mortality 6 hours after a 10-minute application of 0,8; 1,6; 3,2 and 16 ppm "Teknar" in rain water. The powder formulation applied at 0,2; 1,0; 2,0 and 30 ppm resulted in a 7; 17; 35 and 100% mortality. In polluted river-water the mortality was 85% with 16 ppm "Teknar" and 80% with 30 ppm B. thuringiensis powder. In the field trials "Teknar" at 1,6 ppm and B. thuringiensis powder at 3 ppm did not cause any larval mortality at flow rates of 3 060 l/min and 2 040 l/min, respectively. However, 24 hours after application of the powder formulation, numbers of S. hargreavesi decreased significantly (P = 0,05) 20 m below the application point. A further 24 hours later, after "Teknar" had been applied, the numbers of S. adersi decreased and those of Chironomidae increased significantly. There was a significant increase in S. hargreavesi 200 m downstream after treatment with "Teknar".

[Varroatosis. A zootechnical system of control]

A managerial method is described by which the number of Varroa mites in a bee colony may be reduced to a level not interfering with economic bee-keeping. This theoretical approach is based on the removal of combs in the month of May, when about ninety per cent of the mites have enclosed themselves in cells with developing larvae and puppae, to multiply.

Pathogenicity of mollicutes for insects: possible use in biological control

Acholeplasmas, spiroplasmas and other non-helical sterol-requiring mycoplasmas of unknown phylogenetic affinity inhabit insects. Of these, only spiroplasmas are known to be pathogenic. Group I-2 spiroplasmas, or Spiroplasma apis, especially in combination with other organisms, reduce honey-bee longevity. Plant pathogenic mycoplasma-like organisms are often found intracellularly in insects. Spiroplasmas are found predominantly in the gut lumen or haemolymph (or both) of their insect hosts. Pathogenicity of mycoplasmas is usually altered by extended passage in unusual hosts, in only one of two alternate hosts, or in culture media. Enhancement of experimental pathogenicity may occur with extended cultural passages, but maintenance of natural pathogenicity must be accomplished by continuous exposure to the usual host. Recent data provide new information on the ecology of pathogenicity. Spiroplasmas from unique habitats also tend to be unique. Spiroplasmas isolated from flowers appear to be adapted to insect species that frequent floral surfaces. Group IV spiroplasmas have been isolated from members of 4 holometabolous insect orders (including Lepidoptera), all of which visit flowers. Social or predatory insects, or insects with an "aggregation" phase in their life histories, also appear to be prone to spiroplasma infection. Some insect species which harbor spiroplasmas also carry infections of other mollicutes, some of which involve the haemolymph. Appearance of spiroplasmas in adult insects in nature is strongly affected by seasonality. Extensive tests of the host ranges of the new insect mollicutes will be required before their suitability for biological control can be evaluated.

Socio-economic problems related to animal trypanosomiasis in Africa

Animal trypanosomiasis is one of the major constraints of rural development in Africa. If trypanosomiasis did not exist in these areas, it is estimated that some regions could carry 3-5 times more livestock. Chemotherapy and chemoprophylaxis are old methods used to tackle the problem and are still valuable, provided the number of interventions is limited. Tse-tse fly control is a very useful tool to combat trypanosomiasis but expensive. The sterile male release technique is not economically feasible on a wide scale. Immunization or vaccination is not yet available. Trypanotolerance is a biological phenomenon; it seems to be a valid method, which allow African countries to keep and increase livestock within economically feasible conditions; but inevitably it will be a long exercise.

Fate of Bacillus sphaericus 1593 and 2362 spores used as larvicides in the aquatic environment

Dry powders produced from insecticidal Bacillus sphaericus strains 1593 and 2362 were applied against Culex tarsalis and Anopheles franciscanus mosquito larvae in small-plot field trials. Good control of C. tarsalis, but not of A. franciscanus, was produced at 0.1 and 0.2 lb/acre [ca. 0.122 and 0.244 kg/ha]. B. sphaericus spores settled rapidly from upper water layers and accumulated in bottom muds. Control of third/fourth-instar C. tarsalis larvae was maintained through day 4 after testing and was related to the presence of at least 100 spores per ml in the upper water layer 2 days earlier. B. sphaericus was shown to recycle in dead larvae both in the laboratory and in the field, producing an increase of 100- to 1,000-fold in spore numbers. There was no evidence of recycling in treated water nor of significant spore persistence upon reflooding of ponds after a very hot, dry period.

The Florey lecture, 1983. Biological control, as exemplified by smallpox eradication and myxomatosis

Biological control is an important method of dealing with plant and insect pests. The control of rabbits by myxomatosis and the eradication of smallpox by vaccination are unusual examples of biological control, in that they involve a vertebrate and a viral pest respectively. Myxomatosis is a benign disease in Sylvilagus rabbits in South America which is transmitted mechanically by mosquitoes. In the European rabbit, Oryctolagus, which is a pest in Australia and England, the virus from Sylvilagus produces a generalized disease that is almost always lethal. Myxomatosis was deliberately introduced into Australia in 1950 and into Europe in 1952. It was at first spectacularly successful in controlling the rabbit pest, but biological adjustments occurred in the virulence of the virus and the genetic resistances of rabbits. After 30 years of interaction, natural selection has resulted in a balance at a fairly high level of viral virulence. Smallpox has been a major scourge of mankind for over 1500 years. It spread from Asia to Europe in the Middle ages and from Europe to Africa and the Americas in the 15th and 16th centuries. Jenner's cowpox vaccine provided a method of control that reduced the severity of the disease during the 19th century but failed to eliminate the disease from many countries before the 1930s. Thereafter it was eradicated from Europe and North America, but remained endemic in South America, Africa and Asia. In 1967 it was still endemic in 33 countries and W.H.O. established a programme for global eradication within 10 years. The goal was achieved in 1977. Problems of the eradication programme and reasons for its success will be described.