Bacteriological larvicides of dipteran disease vectors

BACTERIOLOGICAL LARVICIDES OF DIPTERAN DISEASE VECTORS

MARIA HELENA N. L. SILVA-FILHA

Centro de Pesquisas Aggeu Magalhães – FIOCRUZ, 50670-420 Recife, PE.

Presented by HELIO B. COUTINHO

The bacteria Bacillus sphaericus (Bs) and B. thuringiensis serovar. israelensis (Bti), display toxic action on mosquitoes and black flies, important vectors of man disease, acting as per os larvicides. These sporulating bacteria show a major advantage over synthetic insecticides: selectivity due to the specific mode of action. Bs is toxic against some species of Culicidae while Bti is also highly toxic against Simuliidae.

Both bacteria produce, during the sporulation, crystals, which contain protoxins. Bti crystals contain four polypeptides of 123-, 135-, 72- and 28-kDa, respectively called Cry4A, Cry4B, Cry11A and CytA. For Bs, crystals contain a toxin (Bin) made of two polypeptides of 42- and 51-kDa, called BinA and BinB, respectively. The mode of action of these proteins on larvae involves the ingestion of crystals and spores in suspension in water. Inside the midgut lumen, under the action of the alkaline pH and proteinases, protoxins in the crystals are solubilized and activated. Released toxins bind to apical microvilli of midgut cells, then cytopathological alterations are observed in midgut cells, leading to the death of larvae. Those toxins need to act in synergy to display the full toxicity and also bind to specific receptors in the larval midgut. Recently, the receptor of the Bin toxin of Bs in C. pipiens larvae was identified as being an a-glucosidase of 60 kDa.

Bti and Bs based larvicides have been produced and successfully used in vector control programs throughout the world. Bti has been mostly used to control species of Simulium and Aedes, vectors of onchocerciasis and dengue, respectively, while, Bs is very effective for controlling Culex species, vectors of filariasis and encephalitis. Data from the operational use of microbial larvicides in several countries, in the past two decades, have confirmed the effectiveness of Bti and Bs and their safety to non-target species. On the other hand, Culex populations intensively sprayed with Bs, can display resistance. Data shows that Bs should be used into integrated programs with other control agents such as Bti, in order to avoid these phenomena. Resistance is unlikely to appear towards Bti due to its multiple toxins. — (May 24, 2002).

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Publication Dates

• Publication in this collection
  09 Oct 2002
• Date of issue
  Sept 2002