Bt Utilization:Nature and Mechanism of Action toMosquito Vectors

A SYNTHESIS

Ian Niel B. dela CruzAugust 14, 2012

Review

Shigetani Ishiwatari; 1901Bacillus sotto

Ernst Berliner; 1911Bacillus thuringiensis

Review

• Bt as pesticide in 1920.• Commercialized spore-based formulations

called Sporine in 1938.• Limitations:• washed by rain• degrade in UV• insects were not susceptible• limited penetration

• discovery of Cry proteins on 1956

Review

• Commercialized in USA on 1958• Registered as pesticide on1961 by EPA• 1977; 13 subspecies that has been

discovered were toxic to Lepidopterans• 1977; followed by the strain that was toxic

to Dipterans• 1983; strain identified to target

Coleopterans

Review

Bti (israelensis)• Mosquito vectors of human disease pose

a constant threat worldwide (tropics) and recent interceptions of exotic mosquitoes with the potential to vector serious mammalian diseases has highlighted the need for agents for use in control and eradication programs.

• The discovery of Bti, a variety specific to Diptera (especially mosquitoes) in Israel in 1978, has led to the development of many products that have been used extensively in mosquito and biting fly control programmes.

Characterisation

• The classification of Bacillus thuringiensis is difficult because of the close genetic relationship between Bt to B. cereus, B. anthracis and B.mycoides.

• The main and unique characteristic separating Bt is the formation of insecticidal proteins.

• During sporulation in Bt , some strains produce one or more inclusions or parasporal bodies within a sporangium. This body is often toxic to specific insect group (Cry proteins; δ-endotoxins).

Characterisation

Phenotypic• Flagellar serotyping, crystal morphology,

biochemical reactions, bioassay

Subspecies and variety• H-serovars

Strains• Molecular biology (DNA sequencing, specific

primed PCR, RAPD, colony hybridisation and rRNA-based probed.

Mode of Action

Mode of Action

Occurrences• Bt apparently exists in the environment as spores• Recycle poorly and rarely cause natural epizootics in

insects

-Rarely grows in soil but is deposited there by insects-May be infective to soil dwelling insects (as yet undiscovered)-May grow in soil when nutrients are available-Affinity with B. cereus

• Alternatively, it was also proposed that Bt is a natural component of the phylloplane microflora and has evolved in a symbiotic or mutualistic association with plants to provide protection against herbivores.

Occurrences

• Although Bti is rarely found causing epizootics, this strains was discovered causing massive death among Culex sp. in Israel.

• Presence of the plasmids which encode endotoxins is related to germination and sporulation .

Bti Formulations

Bti Formulations

• Highly concentrated liquid formulations are available for control of floodwater mosquitoes.

• Formulations which float for as long as possible have been developed for use in fast-flowing or turbulent waters.

• Formulations which settle and persist at the bottom are required for bottom feeders.

• Granules which float on the surface are most effect against surface feeders.

• Briquettes to continual successive generations like Culex are useful for overcoming lack of persistence.

Bti Formulations

Other types:

• Superabsorbent polymer controlled-release system

• Combinations of chemical and biological agents (insect growth-regulator)

• Sprayed-dyed Bti powder as tablet

• Floating bait formulations• Carrier particles

Host range

• Highly pathogenic against Culicidae (mosquitoes) and Simuliidae (blackflies).

• Generally regarded as specific to larvae of Nematocera (also include midges, craneflies and gall flies).

Different preparations shows differing levels of toxicity

• Culex and Aedes are highly susceptible• Anopheles are less susceptible, but can still be killed• However, in one genus, some species are more

susceptible than others.

Host range

Persistence and Activity in the Environment• Persistence

Generally, toxicity to mosquitoes persists for only days and efficacy can be reduced within 24 hours. Formulation and application techniques can extend the persistence of activity for over one month in some situations, but activity remains sensitive factors like UV degradation.

• Application Methods and RatesApplications can be made by pouring into water,

spraying (ultra low volume), aerial techniques and special formulation as briquettes.

Persistence and Activity in the Environment• Gene Transfer

Studies on transfer of genetic elements in the environment are in their infancy, but it has been demonstrated in the laboratory that Bt strain can transfer toxin-encoding plasmids to other bacterial species.

• ResistanceResistance to Bt has arisen especially in

Lepidopterans. However, only low level resistance has ever been detected with this dipteran-specific subspecies. The reasons could be related to the synergistic activity of the multiple toxic proteins of Bti.

Host and environmental factors affecting efficacy• Feeding behaviour of host• Inoculum and host density• Developmental stage of larvae• Other organisms• Water quality• Temperature• Vegetative cover and food• pH• UV• Bacteriophage• Efficacy comparisons

References

• Balaraman K., 2005. Occurrence and Diversity of Mosquitocidal Strains of Bacillus thuringiensis. Vector Control Research Centre (ICMR), Indira Nagar, Pondicherry, India

• Boisvert M., 2005. Utilization of Bacillus thuringiensis var. israelensis (Bti)-based Formulations for the Biological Control of Mosquitoes in Canada. Société de Protection des Foréts contre les Insectes et Maladies, Québec, Canada

• Cranshaw W., 2008. Home and Garden (Bacillus thuiringiensis). Department of Agriculture, Colorado State University Extension, Colorado, USA

• Glare T., 1998. Report for the Ministry of Health – Environmental and Health Impacts of Bacillus thuringiensis israelensis. Biocontrol and Biodiversity, Grasslands Division, AgResearch, New Zealand

Acknowledgments• Dr. Franco G. Teves; OVCRE, MSU-IIT• Prof. Faith B. Amorado, DBS; CSM, MSU-IIT• Prof. Angelie Valera- Mag-aso; DBS, CSM, MSU-IIT

• Dr. Henry I. Rivero; DBS, CSM, MSU-IIT• Bio 296 Co-presenters• Bio 296 Class