Bacterial Larvicide Strain Designations

Per US EPA requirements (US EPA Data Requirement for Registration 40 CFR Part 158: Subpart V: Microbial Pesticides 158.2100): “Each new isolate of a microbial pesticide is treated as a new strain and must be registered independently of any similar registered microbial pesticide strain and supported by data required in this subpart.” Any manufacturer that registers a bacterial-based biopesticide with the US EPA must assign a unique strain number for that biopesticide. This policy holds true even for manufacturers that use a competitor’s strain for their own products. In addition, the WHO Pesticide Evaluation Scheme requires that Bacterial Larvicides (BL) that pass their rigorous specifications process be listed by strain (see link to WHOPES larvicides table — PDF).

Why strain numbers are important for bacterial-based larvicides

Bacterial products such as Bti and Bs are like all living organisms: no two “cells” are perfectly identical. While genetics plays a crucial role, the environment in which an organism grows can have a significant impact on performance. This is analogous to identical twins being split at birth. While many traits may be similar, different environments can result in distinctly different individuals. The same holds true for the same species of bacteria being produced by two different manufacturers.

The manufacturer’s unique strain number for bacterial-based biopesticides is a critical link to product performance and quality expectations. The published literature demonstrates that identical strains produced under different fermentor conditions can affect performance. Even when the same strain is used, differences or changes in the use of raw materials, fermentation, recovery, and formulation processes will greatly affect not only the quality of end-use product, but will also likely affect the biological performance (Devisetty 1993). Fermentation-recovery methods can be different from manufacturer to manufacturer (e.g. precipitation, centrifugation, ultrafiltration) and can ultimately affect critical factors such as particle size, which drives behavior in the water column and the increased probability of the insect ingesting the toxin (Devisetty 1993; Clark et al. 2006; Clark et al. 2007). Furthermore, since this is a fermentation process, sub-standard quality controls can lead to undesirable growth of contaminants that could cause end-user and environmental safety hazards.

In addition, quality control (QC) requirements for fermentation are critical to ensure consistency and safety. The same QC measures are not utilized by all manufacturers of bacterial-based larvicides and as such, it is critical to differentiate these products based on the manufacturer’s unique strain number from ubiquitous strain identifiers (e.g., using H14 as an “identifier” for Bti-based products). Use of ubiquitous strain identifiers does not guarantee identicalness of product performance or environmental safety.

Bacillus sphaericus 2362, strain ABTS-1743 — strain history

The following table derived from de Barjac and Sutherland’s editorial book, entitlled Bacterial Control of Mosquitoes and Black Flies: Biochemistry, Genetics and Applications of Bacillus thuringiensis israelensis and Bacillus sphaericus, highlights the history of the development of key Bs strains.

Key Strains

Historical Significance of
Key Strains

Other Strains and
Relationship to Key Strains

K (U.S.)

First reported active isolate; work discontinued after brief program

Q (U.S.); Phage group 1; low
larvicidal activity

SSII-1 (India)

First active isolate universally available; fermentation and population-stability problems

1404 (Philippines); 1883, 1885–1893, 1895, 1896 (Israel); Phage group 2; moderate larvicidal activity

1593 (Indonesia)

First fermentation and population-stable strains; one of key field candidates

1691, 1881 (El Salvador); 2013–4, 2013–6 (Romania); 2117–1 (Philippines); 2500, 2501 (Thailand); Phage group 3; high larvicidal activity

2362 (Nigeria)

First highly active African strain; the prime field candidates


Lysenko Isolates: 2537–2, 2533–1 (K1), 2533–1 (K2) (Guyana); 2601 (Hungary); 2602 (Czechoslovakia)

First active types isolated from non-mosquito terrestrial sources


2297 (Sri Lanka)

Crystal first noted in this strain of key field candidates

2173, 2377 (India), 2317–3 (Thailand); Phage group 4; mixed larvicidal activity; 1894 (Israel); Phage group 5; 2115 (Philippines); Phage group 6; 2315 (Thailand); Phage group 7; moderate larvicidal activity

 

By 1988, 186 strains had been isolated worldwide, of which 45 had shown some toxicity to mosquito larvae (the most commercially interesting of which were strains 1593, 2297, and 2362). A number of these isolates were maintained at the World Health Organization (WHO) Collaborating Centre at the Institute Pasteur, Paris, and were available to scientists for many years.

The Bs 2362 strain was received as a slant culture by Abbott Laboratories (former parent company of Valent BioSciences Corporation — VBC) in 1982, and the first lyophilized batch was a mass transfer of sporulated cells. Over the course of the 1980s, this strain was preserved for long-term use and is currently used to provide inoculum for large-scale commercial fermentation of Bs by VBC. The 2362 strain was provided a unique strain number, ABTS-1743, in 1986 by Abbott Laboratories.

In 1991, Abbott received registration of Bs by the US EPA. The registration package submitted to the US EPA designated the WHO strain 2362 as strain ABTS-1743. However, the US EPA registered the technical product as strain 2362 at that time. As such, for over 20 years Bs strain 2362 has been associated with VBC’s VectoLex® (Bacillus sphaericus 2362, strain ABTS-1743) and VectoMax® (Bacillus thuringiensis subsp. israelensis, strain AM65-52 + Bacillus sphaericus 2362, strain ABTS-1743) brands worldwide and is recognized in the literature as associated with VBC. In 2010, the US EPA decided that the 2362 designation is a ubiquitous strain identifier and belongs to the WHO; as such, no single company can claim exclusive use of this strain in the U.S. Thus VBC was mandated to amend their registration by adding the strain isolate number ABTS-1743 to the active ingredient name; the same strain isolate number that was submitted to USEPA as part of the original registration dossier for Bs in the 1980s. In addition, USEPA mandated that all registrants of Bs products amend their registrations to provide a unique strain designation for their specific Bs active ingredient.

VBC Biorational Strain
VectoBac 12AS   Bacillus thuringiensis subspecies israelensis, strain AM65-52      
VectoBac WG/WDG  
VectoBac G/GS/GR  
VectoBac DT  
Bactimos PT
Teknar SC Bacillus thuringiensis subspecies israelensis, strain SA3A
VectoLex WDG   Bacillus sphaericus 2362, strain ABTS-1743    
VectoLex CG
VectoLex WSP
VectoMax CG/G   Bacillus thuringiensis subspecies israelensis, strain AM65-52 + Bacillus sphaericus 2362, strain ABTS-1743  
VectoMax WSP