Use of Bti considered safe to control aedes mosquitoes

Mosquito control is a major public health concern, as mosquitoes transmit many severehuman diseases such as malaria, filariasis, dengue, yellow fever, West Nile virus and the chikungunya virus. These diseases represent a major health threat and economic burden indisease-endemic countries, and are currently in expansion due to increased worldwide exchanges, urbanization, and global warming. The only effective way of reducing the incidence of these diseases is to control the vector mosquitoes, mainly by application of insecticides to their breeding places. Since the 1950s, the massive use of chemical insecticides has led to undesired toxicity on non-target organisms and the selection of insecticide resistance mechanisms in mosquito populations (Hemingway & Ranson, 2000).

A safe alternative to chemical insecticides is to spray toxins produced by the bacteria Bacillusthuringiensis subsp. israelensis (Bti) over mosquito breeding sites. Bti represents today the best alternative to chemical insecticides in controlling mosquitoes. Bti toxins are safe for non-target species and human health and are believed to show low persistence in the environment, and so far, no resistance was detected in mosquito populations. Bti is the only insecticide allowed against mosquito larvae in Europe. To ensure a long term efficiency of this bio-insecticide, it is however necessary to evaluate the risks associated to its intensive worldwide use. The two main risks are (1) the accumulation of spores and toxins in the environment, and possible proliferation of Bti a long time after spraying, which may have an impact on the whole ecosystem functioning, and (2) the evolution of resistance to Bti in mosquitoes, rendering the treatment inefficient. It is therefore necessary to develop monitoring tools to follow the fate of spores and toxins in the environment and the evolution of resistance in target mosquito populations. Bacillus thuringiensis subsp. israelensis (Bti), serotype H14, is a subspecies of the diversified.

Bacillus thuringiensis species, an entomopathogenic bacterium able to survive in the environment as a spore and producing insecticidal toxins within an inclusion body duringthe process of sporulation. Bti was first isolated from a water pond in the Negev desertin southern Israeland was the very first strain described for having insecticidal activity outside Lepidoptera. Bti, like other B. thuringiensissubspecies, is a member of the Bacillus cereus complex. The characteristic of B. thuringiensis is the presence of an inclusionbody or crystal.The specificity of the B. thuringiensis insecticidalproteins is central in the wide use of Bt as an alternative to chemical insecticides for thecontrol of insect pests in forestry, agriculture, and public health. The serotype H14, Bti,produces 4 main toxins (Cry4Aa, Cry4Ba, Cry11Aa and Cyt1Aa) specific to dipterans(mosquitoes, blackflies and chironomous midges).
All Bti insecticidal proteins are produced as protoxins and all must be activated in vivo byinsect midgut proteases prior insecticidal activity. Following this initial activation, thesubsequent receptor binding step is still not fully resolved.

The activity persistence of Bti and the induced side-effects on non-target organisms depends on the type and the characteristics of the formulation, the frequency of application as well asthe environmental factors such as the temperature, the water depth or the vegetation. A 5-years study including three years of intensive Bti treatments showed a reduction of the taxonomic richness and the totalnumber of invertebrates. These changes should haveinterfered with the trophic network involving the invertebrates but did not affect the populations of zooplankton and nesting birds. In a 4years study conducted by Lagadicet al. (2002) in Morbihan, France, revealed that no significant effect the health, the number and theabundance of non-target aquatic invertebrates present in mosquito breeding sites treated annually with Bti (VectoBac® 12AS).

Resistance to Bti toxins is nothing else than the interruption at one place or at several placesof the cascade of events known as "mode of action". This interruption of the mode of action,i.e., mechanisms of resistance, can occur at different levels including toxin lack of activation,toxin proteolysis, precipitation of the toxin, modification of the receptor but also through alack of pore formation after specific binding. However, the mostfrequently encountered mechanisms of resistance were receptor mutation and alteredbinding on one hand and lack of proteolytic toxin activation on the other hand. It is alsoimportant to consider that almost all these accounts are laboratory works under forcedartificial conditions where resistance can be easily selected. With respect to Bti, there is,despite a significant amount of research over the last twenty years, no resistance to thewhole Bti crystal in laboratory or in the field.

Numerous studies have assessed the persistence of Bti toxicity after treatment. Several environmental factors such as solar radiation, temperature, type of substrate, presence of vegetation, salinity, pollution, water height, were shown to influence the persistence of Bti toxicity in the environment. Depending on all these factors, Bti toxicity was shown to decrease with highly variable patterns in the field, from a few days to several weeks. In contrast, very few studies looked at the fate of Bti spores in the environment after spraying and no studies exist so far on the fate of Bti end toxins in the environment. Reduced toxicity over time does not mean that Bti spores and toxins are quickly eliminated in the environment; they may accumulate, or even proliferate in soil, or in decaying vegetation at the bottom of mosquito breeding sites. If many studies have investigated the fate of Cry toxins produced by genetically modified plants (GMPs) and released into agricultural soils, such studies are missing for Bti toxins.

Bti products present low risk for the human health through direct or indirect exposure. Laboratory studies have demonstrated that Bt and Btproducts are toxic to mammals only at a dose higher or equal to 10 8 colony forming unit per mouse. The pH and presence of receptors in the midgut determine the specificity of the larvicide action of Bti. Since the discovery of its insecticidal potential in 1976, the innocuity of Bti for micro- and macro-invertebrates, fishes, batracians, and other vertebrates sharing the same habitats as mosquito larvae is well established at dosage rates used at the operational scale. All the studies carried out in laboratory and in field conditions show that Bti has a main target effect on the Diptera.

At the end of this scientific discussion, Bti appears to be a safe and efficient bio-insecticide against mosquitoes. Although Bti still comes at a higher cost than chemical insecticides, presumably due to its so far limited use, there is no technical reason for this high cost. The successful use of food processing organic industrial wastes such as chicken feathers as a nutrient medium to grow Bti opens avenues to a future low-cost production of this bio-insecticide, with theadditional benefit of effective recycling of bio-organic wastes from the environment, shown by a research by Poopathi & Abidha in 2007.There is an urgent need to develop efficient,easy-to-use and low-cost diagnostic tools to evaluate the fate of Bti in the environment, and to monitor Bti resistance evolution in mosquito populations.

The writer is Professor and Head, Department of Entomology, National Institute of Preventive and Social Medicine, NIPSOM