Management of honeybee diseases and parasites in Tunisia by exploiting
environment-friendly novel biocontrol microbes and symbiotic control strategies

Tunisian apiculture, especially that in the south of the country, is an important economic source and in the sub-arid zones play an important role for pollination of crops and wild plants. However, the honeybees have to withstand the harsh environmental climate conditions in summer and honeybee pathogens and parasites.

American foulbrood disease (AFB) affects larval and pupal stages of honeybee (Apis mellifera L.) and it can destroy entire insect colonies. AFB causative agent is Paenibacillus larvae subsp. larvae, a gram-positive, spore-forming bacterium worldwide distributed and extremely resistant to control treatments. These characteristics make AFB the most virulent disease of honeybee with a high socioeconomic significance (a list B disease according to the classification of the Office International des Epizooties). AFB is found in many bee-keeping areas of Tunisia where it causes important economical losses.

As all the animals also insects host a quite diverse microbial communities associated to different organs. In particular many insects have established during evolution stable symbiosis with the insect host. The symbiosis can be obligate and the microbial symbiont cannot live outside the host providing a necessary function, typically by the production of nutritionally important factors that are lacking from the diet and cannot be produced by the host. Among the microbiota described in association with the honeybee there are sporeformers of the genus Bacillus and related genera [Evans and Armstrong, 2006].

In the last twenty years, research in the field of insect/microbe symbiosis has received a strong boost, due to the use of molecular techniques that enlarged our perspective of the microbial diversity associated with insects. The importance and prevalence of the symbiotic microorganisms has stimulated a wide basic research on the host-microbe interactions, but has also envisaged potential new applications for the use of symbiotic microbes. A completely novel approach termed "Symbiotic Control" has been proposed. Symbiotic Control uses symbiotic microorganisms to: (i) control pest populations, (ii) deliver anti-transmission modalities in vector populations and (iii) to block the virulence of microbial pathogens causing diseases in the useful insects. Modern methods allow the development of both genetically altered symbionts and non-transgenic strategies.

This can be the case for the AFB of honeybee. Symbiotic microorganisms have been proposed for controlling the infection and/or the virulence of Paenibacillus larvae subsp. larvae [Cherif et al. 2007]. Other bacteria that could be employed for this purpose are those of the species Bacillus thuringiensis, the most important biopesticide worldwide. Bacillus thuringiensis is a Gram-positive spore-forming bacterium producing characteristic proteinaceous crystalline toxin (-endotoxin) with specific activity against certain insect species. It can also produce other extracellular compounds such as -exotoxins, chitinase and vegetative insecticidal proteins that contribute to killing insects. B. thuringiensis constitutes the basis of over 90% of the commercially available biopesticides used for the control of many agricultural insect pests and vectors of human diseases. The actual strategy of application of B. thuringiensis is based on the direct antagonism of the bacterium or its insecticidal toxins against an insect pest. However it can produce other antagonistic factors including antifungal and antibacterial compounds like zwittermycin-A and bacteriocins [Cherif et al., 2001; Cherif et al., 2003; Cherif et al., 2008]. Such capabilities are attracting a growing interest in insect biocontrol both as synergistic factors of insecticidal toxins, and recently as potential antagonists of microbial pathogens of useful insects. Since B. thuringiensis and the strictly related species of B. cereus can be considered normal inhabitants of the arthropod gut, the capacity of producing antibacterial agents together with the ecology related to the insect gut, make these bacteria interesting candidates for developing a symbiotic control approach for blocking the AFB of honeybee.