Bacteria are simple microorganisms with individual prokaryotic cells that reproduce by simple cellular division (simply known as binary fission). Their size oscillates between 1 to 5 μm and they may be saprophytes until obligedly parasites (extracellular pathogens). They prosper in aerobe and anaerobe environments, in cold or warm conditions, in bright or dark places, dry or humid.
Few species of bacteria have been reported as insect pathogens, something that has attired the attention due to their potential as pest control in agriculture, being the most renowned species as pathogen for insects the following families: Bacillaceae, Enterobacteriaceae and Streptococcaceae.
They are Gram-positive, flagellated, ubicuous and sporulant bacteria with a long morphology (resembling sticks), which produce toxins that are different upon the species. They may form crystals (paraspores) of toxic proteins when they sporulate, such as the B. Thuringiensis and B.Popilliae. These ones lyse erythrocytes of different species of animals (emollolines), attrack proteins (proteolytic activity), degrade (lecithins) by the action of lecithins; while others possess pathogen properties for insects. Proteins that compose them are called delta-endotoxins, of two different types: Cry and Cyt. Their unique feature is their hability to produce endospores under stressing environmental conditions (physical or chemical), capacity that allows them to remain inactive for long periods of time
They may be found on the ground, water, dust… They can be strictly aerobes or facultative anaerobe; en el suelo, agua, polvo, etc., pueden ser aerobias estrictas o anaerobias facultativas; they include free-life species as pathogens and they can be used industrially (they are the base of some commercial biological insecticides) or in the fields.
The most used for biological control are forming spores of the Bacillaceae family, especially the gender Bacillus, which includes 51 species. They are natural agents and the toxins have been used to be able to classify them. The formation and resistance of their endospores, production of antibiotics, toxicity of their spores and the production of toxic protein crystals to several kinds of insects make this gender very important in medicine, agriculture, biochemistry and the pharmaceutical industry.
Only four species of bacteria have been produced and are commercially widely found (B. popilliae, B. sphaericus, B. thuringiensis, B. entomophila); being the most relevant insect pathogen species used in the biological pest control: B. sphaericus (now Lysinibacillus sphaericus), B. laterosporus (included in the gender Brevibacillus), Paenibacillus larvae, P. lentimorbis, and P. popilliae (species described as Bacillus), which are invasive pathogens, plus B. thuringiensis, which forms a parasporal body toxic to insects. They all show a wide spectrum and activity levels correlating to the nature of the toxins, which are produced during sporulation.
Some species of the Enterobacteriaceae family are know pathogens for insects suchas the Serratia entomophila. This non-sporulating species provokes higher mortality and kills blocking the instestines of its host. Otehrs with entomopathogenic activity include the Photorhabdus luminescens, Xenorhabdus nematophilus and Yersinia enterocolitica. The species not included inside the Enterobacteriaceae family that have pathogen activity for insects are: Pseudomonas entomophila, Erwinia carotovora, Dickeya dadantii.
The first effects are reflected upon the paralyzation of the digestive tube, loss of apetite, diarrhea, which provokes slow movements, vomiting, convulsions and general paralysis, to subsequently invade totally the host, causing death. They acquire a fetid smell, turning into dark, black coffee colour. Larvae affected change colour, generally from black to brown, their heads becoming longer, and they normally die in a few days, maximum in a week.
Dispersion of the inoculum
Dispersion it is carried out through abiotic factors such as the wind, rain, watering, where spores are transported. Biotic as parasites, predators, adults of the host, dead larvae where they remain, being the initial inoculum for future infections, usually must be ingested by hosts to get into the body through the intestine.
A great variety of subspecies of Bacillus thuringiensis has been investigated. These ones control different species of pest insects and several strains have been reproduced and traded in an industrial scale.
Bacillus thuringiensis var. Aizawai: controls diverse larvae stages in pests such as cotton leaf worms, asparagus, leguminous, eating work in broccoli and Spodoptera frugiperda in corn.
Bacillus thuringiensis var. israelensis: they are sporangia bacteria, producers of endotoxins. They control dipterous (mosquitoes), they are specific for larvae, like Psorophora Confinnis, Aedes taeniorhynchus, Mansonia sp., and Simuliidae.
Bacillus thuringiensis var. Kurstaqui: HD-1 used for lepidoptera pest control, forest and agricultural pests. Characterised by carrying the following cry genes antilepidoptera: cry1Aa, cry1Ab, cry1Ac, cry2Aa, cry2Ab and cry1Ia. It is also toxic for 8 superfamilies: Sphingoidea (Sphingidae), Noctuidea (Noctuidae, Arctiidae), Notodontidea (Notodontidae), Yponomeutoidea (Plutellidae), Tortricoidea (Tortricidae), Pyraloidea (Pyralidae), Papilionoidea (Papilionidae, Pieridae, Nymphalidae) and Bombycoidea (Saturniidae).
Bacillus thuringiensis var. Tenebrionis: To handle coleopterans.