Quick Primer--Entomopathogenic Fungus Beauveria bassiana

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8-20-17

This video is intended to be a short primer on entomopathogenic fungi such as Beauveria bassiana, their physiology, and their mechanism of action.

Spores must make contact with a suitable host--some species are more host-specific than others, but this isolate of Beauveria bassiana can affect species across 8 orders of insects, as well as some mite species, which is considered rather extensive, though infection efficacy is not the same for every species. The spore will germinate when it detects the right conditions, oftentimes on the cuticle of a suitable host, though they can be ingested. In the case of ingestion of spore or hyphal matter, the resulting mycosis can be very dramatic. In one study published by the International Journal of Plant Protection, the Bannana Weevil Odoiporous longicollis, was found to be highly susceptible to Beauveria bassiana as an endophyte in banana (M.K. Prabhavathi, 2014) . Within 10-15 days of exposure to endophytically-colonized Banana, specimen mortality was between 50-70%. Damage was also reduced by more than 50%, so it is clear that the infections were disrupting herbivory.

Externally, the fungal material makes contact with the cuticle, by piercing the waxy top layer known as the epicuticle with special penetrative structures called haustoria. The haustoria go on to dissolve the chitinous cuticle and ingress into the body from that point. Many insects are very hygienic and constantly clean themselves, which helps to prevent some infection, but awkward infection sites like the back of the head or abdomen can be more difficult to deal with. It is for this reason that the greater the spore contact density is, the higher (and many times quicker) the rate of mortality: a greater potential surface area will be infected, hygiene efforts are neutralized, and hyphae will work synergystically, overmatching the insect's immune system.

Insects in fact do not have an acquired immunity like humans do. There are haemocytes that are able to phagocytose living microorganisms, however, and some enzymatic cascades that occur which react to lipases, and the cells walls of bacteria and fungi to make compounds that would damage invading pathogens, though non-self recognition in the body is not well-understood at the molecular level.

Over time, the mycelial body inside the host may start to sporulate: reproductive and hyphal structures may start to emerge from openings in the exoskeleton. These spores will be passed through contact or by air currents, and the cycle continues, though this sporulation does not always occur, and is not indicative of efficacy. Certain environmental conditions must be met which may not be detected by the fungus.

Knowing the physiological interaction between entomopathogenic fungi and their hosts can help increase the effectiveness of treatment when used as a biocontrol agent. Application of some entomopathogenic fungi with an agent that aids in their penetration into the host would be a synergistic combination. Multiple microbial biopesticides could be mixed and applied as a sort of general-purpose anti-pest cocktail. My favorite use of entomopathogens are as endophytes, but that isn't always possible to initiate, but dipping plants into spore suspensions with high spore density can be an effective barrier. Insects that land on the coated leaves will have a high chance of making contact with the spores, especially slow-moving pests species. Pests that have chewing mouthparts are likely to ingest the spores, which can quicken infection.