Virus Melts Caterpillar!

I recently saw a presentation on the molecular biology of baculoviruses and was blown away by the missed opportunity to discuss their ecology.
If the speaker had introduced this fascinating group of viruses properly, the audience would have been much more interested in what he had to say.

The lifecycle of a typical baculovirus begins when a caterpillar ingests virion particles while feeding on a leaf. The virus exists in two different forms - a tough protective structure that can survive the elements and an infective structure that travels quickly through the insect's body.

The former version of the virus can withstand harsh environmental conditions -heat, solar radiation and rain - but it breaks down in the incredibly alkaline environment of an insect's midgut (pH as high as 10 or 12!). This alkalinity (on par with ammonia!) dissolves the virion's protective matrix and allows the virus' nucleic acids to climb into the insect's gut cells. The virus then transforms to its latter form and spreads throughout the body by budding off each infected cell in little bubbles of cell membrane.

In Manduca sexta (pic.), this virus also seems to induce the caterpillar to wander up high into branches before it dies. This was noticed in Germany in the 1900s, where it was called "wipfelkrankeit," or, tree-top disease. At this point the virus enters its lytic phase, bursting cells and inducing the caterpillar's own cells to produce chitnases that digest the exoskeleton. The sick caterpillar literally melts, leaving only a black, virus-filled stain on the leaf. The slurrified caterpillar drips and falls down through the branches, contaminating every leaf it touches. Our very hungry caterpillar got infected by eating the liquefied corpse of another.

Because these virion particles are water soluble, they can likely be found on every land mass and in every body of water on Earth - and are certainly present on most of the fruits and veggies you eat! Luckily, they're harmless to us. In fact, they're so incredibly species-specific that they haven't been very useful as biopesticides since growers usually need to control more than one bug at a time. There are just a few commercial preparations, including one that the USFS uses to control gypsy moths. Baculovirus biopesticides (like many forms of biocontrol) are kinda tricky to use, and therefore expensive, but a few labs are working on discovering/engineering more effective viruses strains.