What is NPV?
Nuclear Polyhedrosis Virus, or NPV, is a virus that infects caterpillars in early instar stages and take action during the later instars or sometime the chrysalis phase. NPV spreads by liquifying the original infected caterpillar, causing the virus particles to spread on nearby plants, where other caterpillars could possibly eat those leaves causing more instances of NPV in caterpillars.
A Monarch caterpillar after dying from NPV, showing the "arch" position.
What are symptoms of NPV?
NPV infects caterpillars by the spreading of virus particles from previous infected caterpillars that land on leaves. After the caterpillar has eaten the infected leaf, the virus particles will remain dormant until later instars or when the caterpillar is getting ready to chrysalize. After the caterpillar has ingested the infected leaf, there is an almost 100% fatality rate.
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Once the caterpillar has gotten to the later instars, the virus "takes over". The caterpillar is now not in control of its body. The virus takes the caterpillar's body to an open, high up area, where the virus then takes full control and puts the caterpillar's body into an "arch" position, with either the back, middle, or front set of legs are hooked onto the surface. Then, the virus liquifies the caterpillar, turning it into a sort of water balloon. The caterpillar is left up there, and eventually that "water balloon" will pop, causing billions of miniature spores spreading all over the surrounding area.
How can we find NPV earlier?
Caterpillars infected with the NPV virus eat little to nothing, and the food they do eat is usually vomited out in a darkish green color and dries up to be a black color. The caterpillars will be noticeably skinner, and when held, they will have significantly less muscle mass. They will also move much more sluggishly, and their bright black, white and yellow bands will look much more desaturated, with grayish, cream-colored and orangish-yellow colors instead.
A monarch caterpillar infected with the NPV virus, showing early symptoms. (Photo credit; Rose Franklin)