Unit's 731 and 100 did indeed attempt to develop this type of weapon.
I seem to recall they had a lot of difficulties due to survivability of the plague based carrier or vector which precluded high altitude and explosive
In terms of 'genetic engineering' I think you would find that there are many toxins which no insect could survive and which would be 'justified for
use' on entire populations of pet species, just as happened in China and Hong Kong with the bird influenza. Many insect species have rather specific
parasitic preferences and to create a universal vector which could act as a disease carrier while breeding outside a given area would be hard if you
scattered them broadly enough to inhibit point countermeasures (In concentrations, the very frequency of infection rate acts as a beacon to both
active pest control and quarantine isolationism as has been the principle die out and scorched earth method of plague containment for /millenia/.)
Perhaps the greatest gift to humanity in terms of biologics weaponization is simply that genetics research and filtration of enhanced cultures down to
an exploitable vector base is NOT a 'home kit' type of effort, requiring many specific equipment items which are either restricted, expensive or
This is not to say that China or Saudi or or or some nation or citizen with money could not develop such a weapon and then hand it over to a
But at some level of social development, once you are functioning past the instinctive animal response to any and all hostile stimuli, you tend to shy
off of 'unknown outcome' type options which are open ended in their ability to escalate out of control. Or be traced back to _just you_.
If America suffered an 'Outbreak' type disaster in a major city with say more than 500,000 dead before a evacuation, isolation and
burnout/starvation could remove the threat (and even insects need to eat to maintain generational survival); the only question would be how long it
took to trace back the altered genetics of the bug or the microbe to a host nation with the capability and the researchers.
There WOULD be a massive (Interpol) level of response and should you ever be found out, the U.S.' official stance of 'nukes only' would likely mean
a countervalue attack that amounted to decapitation and replacement of the government.
Quite beyond the difficulty people have in envisioning medieval level attrition from a Red Death type of uncontrolled vector progression, NOBODY wants
-that- (fireball explicit) kind of idiocy coming back to haunt them.
Or at least so we commonly hope.
Unlocking A Deadly Secret
Japan's Gloriously 'Unacknowledged' WWII History