When people discuss historical uses and development of biological weapons, much emphasis is rightly given to the Soviet, US, and Japanese bioweapon programs. However, there are some other efforts that are very notable but little discussed, including those of Germany.
While most know of Germany's efforts, led by Anton Dilger, to infect American horses bound for the war with glanders (in Baltimore) not much is known about further German efforts. There is a general consensus among scholars the German Chancellor Hitler's experience with chemical weapons as a solider in WWI largely biased him against chemical and biological weapons.
A recent news article, however, reports National Socialist Germany's efforts under Heinrich Himmler to use malaria-infected mosquitoes during WWII. The article reports that the mosquitoes were bread, but not deployed.
As a bioweapon during a war, I tend to think mosquitoes would be a poor choice given the unpredictability of their flight patterns which could easily place one's own soldiers at risk. Moreover, malaria is a treatable and recognizable condition making it less attractive.
However, entomological warfare with the covert release of mosquitoes infected with, for example, dengue (as Fidel Castro accused were used against him) might work to create havoc and make a region inhospitable for living. As such, it is a concern that merits consideration.