This is a tuff one to answer depending on how you are looking at it, I have listed some examples below. Mosquito the mosquito is responsible for millions of human deaths yearly by transmitting a variety of diseases including; west nile, malaria, dengue fever, encephalitis, yellow fever, heartworm (in dogs), and rift valley fever. We were particularly interesting in learning HOW the diseases were transmitted, whether cells were transmitted on the outside of the proboscis, inside the proboscis, or in fluids that are exchanged. Please read the http://www.monarchlab.umn.edu/InsectFair/PreviewProjects.aspx?id=37 African Army Ants Army ants kill and eat up to 100,000 animals in a day. Together they can kill lizards, snakes, chickens, pigs, goats, scorpions, and many other animals. They also climb trees and eat birds plus insects that may live in trees. Please read http://www.insecta-inspecta.com/ants/army/ The tsetse fly kills another 66,000 people annually.

In much of the world, mosquitoes are a major public health problem; they are estimated to transmit disease to more than 700 million people annually, and will be responsible for the deaths of about 1 in 17 people currently alive. In New Zealand, the UK, Scandinavia, and other temperate countries, mosquito bites are mostly just a nuisance. [1] The mosquito genus Anopheles carries the malaria parasite (see Plasmodium). Worldwide, malaria is a leading cause of premature mortality, particularly in children under the age of five, with around 1.3 million deaths annually. Most species of mosquito can carry the filariasis worm, a parasite that causes a disfiguring condition (often referred to as elephantiasis) characterized by a great swelling of several parts of the body; worldwide, around 40 million people are living with a filariasis disability. Most species of mosquito can carry the viral diseases yellow fever, dengue fever, epidemic polyarthritis, Rift Valley fever, and West Nile virus. Fortunately, mosquitoes do not transmit HIV or AIDS. Viruses carried by arthropods such as mosquitoes or ticks are known collectively as arboviruses. West Nile virus was accidentally introduced into the United States in 1999 and by 2003 had spread to almost every state. A mosquito's period of feeding is often undetected; the bite only becomes apparent because of the immune reaction it provokes. When a mosquito bites a human, she injects saliva and anti-coagulants. For any given individual, with the initial bite there is no reaction but with subsequent bites the body's immune system develops antibodies and a bite becomes inflamed and itchy within 24 hours. This is the usual reaction in young children. With more bites, the sensitivity of the human immune system increases, and an itchy red hive appears in minutes where the immune response has broken capillary blood vessels and fluid has collected under the skin. This type of reaction is common in older children and adults. Some adults can become desensitized to mosquitoes and have little or no reaction to their bites, while others can become hyper-sensitive with bites causing blistering, bruising, and large inflammatory reactions. Mosquitoes in flight emit a distinctive high-pitched buzz, which can interrupt sleep.

mosquito

The mosquito wins hands down; but if you are looking for an insect that can kill with venom ...bees kill many people every year through anaphylactic shock in severely allergic people, and several ants can too ... but there is an ant that can cause fatal anaphylaxis in people not previously known to be allergic ...and yes, the prize goes once more to Australia, where the Tasmanian Bulljoe, or Jumping Bulldog Ant delivers a sting that will have you crying for your mama and hoping you aren't sensitive.

Black widow.

I'm gonna go with the Florida state bird... the humble mosquito. However, have you seen these bad dads?