Mosquitoes are a nuisance no doubt but they are useful for causing large scale diseases that can kill humans who by any measure are overpopulated.
Believe it or not, mosquitoes are pollinators. In fact, mosquitoes' primary food source is flower nectar, not blood. Just like bees or butterflies, mosquitoes transfer pollen from flower to flower as they feed on nectar, fertilizing plants and allowing them to form seeds and reproduce.
There are about 3,500 mosquito species, many of which want nothing to do with biting humans or any other animal. Even in species that bite, it is only the females that do so and just to develop their eggs.
The fundamental food of all adult mosquitoes is plant sugar and its associated nutrients, most often in the form of floral nectar. In the process of looking for nectar, mosquitoes pollinate many of the flowers they visit — this is one of the most commonly overlooked ecological functions of mosquitoes.
I don't know anything about wasps or hornets since they are not native to my area.
Ditto on mosquitos contributing to pollination, and not all of them being sanguivorous. In fact, not only are adult elephant mosquitos (Genus
Toxorhynchites) completely herbivorous (feeding on sap, nectar, juices, honeydew, and fruit), but their larvae are predaceous and feed on other mosquito larvae.
Some mosquito larvae are detritivores, breaking down dead matter and cycling nutrients back into the aquatic environment. Mosquito larva that feed on algae and bacteria help control the growth of these organisms. Mosquitos of all life stages are food sources for other animal species like spiders, dragonflies, freshwater fish, diving beetles, birds, and bats.
As for the ecological roles of hornets and wasps, they are predators and parasitoids that help control various insect populations, including "crop pests". Some wasp species are important pollinators for plants like figs and orchids. Some wasps aid in decomposition by feeding on rotten fruit and taking bits of carrion to their larva. Of course, wasps and hornets are preyed on by many animals like spiders, mantids, bears, and birds. They even feed carnivorous plants like Venus flytraps and pitcher plants.
As for the original post stating that herbivores are "harmless" and that a world consisting of only herbivorous animals would be "in harmony and peace", that couldn't be further from the truth. Like all organisms on Earth, herbivores exploit and compete against other organisms (including members of their own species) for their survival and reproduction. Oftentimes, they will use violence for self-defense, protecting their offspring, protecting the herd, and competition for food, water, territory, and mates. The most aggressive and dangerous herbivores include elephants, hippos, moose, bison, cape buffalo, and honeybees.
Even without the existence of predators, parasites, or parasitoids, herbivores will still experience suffering in countless ways including anxiety, fear/terror, pain, disease, injury, starvation, dehydration, drowning, natural disasters, getting killed by other herbivores, rape, separation from their herd, and in extreme cases, cannibalism.
Without carnivorous scavengers and detrivores like vultures, jackals, fly larvae, and burying beetles, dead herbivores would pile up all over the land and water, leaving no room for new organisms to grow, survive, or thrive. Their carcasses would increase the growth and spread of deadly microbes that cause diseases like anthrax. The nutrients in their dead bodies couldn't be (efficiently) broken down and cycled back into the substrate to nourish plants, algae, and fungi.
Planets desolate of life are the only peaceful environments. For there are no organisms that will experience or cause any kind of suffering.
