Arthropods become pests when they spread diseases to humans, attack human food crops or otherwise annoy humans.
Insects are the arthropods that cause most harmful effects.
This is not surprising because insects are the largest group of arthropods. Often these were due to human intervention or destruction of natural habitats, causing population explosions of the insects, or depriving the insects of their natural hosts and forcing them to turn to humans instead. Thus, in undisturbed habitats, there are generally no infestations of pests. In a natural habitat, such foodplants usually occur widely spaced apart, and the pests are preyed on by natural predators. The problem is usually aggravated by the human tendency to breed plants which are especially yummy and have bigger leaves and fruits.