Common on backyard trees, ornamental shrubs, greenhouse plants and houseplants, over 1,000 species of scale insects exist in North America. They are such oddly shaped and immobile pests that they often resemble shell-like bumps rather than insects. In many cases, heavy infestations build up unnoticed before plants begin to show damage. Large populations may result in poor growth, reduced vigor and chlorotic (yellowed) leaves. If left unchecked, an infested host may become so weak that it dies.