Aphids are often found in clusters on the ends of stems. They are about the size of salt grains on a pretzel and have soft bodies that squash easily. If they are only on the end of a stem, just prune off the tip and step on it. Colonies of aphids can cause the new leaves on a plant to be curled and distorted. Aphids that move from one plant to another can spread viral and other diseases.
They have an interesting life cycle. The egg hatches in the spring into what is sometimes called a stem mother. She is wingless and gives live birth to more female aphids that are often born pregnant. A new generation comes in only days and so there are many generations each summer. Some are born with wings so they can fly to other plants. At the end of the season, a stem mother gives birth to males and females that mate. This female lays fertilized eggs that hatch in the spring as stem mothers. In warm climates and greenhouses, there can be years of generations without fertilized eggs being produced.
Scale insects look like scabs. Sometimes, they look like seashells, and other times, like tiny cotton balls. Under the scale is an adult insect that is protected by the covering. The female lays eggs that hatch into "crawlers" that move around. Once they settle down, they develop the scale covering. During the crawler stage, they are much more susceptible to insecticides. The overwintering scale can be killed by spraying the plant with horticultural oil that smothers the scale.
Mealybugs are a soft-bodied relative of the scale insects. They are usually covered in a white or gray fuzzy wax. A bunch of them together look like cotton balls. They are active when young, but as they mature, they tend to hide in the joint where a leaf meets the stem.
Whiteflies are so tiny it takes a dozen or so to make a line an inch long. You will see them fly around a plant that is infested when the plant is bumped. They land on the bottoms of leaves. Hundreds may be living on the bottom of a single leaf. They have short life spans, and dead ones may remain on the leaf when it is turned over, while the live ones scatter like snowflakes. They are often common in greenhouses. They can't survive the northern winter outside like all the other insects mentioned. They have a crawler stage that feeds on the leaf. They are resistant to insecticides in the egg and adult stages, so a repeat spray every few days will be needed to control them.
Sucking insects are very common, and the ones listed here are the most common kinds that cause lots of damage. Leaf hoppers, cicadas, and many others do cause damage in some years and on some plants. The best cure for them all is a systemic insecticide that soaks into the plant to get them wherever they are. Other kinds of sprays tend to not get through the scale covering on scale insects and not get on the undersides of leaves to get the other kinds.
Spider mites are about the size of grains of pepper dusted on food. They produce tiny webbings that cover the bottoms of leaves. They are not insects but are classified with the spiders. Some insecticides state on the label that they kill spider mites. Animals classified as insects have six legs and the spider group have eight. They may be tiny, but they can cause a great deal of damage. Plants with mites will look off-color. They will look dry and brown, and the webs will be between or on the leaves. The top of an individual leaf will have tiny, discolored speckles. The mites can be discovered by knocking the bottom of a leaf on a white piece of paper and looking for "pepper grains" that are walking around.
The groups listed today tend to not be much of a problem in areas that are getting regular rains, as much of the population can get washed away during a rain. Plants in dry landscapes not only get more insects, but the insects cause more damage since the insect is sucking out the sap and the plant can't replenish it quickly.