Aphids
What They Look Like Aphids are easily visible to the eye, but are fairly small. They look similar to fat bodied ants and are rather pear shaped. Aphids come in several colors and you may find several sizes on a plant at the same time. They are soft-bodied and rarely have wings. Often the first sign of aphids on plants is sticky, shiny spots on lower leaves. Aphids (like scale insects) cannot digest the sugars in the plant sap and they exude that liquid as "honeydew" that can coat the lower leaves or the table below or a car parked under an infested tree.  Ants are often associated with aphids because they are drawn to the sweet honeydew.

Type of Damage By sucking on the soft, tender tissue of plants, aphids can damage or even kill new growth. A large, prolonged infestation may even weaken a plant enough to lead to its death. Because they need tender tissue, look for them on the youngest growth. They hide on the underside of those new leaves and are often lined up along the main veins. They almost always stunt the new growth and may cause puckering or curling.

Plants They Target There is hardly a plant that isn't a target for some type of aphid. In the garden, they will target some trees and shrubs and many annuals and perennials. Roses are a favorite. So are shrub dogwoods. Indoors, aphids seem to avoid most ferns and succulents but like almost everything else.

Life Cycle Aphids do vary some, but they all have basically the same life cycle. If you start the cycle when they are eggs in the soil, both winged males and females will hatch. The females will immediately go in search of a host plant. Once they find food, they begin giving live birth (not eggs) to baby aphids that are almost always all females. These female baby aphids are born pregnant and within a very short time (a few days) they can begin giving birth to their own pregnant female babies. When the source of food is exhausted, a few aphids will be born with wings to fly off in search of another host plant. At the end of the season, the male and female aphids get back together to lay eggs for next season. Because they are capable of such incredible multiplication, it is easy to see how a simple problem can become overwhelming very quickly.

Treatment Options The good news with aphids is that they are very susceptible to almost every insecticide. In fact, the first defense should be to simply wash them off the plant with a spray of water. Since they are soft-bodied and wingless, they are unlikely to travel back up from the ground to re-infest the plant. Being soft-bodied, they are easily killed by most insecticides including the milder types such as insecticidal soap and pyrethrins. Repeat the treatment in another 3 or 4 days then monitor the plant closely. The key is to be sure you spray them all, since missing just a few might lead to a substantial re-infestation in just a matter of days.  Ladybugs are a natural predator.

Spider Mites
What They Look Like: Technically, spider mites aren't even insects. They are almost too small to see. In fact, if you can see it easily and it looks like a spider, it's one of the good guys (spiders eat spider mites). Spider mites can only been seen by the naked eye if you take a sample of the affected plant and tap it hard on a piece of white paper to dislodge the insect. Then look closely at the paper to see if any of the tiny dots (like the smallest mark you can make with a sharp pencil) start moving across the paper. Those are the spider mites. Occasionally, on an indoor plant, they can become so prolific that you might notice a webbing.

Type of Damage: Spider mites infest a plant by the thousands and suck sap from the foliage. They are tough pests and target almost all leaves or needles, not just tender new growth. They tend to like hot and dry weather, so they are usually worst in mid to late summer and indoors in winter. Spider mites often weaken a plant, sometimes severely. Their damage makes the surface of the leaves or needles look speckled or rough.

Plants They Target: Outdoors, they tend to like evergreens, especially spruce and arborvitae. They will target various deciduous plants too, including roses, oaks and raspberries. Indoors, they love ivies and citrus but will bother other plants too, especially when the air is dry.

Life Cycle: Spider mite eggs overwinter in cracks and crevices in the bark on plants and hatch when the temperatures are right. Once they find a host plant, they will go through several generations from eggs to larvae to nymphs to adults and back to eggs again on the same plant.

Treatment Options: You can't just ignore a spider mite infestation. First step is always to wash the foliage with water, ideally several times over several days. Spider mites are not susceptible to all insecticides. Use a suffocant such as horticultural oil or a specific miticide. Always check the label to see if the insecticide is recommended for the pest and specific plant you are treating. For instance, horticultural oil is recommended on spider mites, but not on blue spruce. The oil would work against the mites, but it would also ruin the glaucous blue coating on the needles. Ladybugs are one of the natural predators of spider mites.

Whiteflies
What They Look Like: Depending on the stage of their life cycle, white flies will either look like a tiny (less than an eighth of an inch long) white flying insect or like a small piece of rice, usually white or pale yellow.

Type of Damage: Whiteflies suck the sap from the underside of leaves. That is also where they lay their eggs and the nymphs live. The sucking will result in a weakened plant and poor growth. It may cause some leaves to yellow and drop. With vegetable plants, the damage will delay and reduce fruiting.

Plants They Target: There are several types of whitefly that target a wide variety of plants. Outdoors, you most often find them on vegetable plants, especially tomatoes and cucumbers. Indoors, they love hibiscus, euphorbias (including poinsettias) and various other plants.

Life Cycle: An adult will fly to a plant where it lays its eggs on the backside of a leaf. The eggs hatch about 5-7 days later into nymphs (crawlers) that find a place to feed and stay there for 1-2 weeks. They phase into a pupa from which an adult emerges about 10 days later.

Treatment Options: Treating whitefly infestations can be tricky. There isn't any one insecticide that will affect all the life stages at the same time. Horticultural oil will suffocate adults, nymphs and most of the eggs, but not the pupae. Insecticidal soap will deal with the adults and the nymphs, but not the eggs or pupae. Stronger insecticides are not any more effective. To treat whitefly, it is best to make three applications of the insecticide three to four days apart. The first treatment should kill the susceptible stages and the follow-up treatments will target the remaining stages as they change into either nymphs or adults. Monitor the plant closely to see if it needs further treatment.