A myriad of pests naturally occur in vegetable gardens. There are many animals or insects--large and small--that feed on garden pests, but the natural balance can be upset when harmful insecticides are used and kill the good with the bad. Learning to differentiate the pests from the creatures that help control the pest population is a good start in being able to manage a garden in a more organic way.
Animals with needle-like mouths are considered "sapsuckers," such as aphids, whitefly or red spider mite. These pierce the outer epidermis of a plant and suck the sap out, thus distorting growth in the plant. These are also known for transmitting plant viruses, as well as bacterial infections from the wounds created by these mouths. The excrement of these pests, is called "honeydew," which can create an especially fungal environment around the plant where mold will increase, according to Iowa State University Extension.
Many common pests graze on the roots of plants. These include but are not limited to caterpillars such as cutworms, beetle grubs like wireworms, vine weevil, chafer beetles, cabbage root fly, carrot flies, as well as microscopic pest nematodes. Some of these pests move between plants while others will focus on one and completely invade the root system. This restricts nutrient and water uptake, thus causing wilting.
The most noticeable damage to plants is often in the form of a bite taken right out of a leaf. Leaf feeders like butterflies, moths, sawfly caterpillars, adult beetles and their grubs, slugs and snails are some of the smaller culprits but there are also larger animals such as birds, rodents and mammals that will feed on the leaves of your plants. This sort of damage reduces the area where photosynthesis can occur, which in addition to wrecking the appearance of a plant can restrict growth. Some of these pests will also go for the stems of plants.
Flower and Fruit Feeders
When pests are hungry, they may go for the blossoms of the plants which can reduce the production of fruit. These pests include earwigs, thrips, blossom weevils and Japanese beetles. Others such as caterpillars, some moths, sawflies and some beetle grubs and fly larvae feed right on the fruit or vegetable that has developed, which causes it to be is unpalatable. It can also cause an opening for a bacterial or fungal plant infection.
The odd, enlarged masses seen on some plants are called galls and are an enlarged mass of the plants own cells. This happens in response to when certain pests feed, such as fly and wasp larvae, and some mites. Often, the pests feed on the gall cells and it doesn't cause any damage to the main plant, but occasionally, according to "Rodale's Illustrated Encyclopedia of Organic Gardening" these can cause injury. And sometimes galls are created by fungal, bacterial, or viral infections.