Most of these “bugs” have mouthparts like a hypodermic needle. They stick their needle-like mouth parts into the veins of plants and suck out the nutrient rich sap. As their populations build they can seriously impact the energy budget of your vegetable plants and limit your crops. Also, just like a mosquito sucking the blood from your arm to give you malaria, these insects can transmit serious diseases to your plants.
Very familiar to most gardeners, aphids (aka green flies in the U.K.) attack almost all vegetable crops. There are thousands of different species in different colors (green, black, gray, yellow or reddish-brown). Sedentary, they don’t move around much. They just sit still and suck. Some species transmit mosaic virus diseases.
Tiny thrips on an onion plant. They have rasping mouthparts. They file away the soft tissue from leaves and flowers causing silvery discoloration. Active flyers, thrips transmit tomato spotted wilt virus.
Mealybugs don’t even look like insects. They look more like q-tips in the leaf axils and undersides of leaves. They are sedentary. They transmit mealybug virus to pineapple plants.
Leafhoppers come in an astonishing array of colors, and some are quite pretty. Common garden variety leafhoppers are generally pale green, wedge-shaped, and jump away when disturbed. These are active flyers with sharp little beaks. They transmit aster yellows (a serious mollicute disease) and viruses like beet curly top and yellows virus.