Aphids and flea beetles are two garden pests that affect vegetables as soon in the season as possible for a bug. They are two of the earliest pests.
If you see ants on your plants, then look for aphids. Ants tend to aphids because aphids secret honey dew, a substance that ants like to eat. Aphids weaken plants and spread diseases. If the honey dew that they produce is not gathered, that will attract mildew. If your garden is small like mine, a simple way to control aphid population and damage is by spraying off the affected plants every day or so. If you have it, a soapy spray suitable for plants will kill the aphids by suffocation.
The image below shows a couple of aphids on a tomato plant. These aphids are red and on the plant stems. Aphids are several colors - green, yellow, etc, and they are more often on the underside of leaves. To check for aphids, look on the underside of leaves. They can be washed off, sprayed with a plant-safe soapy spray, or squished.
Flea beetles weaken or kill young plants by eating holes in the leaves and stems. They also spread some plant diseases. They are small dark beetles that jump away when they see you. They start feeding on young plants or seedlings as soon as the bugs find them. Flea beetles eat tomatoes, peppers, potatoes, beets, cabbage, corn, and related crops.
This image shows flea beetle damage.
The image below shows a flea beetle on a tomato seedling.
In my garden, flea beetles appear every year. Last year, I walked through my garden at least twice a day to scare them off the plants. Some days, I sprayed the plant leaves with water. That was all that I did. If I had cabbages, I would cover them with a floating row cover as soon as the plants or seeds are planted.