Now that your tomatoes are planted and growing, what do you do when the aphids show up? They are quite small, pale green insects. They tend to congregate on the new growth. There are a few options. My preference is to collect the new leaves that are infested with the aphids. Put them in a blender and add water. Blend it up well and let it set over night. Then run it through a fine strainer and spray it on your tomatoes. The idea is that there are a few sick aphids and by blending the mix, you will spread the sickness to the rest of the aphids. You will be spreading the disease that is particular to that insect.
You can also wash them off the plant with a blast of water. But they reproduce so quickly, that you'll have to do this more than once.
I have not had aphids for several years now. I suspect that a certain amount of that is because the organic matter continues to rise, probably due to the leaf mulch that we use every year with our heirloom seed The healthier your soil is, the more your plants will resist insects and disease.
We tend to look at insects as our enemies. In reality, they are not our enemies. They are better looked at as a clean up crew. Insects feast on the weak plants. When the weak plants are devoured, they will not live to produce seed. This should be looked on as a blessing, because we do not want weak seed. Have you ever noticed how a broccoli or cabbage can be without worms right next to a plant that is full of worms? The seed may have come from a weaker parent, or possibly in transplanting, was weakened. Somewhere in its life, it lacked nourishment and growth was slowed and stunted. When you go to collect seed, choose the healthiest plants. They will produce your most vigorous seed. When you save your own seed, it will become acclimated to your growing conditions. We save some of our seed and they usually produce big healthy seedlings.
One last word; the blender you use to make the bug juice should either be cleaned very, very thoroughly, or set aside for only making bug juice and not used in your own food preparation.