Asked January 11, 2015, 7:05 PM EST
Denver County Colorado
There is nothing that you can really put in your soil to prevent the insects from coming back. Aphids do not inhabit the soil, they need plant material to provide the food necessary for survival. Depending on how cold it gets in the winter, aphids may or may not live through the winter on plant material. Aphid populations rise and fall due to the current climate conditions. For example, wet springs which produce lush growth followed by periods of hot dry weather can cause big increases in aphid populations.
One idea is to watch carefully in early summer for the arrival of aphids on your plants. As soon as you see them take the hose with a nozzle and give your plants a thorough spraying with water (make sure you get underneath the leaves) as often just knocking the aphids off the plant will greatly reduce their numbers as they are not able to climb back up. You can also use an insecticidal soap that is safe to use on vegetables.
Mealy bugs are also plant insects and are almost always found on the plant, not in the soil. They suck on plant juices, so they are usually found usually where the leaf joins the stem or along leaf veins. You can try using a horticultural oil or insecticidal soap if they emerge in the spring.
Here is a fact sheet on where Insects go in the winter.
Regarding the ladybugs, here is an excerpt from our fact sheet on ladybugs: “Unfortunately, the record for effective use of lady beetles released in a garden is typically poor. One reason for this is that the beetles are highly dispersive, poised to migrate long distances from their collection areas. With few exceptions, expect released lady beetles to fly away from gardens where they have been introduced. Furthermore, these field-collected lady beetles usually are in a state known as reproductive diapause, during which time they do not produce eggs. Only after feeding for several weeks after release do they resume egg laying.”
Here is the entire fact sheet: http://www.ext.colostate.edu/pubs/insect/05594.html
Please let me know if you have any questions.