Two problems I have in the butterfly garden are aphids on milkweed and mildew on many plants. One idea that I saw recently was using Dormant Oil Spray.during late winter and early spring. While the video was mainly for fruit trees, the process would also be similar in my garden.
In particular, the area where I grow milkweed, might benefit from spraying the soil where aphid eggs would over-winter. I remove any milkweed plants around March 1st so my plan is to spray the soil around Feb. 15th then once again after I clean out the garden.
Note – I remove any chrysalises I find in the fall and keep them in the garage.