Answer: Yellow leaves can be caused by many things including lack of nitrogen, insufficient light, water-logged soil (plant roots need oxygen to thrive), dry soil, or iron deficiency. Insects may cause yellowing if they suck the sap from leaves, which can leave behind tiny pinprick holes. Try to isolate each of these possibilities one at a time to determine the problem. It wasn't clear to me from your question if you actually saw insects or were just spraying as a possibility. Aphids can usually be controlled by less toxic methods than an insecticide. (More on that below.) If the older bottom leaves are yellow, but new growth is green, it's usually a lack of nitrogen. If new leaves are yellow, with green veins, it's usually a lack of iron. (Lack of nitrogen is a more common problem than lack of iron.) Soil should be kept moderately moist (but not wet). Transplant shock can contribute to yellowing. If new growth shows up as green, that might be the problem. Also, gardenias prefer acidic soil conditions and can show signs of stress if the soil pH isn't too their liking. You might want to try fertilizing with a product for acid-loving plants, such as Miracid.
It's really important to identify exactly what is eating your plants before attempting to spray something on them. You may do more harm than good. I always start with the simplest method first, and if that isn't successful, move on from there. Healthy vigorous plants will withstand insect attacks best and sometimes a little damage is acceptable. If your aphid problem is not too severe, a strong blast of water from the hose should work. Spray underneath leaves, in between, etc. Do this daily.
If that doesn't work, try a soapy water spray. Use 1 teaspoon to 2 tablespoons of liquid detergent soap per gallon of water. Use regular, not concentrated soap. Don't use soaps with lemon, as the citric acid can burn plants. Start with the lower amount and work up as needed. Spray as often as needed. As with any spray you might wish to test it on a few leaves first before you treat all your plants. Next on my list would be an insecticidal soap spray.
Ladybugs and their larvae are voracious eaters of aphids. They often "arrive" a week or two after the aphids, so not spraying with chemicals is a good idea if you'd like to attract them to your garden to consume aphids for you. Good luck!
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