The Bayer 3 in 1 is not an acceptable product for an organic grower to use and since you are posting in an organic gardening forum I presume you want organic solutions.

Where in the world are you?

What has the weather where you are been like?

What is the soil these plants are trying to grow in like? When was a good reliable soil test done and what did it show?

Pest often are drawn to plants in stress, although they do also eat many other plants.

kimmq is kimmsr

aqsack, you need to know that guava has its own group of serious pests and doesn't need a gardenia to bring them in. Just sayin'.......

You are absolutely correct in your evaluation of gardenia....they do attract certain pest insects. In my experience with this plant, whitefly has been the primary pest. The insect infestation is the cause of the black sooty mold, which grows on the sugary excrement from certain insect pests.

My favorite treatment for pests like whitefly, scale, and mealybugs is repeated applications of a horticultural oil, sprayed directly on the underside of the leaves and stems as well as over the top of the plant. Oils should not be used on broad leaf evergreens during temperature extremes, hot or cold, but can be used year round if you're watchful of the temperatures.

Horticultural oils smother the insects in all stages of development. It's especially useful, I think, in smothering the overwintering eggs of many pests, preventing a big hatchout in the spring.

I might suggest getting rid of the gardenia, and finding another shrub to go into its place that shares some of the characteristics that you like. There are other shrubs with pretty flowers, glossy foliage, etc. that won't constantly put you in such an adversarial position. Sometimes plant choice is the tool you most need.