Help! Why Do Marigolds As A Companion Planting Fail To Deter Grasshoppers?

Help! If planting marigolds (Tagetes spp.) in Australia to deter grasshoppers helps these pests to breed, why do well known authors and garden clubs pass on misinformation? Are marigolds useful for anything other than ornament? How do I control the grasshoppers plaguing my garden? asks Vivienne in Queensland.