Butterfly Bush Doing Poorly May Be Grubs in the Soil

Q. I have a butterfly bush that is struggling. It looked wonderful this past spring, now the leaves and blooms are alot smaller. There are alot of leaves turning black. The bush has full sun and 

gets watered regularly. Do you know what the problem might be?

This is from a previous question regarding the same problem on Lantana. She is pointing to grubs in the soil eating the roots and damaging the plant.

A. Check for grubs (worms) eating the roots. This is a common problem on butterfly bush this time of year. Use a liquid insecticide as a soil drench to wash the roots and kill the grubs. You can also use granular insecticides and lightly watered into the soil at the base of the plant. Look for one that is for grubs at your nursery or garden center.