The Arena .25 Granules application coverage rate depends on what you are treating. Please review the product label as the the turf grass application chart will explain how much to apply to treat different types of pest.
Arena .25 Granules is safe for use around landscape ornamentals plants such as roses for Japanese beetle grubs and other insects on the product label.
Unfortunately, we do not have the spreader settings for that specific combo and the manufacturer does not list that spreader on their bag. Due to the thousands of granules on the market and just as many different types of spreaders, you will want to calibrate your spreader to the product you are using and its application rates. You can see a short video on How to Calibrate your Spreader Video.
For further assitance on correct spreader settings, contact Scotts directly at 888-270-3414
Per the label, you will use 0.5-1lb/height of the shrub (5-10 lbs of Arena 50 WDG per 10 cumulative ft in height). Please see the label for complete application instructions.
Because Arena .25 Granules is an insecticide, there should not be a negative effect on your St. Augustine grass. Unfortunately, you have exceeded the yearly rate of Arena .25 Granules in the area used. Per the product label, for turfgrass applications you should not apply more than a total of 160 lbs per acre per season, which means no more than 40 lbs per 11,000 square feet.