May I encourage you to stop using Bonide? Here's a snippet from their label:

"This pesticide is extremely toxic to fish. Do not apply directly to water. Avoid contamination of ornamental fish ponds. Drifts and runoff from treated areas may be hazardous to aquatic organisms in treated areas. Do not contaminate water when disposing of equipment washwaters. This product is highly toxic to bees exposed to direct treatment on blooming crops or weeds. Do not apply this product or allow it to drift to blooming crops or weeds while bees are actively visiting the treatment area."

The part about bees is especially disconcerting, given the pressure bees are under these days.

Aphids can usually be controlled with a sharp water spray, which at least knocks them off the plants, and frequently kills them. They can also be controlled with insecticidal soap, which is much less harmful to the garden environment.

Since you're using Bonide, go ahead and spray it on the ornamental grass. Just do it when it's not covered with honeybees! Which shouldn't be a problem since ornamental grass doesn't seem to attract honeybees.
Seriously, if the ornamental grass is the only thing you get the spray on, everything environmentally will be fine. If you have any blooming plants or weeds, pull the weeds first, and use a big piece of cardboard to shield the blooming plants so that they don't get spray on them.
We've noticed aphids and spider mites happening on some of our ornamental grasses too and other miscellaneous things, even ones that don't usually attract pests are attracting them this year. Those little buggers can sniff out plants under stress I think. What a awful hot and dry summer!

I'm just as concerned about the environment as anyone, but there are times when chemicals have to be used. We're extremely careful when using the chemicals. Our Black Hills Spruce tree was almost eaten away by saw flies. Many of our flowers were badly eaten as well.

I'm aware of the issue with the bees and only apply when they're not active. The aphdis haven't been too bad on the ornamental grasses. Some of the grasses have leaf spot and/or disease. Webworms have eaten a lot of the leaves of my Moudry fountain grass.

Our grass collection continues to grow and there's now over 50 grasses in our garden.