The main ingredient of insecticidal soap products is Potassium Salts of Fatty Acids. This ingredient is very low to non toxic and is biodegradable in 48hours. Although it has potential for skin irritation and stomach irritation, it shouldn't be a problem. Your dog would have to ingest quite a bit before it would be come a problem. My dog occasionally eats grass and throws up a green foam so I wouldn't be too concerned. Since your dog is a curious chewer, you might want to put the plant up for 48 hours anytime you spray it with insecticidal soap. Insecticidal soaps are one of the safest insecticide options.

After you catch your dog eating a plant that they shouldn't watch for these signs:
Rapid breathing
Excessive Drooling
Vomiting
Excessive Panting

If any of these things occur immediately call you Vet.

A slight bit of vomiting may occur without concern but violent vomiting indicates a need for medical attention.

Good luck and please keep me posted.