There are two safe pesticides you could try. I don't know if either will balk your unknown pests but they're worth a try. At the least you can tell us what happened. One is neem oil, sprayed on the zinnias. It seems to make insects lose their appetite. The other is Safer's Soap, basically a concentrate of the elements in soap that kill insects. You mix neem or SS with water and spray the dilute mixture directly on your plants. You have to keep agitating the neem mixture to keep the oil and water from separating. Hope this helps.