To control insects on houseplants we recommend Insecticidal Soap Spray

For Gnats on houseplants

Fungus gnats are more than annoying pests flying around. The can cause serious damage to plants and should be controlled before their population becomes too large. There are effective biological control measures you can take to detect, deter and control fungus gnats around your plants. We recommend a course of insecticidal soap or organic houseplant spray every 2 days applied to the soil until the infestation ends.