Regulations

• There are plenty of regulations when it comes to laws and guidelines around the production of insecticides and repellants. Many of them deal with synthetic compounds instead of natural plants. The EPA lists thyme oil as a plant product -- and since there are no known toxic effects from using the oil, there are no regulations. The EPA may start regulating products that claim to be effective insecticides, forcing them to prove their claim with scientific results.
  
  

Organic

• The nice thing about using plant-based insecticides or repellents is that they tend to be organic. In our society of mass-produced plants, most require pesticides. However, many of the herbs like thyme with volatile oils do not require pesticides to grow, as they are innately bug resistant. A spray made of thyme oil will not damage the environment.`
  
  

Concentrations

• Thyme oil derived from the fresh plant tissues is highly concentrated. Obviously, you want the concentration to be high enough to repel the mosquitoes, but diluted enough to be economical while still effective. In tests where the researchers applied the oils to the skin, concentrations equal to or over 25 percent were too strong smelling, while concentrations lower than 10 percent were not effective. For use as a yard spray, the 25 percent concentration would not be so strong smelling, as the air would dissipate some of the odors.
  
  

Effectiveness

• Thyme oil is made up of five different monoterpenes: thymol, p-cymene, carvacrol, linalool and alpha-terpinene. Different plants have different monoterpenes. The thyme oil that contained the alpha-terpinene in a 2 percent concentration proved to have the highest repellency against mosquitoes in a study comparing the effectiveness of Eulcalyptus globulus, Lavender officinalis, Rosemarinus officinalis and Thymus vulgaris as repellents.