Sometimes there are antagonistic responses from combining pesticides. This can result in no activity of one or both of the products. Other times, synergistic responses may occur. This often results in spray burn. Many pesticide labels list products that can be mixed safely.
Many products suggest the addition of a surfactant.
A surfactant is a chemical that reduces the surface tension on leaves and thus allows the pesticide to cover the leaf more uniformly. Some surfactants also aid in the uptake of the product into the plant. Pay attention to the type of surfactant required. Not all products use the same surfactant. Pesticides can be useful in preventing or controlling various pests in the garden and commercial vegetable production when used properly.