Combination products contain two or more active ingredients. One active ingredient generally has fast knockdown characteristics, while a second active ingredient is slower acting with a longer residual life. In some cases, a synergist, such as piperonyl butoxide, is used to prevent recovery.
Some mosquito abatement districts, which conduct ULV applications at cool temperatures (about 60°F), use a mixture of seven parts oil, three parts malathion, and one part Scourge (resmethrin + piperonyl butoxide-18% + 54%). Malathion has a positive temperature coefficient. As the temperature decreases, the efficacy of the malathion will also decrease. Conversely, resmethrin has a negative temperature coefficient. Its efficacy will increase as the temperature decreases. The mixture works well in cool and warm weather, provided it is warm enough for mosquitoes to be flying.
The following are chemicals commonly formulated with another active ingredient and a brief description of their general characteristics:
Pyrethrins - A botanical insecticide with fast knockdown action and a very short residual life. Insects may recover from doses sufficient to produce knockdown.
Resmethrin - A synthetic pyrethroid with fast knockdown action and a very short residual life. Insects generally will recover from a sublethal dose.
Chlorpyrifos - An organophosphate insecticide with moderate residual activity.
Permethrin - A synthetic pyrethroid with fast knockdown action and moderate residual activity (usually greater than two weeks). Insects may recover from doses sufficient to produce knockdown.
Diazinon - An organophosphate insecticide with moderate residual activity.
Malathion - An organophosphate insecticide with some residual activity when applied as a mist or residual spray.
Methoxychlor - A chlorinated-hydrocarbon insecticide with moderate residual activity.
Note: Residual activity of these products depends on the method of application and environmental conditions. Ultra low volume and thermal fog applications are not residual type applications. Typically, these will leave negligible residues.