The use of selective pesticides or alternative pesticides is an alternative away from the typical conventional broad-spectrum materials that can be harmful to natural enemies, and can be harmful to people, too.
Insectidal soaps are desiccants. When you spray them on a plant, they break down the membrane of the insect and it basically loses all it’s water. Oils, on the other hand, are what we call suffocants. When you spray them, they cover the breathing pores, and the insect basically dies of suffocation or asphyxiation. So that’s the difference.
They both primarily work very well in the soft bodies insects such as aphids, mealy bugs, white flies, and spider mites. Oils will kill the eggs of certain insects and mites, so that gives them a little spectrum activity. Where the soaps primarily kill the nymphs, the larvae, or the adult stage.
The advantage is that they’re selective and they have short residual activity. However, that short residual activity can be a disadvantage because that means you have to apply them more often. They do break down under UV light, sunlight and rainfall. So, that means you have to apply frequently.
They have less indirect impact on natural enemies. But, if you spray them on a natural enemy like a green lacewing, or ladybug larvae, they will kill it. They’re non-discriminatory about that.
First, get the pest identified. And then you can go to garden center or nursery and find the product that has that specific pest on the label.
The key when using these products, soaps and oils, is two-fold. First, you must have good contact and thorough coverage of the plant parts. They’re a contact activity – whatever they touch, they’ll kill. So, if you miss some aphids and spider mites, they won’t die.
The other key is frequency of application. You have to apply them frequently because there is very low residual activity. So again, the key is through coverage of all plant parts and multiple applications if you want to effectively deal with insect or mite pest populations.
This feature story prepared with Raymond Cloyd, Kansas State University Research and Extension Professor of Entomology. For more information, visit your local county extension office or visit our website at KansasGreenYards.org.