Insecticidal Soap For Cannabis: How It Works And How To Use It
Quick answer: Insecticidal soap for cannabis is a spray made from potassium salts of fatty acids. It kills small, soft bodied pests such as aphids, whiteflies, mealybugs and mites, but only the ones it wets, and it has no residual effect once it dries. Read the label, test it on a few leaves first, cover leaf undersides well and repeat at short intervals when the label allows.
What Is Insecticidal Soap?
Insecticidal soap is a soap made for plants, not for dishes. Colorado State University Extension says modern insecticidal soaps, or soap salts, are potassium salts of fatty acids, molecules made of potassium attached to long carbon chains. Products are sold under many trade names, but the active ingredient in all of them is potassium salts of fatty acids.
NPIC, the National Pesticide Information Center at Oregon State University, says these salts are made by adding potassium hydroxide to fatty acids found in animal fats and plant oils. Colorado State University notes that soaps have been used against insects for more than 200 years.
How Insecticidal Soap Works
NPIC says fatty acids penetrate an insect’s body covering and disrupt its cell membranes, so the cell contents leak out and the insect dehydrates and dies. Colorado State University adds that soaps may also remove the protective waxes that cover some insects.
The catch is that it only works on contact. Colorado State University says soaps act strictly as contact insecticides, with no residual effect. Sprays must be applied directly to the pests and must thoroughly cover them, which usually means spraying the undersides of leaves. Pests that cannot be fully wetted, such as aphids inside curled leaves, will not be controlled.
Which Pests Insecticidal Soap Controls
| Pest group | Soap effect | Source |
|---|---|---|
| Aphids | Susceptible when wetted | Colorado State University |
| Whiteflies | Susceptible | Colorado State University |
| Mealybugs and young scales | Susceptible | Colorado State University |
| Mites, pest and beneficial | Both affected | Colorado State University |
| Caterpillars, sawflies, beetle larvae | Generally immune | Colorado State University |
| Lady beetles and bees as adults | Generally not affected, larvae may be | Colorado State University, NPIC |
Colorado State University says insecticidal soap sprays are effective against small, soft bodied arthropods, such as aphids, young scales, whiteflies, psyllids, mealybugs and mites. For pest by pest help, see our guides to aphids on cannabis leaves, spider mites and whiteflies on cannabis.
Before You Buy Insecticidal Soap
Read the whole label before you buy anything. The label tells you which plants and pests the product covers, how to mix it and how often you may spray. Follow the product label, and check that any pesticide is allowed for cannabis in your state.
Colorado State University stresses that it is always the applicator’s job to read and follow the label, and that pesticide recommendations can change at any time.
What You Need
- A labeled insecticidal soap product
- A clean hand sprayer or pump sprayer
- Water for mixing, as the label directs
- A hand lens to confirm the pest
- A notebook or phone to log the date of each spray
How to Use Insecticidal Soap on Cannabis
- Confirm the pest. Soap only helps against small, soft bodied pests, so check leaf undersides with a hand lens first.
- Read the label and your state’s rules. Make sure the product and your state allow the use you have in mind.
- Mix exactly as the label says. Do not make it stronger, and do not swap in household soap.
- Test a small area first. Colorado State University advises testing on a small area a day or two before spraying a larger area, because certain plants are sensitive and may be injured.
- Wait and check the test leaves. Look for spots, browning or curling before you spray the whole plant.
- Spray to wet the pests. Cover leaf undersides and other hidden spots, since Colorado State University says soap must thoroughly cover the insects to work.
- Repeat only as the label allows. Colorado State University says repeat sprays may be needed at short intervals of four to seven days for pests such as spider mites, because soap has a short residual action.
- Stop if leaves show damage. Colorado State University warns that leaf damage can build up with repeated sprays, so limit the number of applications.
When to Hold Off
- The test leaves show injury. Some plants are sensitive to soap, Colorado State University says.
- The leaves are very hairy. NPIC says plants with hairy leaves may hold the soap on their surfaces longer, resulting in a burn.
- You have already sprayed several times. Leaf damage can accumulate with repeated exposure, according to Colorado State University.
- The pest is hard bodied or hidden. Beetle larvae and caterpillars are generally immune, and pests in curled leaves will not be wetted.
Insecticidal Soap as Part of a Wider Plan
Think of soap as a short acting tool. Because it leaves no residual effect, it works best alongside regular scouting, clean plants and good airflow. If you are weighing a dry powder such as diatomaceous earth instead, it works in a different way and is covered in a separate guide.
Troubleshooting
Leaves show spots or burn after spraying. Colorado State University says certain plants are sensitive and that damage can build up with repeat sprays. NPIC adds that hairy leaves can hold soap longer and burn. Stop spraying, check the label directions and test a smaller area before any later spray.
The pests are back in a week. Soap has no residual effect, Colorado State University says, so pests that hatch or arrive after the spray dries are not affected. That is why repeat sprays at four to seven day intervals may be needed for pests such as spider mites, where the label allows.
The spray did not seem to work. Check what you are spraying. Colorado State University says beetle larvae and caterpillars are generally immune, and that adult lady beetles and bees are generally not affected. Also make sure the spray reached the pests, since insects inside curled leaves are not controlled.
Only some leaves cleared up. Coverage was patchy. Colorado State University says thorough coverage usually means spraying the undersides of leaves and other protected sites, so reach those areas on the next spray.
Sources
- Colorado State University Extension, Insect Control: Insecticidal Soap: https://extension.colostate.edu/resource/insect-control-insecticidal-soap/
- National Pesticide Information Center (Oregon State University), Potassium Salts of Fatty Acids General Fact Sheet (PDF): https://www.npic.orst.edu/factsheets/psfagen.pdf
FAQ
Is insecticidal soap safe to use on cannabis plants?
It can be, if you follow the label and test first. Colorado State University says some plants are sensitive to soap sprays and may be injured, so test a small area a day or two before spraying more. NPIC notes that hairy leaves can hold soap longer and burn. Limit repeat sprays to avoid building up leaf damage.
How often should you spray insecticidal soap on cannabis?
Only as often as the label allows. Colorado State University says soap has a short residual action, so repeat sprays may be needed at intervals of four to seven days for pests such as spider mites. It also warns that leaf damage can build up with repeated exposure, so keep the number of sprays low.
Can you use dish soap instead of insecticidal soap?
Colorado State University advises against it. It says dish soaps contain powerful detergents that can injure plants, damage soil and contaminate waterways, and that homemade soap mixes are not consistently formulated or tested. It also says household soaps used as pesticides are of questionable legality. Use a product labeled as an insecticidal soap instead.
Does insecticidal soap harm natural enemies of cannabis pests?
Some of them. Colorado State University says both beneficial and pest mites are affected by soap sprays. Large adult insects with stronger shells, such as lady beetles and bees, are generally not affected, but their larvae may be susceptible. NPIC agrees that immature, flightless stages are more vulnerable, so spray only where pests are.
Does insecticidal soap keep working after it dries on cannabis?
No. Colorado State University says insecticidal soaps act strictly on contact with no residual effect, so they only affect pests the wet spray touches. NPIC says potassium salts of fatty acids are not persistent and have a soil half-life of less than one day. Pests that arrive later need a new spray.
