Spraying Insecticidal Soap on Aphids With a 2-Gallon Pump Sprayer: Leaf Undersides, Early Morning, Below 90°F, and a 24-Hour Test Patch

Pump sprayer wand aimed at aphids on the underside of garden plant leaves

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Insecticidal soap only kills the aphids it actually wets. Most aphids feed on the undersides of leaves, so how you aim the wand matters more than how much you spray.

How do you spray insecticidal soap on aphids?

Use a soap labeled as an insecticide, mixed at the label rate. Spray in early morning or late in the day below 90°F, wetting the undersides of leaves where aphids cluster. Test a small area 24 hours first, and repeat every 4 to 7 days only as the label allows.

Clemson Extension's insecticidal soap fact sheet says soaps are usually used as a 1 to 2 percent solution, 2½ to 5 tablespoons per gallon, following the label. It says not to spray in full sun or above 90°F.

Colorado State University Extension's insecticidal soap page says soaps have no residual effect and must contact the insect directly. It says aphids inside curled leaves will not be controlled.

Insecticidal Soap Numbers

Item Figure Source
Usual strength 1 to 2% (2½ to 5 Tbsp per gal) Clemson
Maximum temperature 90°F Clemson
Repeat interval 4 to 7 days, per label Clemson, CSU
Test patch wait 24 hours (Clemson); a day or two (CSU) Clemson, CSU
Hard water test 15 minutes in a jar Clemson
Residual effect None once dry CSU, Clemson

Why Timing and Aim Matter

Clemson says the spray only works while wet, so slow-drying conditions in early morning or late day give better control. Spray both leaf surfaces, especially the undersides.

CSU says soaps work on small, soft-bodied pests such as aphids, young scales, whiteflies, and mites. Larger insects such as caterpillars and beetle larvae are generally not affected.

Sensitive Plants

Clemson lists sensitive plants including sweet pea, cherries, plum, Japanese maple, bleeding heart, nasturtiums, and gardenias. It says plants under water stress should not be sprayed.

If plants wilt or show browning leaf edges within a few hours, Clemson says to rinse them with clean water.

Materials and Tools

  • 2-gallon pump sprayer
  • A soap product labeled as an insecticide for your plant
  • A clear glass jar for the hard-water test
  • Gloves and safety glasses, or the PPE the label lists
  • A hand lens for checking leaf undersides

Spraying Steps

  1. Confirm the pest. Look for aphid clusters on leaf undersides and new shoots.
  2. Read the label. Check the rate, the plant list, and any warnings.
  3. Test the water. Mix a small batch in a jar and let it stand 15 minutes.
  4. Test a patch. Spray a few leaves and wait 24 hours.
  5. Pick the hour. Spray in early morning or late in the day.
  6. Mix in a clean sprayer. Add the label amount to water.
  7. Spray from below. Angle the wand up into the undersides.
  8. Wet, do not drench. Cover leaves without heavy runoff.
  9. Recheck in a few days. Repeat only as the label allows.

Mistakes and Hazards

  • Using dish detergent or laundry soap instead of a labeled product
  • Spraying in afternoon sun or heat
  • Coating only the tops of leaves
  • Spraying drought-stressed plants
  • Repeating too often, which builds up leaf damage

Aphids are common on vining crops, like those in building two 50-foot cucumber trellises. A frame like building a wire-topped bird net frame lifts netting so you can reach leaves to spray.

For choosing wire for plant frames, see the hardware cloth mesh size and gauge guide.

FAQ

Can I just use dish soap?

CSU says homemade soap sprays are not consistently formulated or tested, and dish soaps contain detergents that can injure plants. It says household soaps used as pesticides are of questionable legality.

Why did my soap spray turn scummy?

Clemson says hard water makes the soap separate out and stop working. If the jar test shows scum, it suggests distilled or bottled water.

Will soap hurt lady beetles?

CSU says adult lady beetles and bees are generally not affected, but larval stages may be susceptible.

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