Will Soapy Water Kill Hornworms? (Read This First)

Is there a non-chemical, cheap way to kill hornworms that reside on your tomato plant?

Some people say that soapy water is the solution, but is it true? Let’s find it out!

Will Soapy Water Kill Hornworms?

Applying soapy water is one of the best ways to kill hornworms on tomato plants. To get rid of hornworms, use 1 to 2 tablespoons of chemical-free liquid dish detergent in 1 gallon of water.

Most people find this method helpful, but you need to know that the success rate is not 100%, and there are situations when you just can’t kill hornworms with soapy water.

Your success will depend on the size of hornworms- the bigger they are, the harder it is to kill them. They often reach easily reach four-to-five inches (10-13 cm) in length.

They can’t bite people, but they are harmful to your tomato plants.

Hornworms consume a tremendous amount of tender leaves and are specialists in quickly decimating tomato plants.

Older hornworms will ultimately attack and injure the vulnerable fruits, so you need to act right now.

How Does Soapy Water Kill Hornworms?

There are 2 theories why soapy water kills hornworms.

One theory says that it dehydrates hornworms by dissolving their waterproof cuticle, or covering. Another one is that soap disrupts insect cell membranes.

Also, others say that soapy water just drowns hornworms with its force.

However, what you should know is that soapy water on hornworms causes them to wiggle and convulse around, ultimately killing them.

Does Soapy Water Kill Other Worms as Well?

Soapy water kills not only hornworms, but also other insects, including:

  • Aphids
  • Thrips
  • Whiteflies 
  • The nymphs of buffalo leafhoppers
  • The nymphs of lygus bugs

These insects suffer from soap and water, because of their small, soft bodies.

There are also bigger insects that you can’t kill with this mixture, like fruitworms, russet mites, stink bugs, lacewings, lady bugs, and predatory wasps.

Like I said at the beginning, it’s not guaranteed that you will kill hornworms with soapy water because some of them might be too big, but if you act fast, you can successfully get rid of them.

How to Apply the Soapy Water to Kill Hornworms?

To apply the soapy water to hornworms, mix 1 or 2 tablespoons of chemical-free liquid dish detergent with 1 gallon of water. Apply it with a compression hand-pump sprayer.

If you’re doing the mixture for the first time, I’d suggest you use 1 tablespoon of chemical-free liquid dish detergent, like liquid dish or Castile soap (you can buy it here on Amazon). If it’s not enough and it doesn’t kill hornworms, use 2 tablespoons during the next application.

The best way to apply the mixture is with a hand-pump sprayer, or a hose-end, because it ensures maximum coverage.

Spritz the plant leaves well until it drips. It kills hornwoms on contact. It should take a couple of hours once you see dead insects.

After the plant dries, rinse it with water to prevent sun damage from soap.

To kill hornworms, you should apply soapy water every 3 days.

After killing them, you might need to handpick dead caterpillars from plant leaves and throw them away.

What Time of the Day Should You Apply Soapy Water?

You should apply soapy water to tomato leaves in the morning, or late afternoon. This prevents the mixture from evaporating too quickly.

Applying soapy water when the temperature is relatively cool lets it dry slowly, which increases the chances of killing hornworms.

Ideally, do it early in the morning, but remember to rinse the soap with water after a few hours. So if you’re not at home in the morning, or noon, you can also do it in the late afternoon, and rinse the mixture at night.

Of course, don’t apply soapy water on a rainy day, because rain will disturb the whole process, and rinse soap quickly.

Can You Harm Your Tomato Plant by Applying Soapy Water?

Usually, you shouldn’t harm your tomato plant by applying soapy water. However, if you use too much soap or the wrong detergent, you can cause damage to the leaves.

Using too much detergent might affect your plant. Always use a max of 2 tablespoons of soap.

Also, using too strong detergent, like laundry or dish detergent, might cause leave spotting or browning.

Some tomato varieties are more sensitive than others, so it might also be the reason that can harm your plant.

The best way to test things out is to spray the mixture on 1-2 leaves and wait 3 days. If the leaves are intact, you can apply the mixture to the whole plant. However, if you see some changes in the appearance of leaves, use a different method.

I would recommend you buy this fertilizer on Amazon. It will kill hornworms for 12 months, and if used correctly will not harm your tomato plant.

For How Long Does This Solution Work?

This solution isn’t everlasting, and it kills hornworms directly but does not prevent them from appearing.

The best way to get rid of hornworms is to kill them once they appear.

You should inspect your tomato plant twice a week. If you see hornworms appear, use a mixture of soapy water.


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