Does Diatomaceous Earth Kill Hornworms?

Will Diatomaceous Earth kill hornworms? The fact is, Diatomaceous Earth (DE) can be effective as one of many organic pest control tools used against hornworms. But, it is not effective in-and-of-itself. 

The best approach to dealing with hornworms is to adopt a holistic, Integrated Pest Management (IPM) program. 

Tomato Hornworm feeding on leaves
Tomato Hornworm munching on leaves | scottiebumich-DepositPhotos

In this article, we discuss control of hornworms with Diatomaceous Earth and an array of other methods and products.

What Is Diatomaceous Earth? 

Diatomaceous Earth is an all natural substance made of microscopic, fossilized aquatic organisms. When you examine the substance under a microscope, you’ll find that it looks like bits of broken glass. 

In addition to desiccating pests, these small, sharp shards damage the soft undersides of pests like caterpillars. More on How To Get Rid of Caterpillars in the Garden.

It’s important to understand that there are two types of Diatomaceous Earth. 

There is a coarse substance used to filter pools, and there is a food grade substance that has a wide variety of household and garden uses. 

The second – the food grade Diatomaceous Earth – is the type you want for pest control. The pool filter type would be completely ineffective and can cause harm if ingested by pets. 

NOTE: Food Grade Diatomaceous Earth is nontoxic and works best in dry conditions.

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What Are Tomato Hornworms?

There are several different types of hornworms (Manduca quinquemaculata) but they all have one thing in common. They are easily identified by the prominent horn which sticks up from the posterior of the caterpillar.

The largest hornworms can be four inches long and as big around as a fairly good-sized thumb. 

Tomato hornworm caterpillars are this size, and they are quite striking in appearance with their bright green, plump bodies and seven jaunty white/yellow diagonal stripes. 

When they are small, you won’t see them, but you may notice their green or black droppings (frass) on plant leaves and littering the ground beneath your tomatoes. 

When they attain full size, they are hard to miss and so are their droppings. 

To find them more easily, try spraying your plants with water. The caterpillars will thrash about, making them very easy to see. 

Integrated Pest Management Iowa State University

What Damage Do Hornworms Cause? 

Even though they are large and flashy and easy to see these garden pests if you’re looking, if you are not vigilant, you may see hornworm damage long before you see a hornworm.

These pests begin causing damage to tomato leaves and fruit during the midsummer months. As summer wears on, their numbers increase. Without intervention, the damage will increase accordingly. 

Heavy hornworm infestation can decimate a tomato plant, leaves, stems, fruit and all, overnight. 

Depending on the type of hornworm and the plants available, these pests may also destroy grape, eggplant, pepper, potato and tobacco plants. [source]

Use Diatomaceous Earth To Deter Hornworms

Apply diatomaceous earth to your plants and the ground by surrounding them a light dusting. Be sure to wear a dust mask! When hornworms walk over the dust, it will affect them just as it affects all insects. 

The diatomaceous earth will become lodged in the joints of the legs and the creatures’ undersides where it will begin desiccating them by drying up their natural juices. 

If hornworms start eating the leaves dusted with diatomaceous earth, they will dry up from the inside out. This is not a fast process. It takes a few days for diatomaceous earth to take effect. But it is effective to help get rid of tomato hornworms.

Conclusion

Diatomaceous Earth is useful in the organic pest control of tomato hornworms and other garden pests.

Even so, it is a valuable tool along with beneficial insects to help naturally control pests in an IPM program. 

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