The tomato hornworm, Manduca quinquemaculata, is a formidable pest that rapidly strips leaves and damages fruit on nightshade plants. This large caterpillar has a voracious appetite, capable of causing significant destruction quickly. Many gardeners use Diatomaceous Earth (DE), a fine, mineral-based powder, as a natural control method. This article explores the science behind DE and provides guidance on its effective application against the tomato hornworm.
How Diatomaceous Earth Kills Insects
Diatomaceous Earth is made from the fossilized remains of diatoms, ancient, single-celled algae that deposited silica-rich skeletons. When processed, these remnants form a light powder composed primarily of amorphous silica. Under a microscope, DE particles have sharp, jagged edges, acting as a physical, non-chemical insecticide.
The mechanism relies entirely on direct contact with the insect’s exterior. As the hornworm crawls over the powder, the microscopic, razor-like edges abrade the insect’s protective waxy cuticle layer. This damage compromises the exoskeleton’s ability to retain moisture, creating small openings in the protective barrier.
Compounding the abrasive action, DE particles also absorb protective lipids and oils from the insect’s body, accelerating water loss. The combined effects of physical damage and absorption cause the hornworm to rapidly lose internal moisture through desiccation. This leads to fatal dehydration, often within a day or two of exposure, provided the product remains dry.
Applying Diatomaceous Earth for Hornworm Control
Diatomaceous Earth is effective against hornworms, but application must be thorough because the pest must physically contact the dust. Hornworms typically feed on the upper parts of the plant and are often found on the undersides of leaves.
Therefore, a simple dusting of the soil surface is insufficient for control. For maximum effectiveness, the entire plant foliage needs a uniform, thin coating of the powder.
Gardeners should use a dust applicator or a bulb duster to puff the powder directly onto the leaves and stems. This method ensures the dust is distributed into a fine layer, which is more likely to cling to the plant and the hornworm’s body. It is necessary to lightly coat both the top and bottom surfaces of the leaves where the pests often rest and feed.
Application should take place when the air is calm, typically in the early morning or late evening, to ensure the powder settles precisely where needed. Since the desiccant action is halted when DE becomes wet, the powder must be reapplied after any rain or heavy morning dew. Maintaining a dry, continuous barrier of DE on the foliage is necessary for sustained control.
Choosing the Right Diatomaceous Earth
Gardeners must select the correct type of product, as Diatomaceous Earth is sold in different grades for various industrial uses. The only acceptable product for pest control and garden use is food-grade DE. This grade is composed of amorphous silica and contains less than one percent crystalline silica, making it safe for use around plants, people, and pets.
Filter-grade or pool-grade DE is the alternative, which is heat-treated to improve filtration properties. This process converts the silica into a high concentration of crystalline silica. This crystalline form is a respiratory hazard and should never be used in the garden or for pest control. Always confirm the label explicitly states “food grade” when purchasing the product.
Even though food-grade DE is considered non-toxic, the fine powder can still irritate the respiratory tract and eyes upon application. Therefore, wear a simple dust mask or respirator and eye protection while dusting plants to avoid inhaling the fine particles. Following these safety practices ensures the control method remains safe.