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The Tomato Hornworm: Identification, Life Cycle, and Biological Defense

Discover organic methods to effectively combat and eliminate the Tomato Hornworm while maintaining the health of your plants.

by BioGrow

The tomato hornworm, also known as the yellow hornworm or tomato caterpillar, is an insect species that can cause significant damage to tomato crops if not effectively controlled. Tomatoes, as we have seen, are delicate plants susceptible to various pests and issues. We have previously discussed pests like the tomato leafminer and the green stink bug, as well as other problems like blossom end rot, downy mildew, and leaf bronzing.
The tomato hornworm causes significant damage, especially during warmer periods, particularly in the months of June and July. In today’s article, we will explore how to recognize this pest and understand its behavior throughout its lifecycle. Naturally, we will also showcase how to defend our tomato cultivation using techniques and organic farming products.

Our consistent goal is to avoid the use of chemical pesticides. Hence, a well-planned defense strategy based on established practices in organic farming can effectively help manage this troublesome pest.

Identification of the Tomato Hornworm

Tomato hornworm adult

Tomato hornworm adult

The tomato hornworm, scientifically known as Heliothis armigera, is an insect belonging to the Lepidoptera order, similar to the tomato leafminer, and falls under the Noctuidae family.
This lepidopteran typically takes on the form of a moth, with a wingspan of about 35 mm. Moreover, the wings of the female adult are brown-ochre with subtle greenish hues, while those of the male are darker. The adults’ appearance is further marked by transverse brownish stripes.
The larvae, responsible for most of the damage, exhibit a wide range of colors. Depending on their developmental stage and diet, they can shift from green-yellow to a darker, brown-black hue.
By the end of their cycle, the larvae can grow up to a considerable size of 3-4 cm.

Life Cycle

Tomato hornworm larva

Tomato hornworm larva

The life cycle of the tomato hornworm starts in the winter, with the last generation of larvae burrowing into the soil to pupate and overwinter.
The adult moths take their first flight in early spring, usually in May. These nocturnal insects are known for their long-distance flights and are considered migratory species.
The mating phase begins before summer, during which the females deposit their eggs on plant vegetation. This species is highly prolific, with each female capable of laying up to 2,000 eggs in her lifetime.
The eggs hatch within 3 to 15 days, depending on ambient temperature. From these eggs, the larvae emerge.
In total, the tomato hornworm can complete 2 to 4 generations. Among these, the second generation is the most damaging, typically occurring during the peak of summer months when plants are producing the most fruit.

Damages Caused

Damage of tomato hornworm on ripe fruit

Damage of tomato hornworm on ripe fruit

As mentioned, the tomato hornworm’s larval stages are responsible for damaging plants and fruits. Initial damage occurs on the vegetation, as the first-generation larvae feed. This often results in irregular erosions on the leaves.
Subsequently, second-generation larvae attack the fruits, both unripe and ripe tomatoes. They usually enter near the stem and create tunnels within the fruit, consuming the pulp. The larvae can even move from one tomato to another, although typically only one larva is found per fruit (due to intraspecific cannibalism). The damage, aside from the tunnel itself, is caused by the release of greenish excrement, which can be quite bothersome.
If the infested fruit is immature, it may fall off the plant. Conversely, if the larvae feed on mature fruits, they are likely to rot.

Biological Defense: Monitoring and Mass Trapping of Adults

To safeguard our plants from the tomato hornworm, a biological defense strategy similar to the one used against the tomato leafminer is essential.
Firstly, monitoring and mass trapping can be implemented in two ways. Firstly, by placing chromotropic traps in yellow, as previously discussed. These traps are highly effective and can be found here.
Secondly, using traps with pheromones, which attract female moths through a mechanism of sexual confusion.
These specific pheromones can be obtained from specialized stores, where you should look for the one developed for this species. When used separately, these two methods allow for monitoring the pest’s presence without affecting plants. When used together, they enable mass trapping. For increased effectiveness, you can also incorporate electroluminescent traps, commonly used for mosquitoes in gardens. This type of trap, when activated at night, will attract the tomato hornworm moths. An affordable product can be found here.
It’s important to note that this type of trap, drawing moths with light, doesn’t distinguish between beneficial (or harmless) insects and harmful lepidopterans (like the tomato hornworm).

Eliminating Tomato Hornworm Larvae

The aforementioned biological defense strategy aims to eliminate adult lepidopterans, effectively halting their reproduction and subsequent generations.
However, it’s important to remember that it’s the larvae causing damage to both plants and fruits. To directly eliminate tomato hornworm larvae, Bacillus thuringiensis var. kurstaki comes to our aid – its biological action mechanism has been extensively explained in previous articles.
In this context, it’s worth highlighting that the kurstaki variety of Bacillus thuringiensis is highly effective against young larvae. Therefore, identifying and targeting them early is essential.
The tomato hornworm larvae are most active at night, making evening applications of Bacillus thuringiensis crucial. This way, plants and fruits will be wet, allowing the larvae to consume the bacterium and perish shortly thereafter. One specific challenge of using thuringiensis is the behavior of the larvae. Once they have penetrated the tomato, they are harder to reach with the bacterium.
The optimal intervention moment is when the larvae have just hatched and are seeking a fruit to bore into. Additionally, predicting when a larva is transitioning from one tomato to another enhances the effectiveness of the treatment.
An important note: Bacillus thuringiensis is highly selective, so it’s essential to purchase the correct variety. The appropriate one for the tomato hornworm is kurstaki (be cautious not to confuse it with other varieties). A reliable product can be found online, such as Bacillus thuringiensis var. kurstaki.
In conclusion, to eliminate tomato hornworm larvae efficiently, careful monitoring is crucial. Recognizing the right time for intervention is of paramount importance. As Bacillus thuringiensis is a biological solution, it doesn’t have a systemic impact on plants (unlike hazardous chemical insecticides), but acts solely through ingestion.

Further Reading

  1. University of Texas at Arlington – Entomology Today: “How to Tell the Difference Between Tomato and Tobacco Hornworms” – The article shows the similarities and differences between tomato and tobacco hornworm caterpillars and moths.
  2. Oxford Academic – Journal of Insect Science: “Tomato Hornworm’s Role in Pollination” – This research highlights the Tomato Hornworm’s contribution to pollination, discussing its interactions with various flowering plants.
  3. University of Arkansas System Division of Agriculture – Cooperative Extension Service: “Beneficial Insects” – The articles debates about the beneficial insect. Inside it explains how parasitic flies lay their eggs on tomato hornworms that would decimate our prized tomato plants.

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