Actias selene caterpillar

Insect behavior: Are moth larvae predators?

Insect behavior: Are moth larvae predators?

Actias selene caterpillar in

Hello everybody! Today we start a series of articles on insect behavior and predation, focusing on the extraordinary behavioral and morphological defense mechanisms of Saturniidae and Sphingidae insect families. In this series of articles we will provide a brief overview on the trophic relationships among insects and explain the link between the environment and the morphology and behavior of insects.

Below the first chapter. What does predation means? What types of predation can we find in nature? How does the environment influences insect behavior?

Insect behavior: Introduction to predation

Why does predation and the interspecific relationship are so important?

Biotic relationships are key to ecosystem functioning because they affect the distribution and the abundance of organisms. Studying how animals, plants and other organisms interact with each other is necessary to understand how they behave as an eccosystem and therefore fosters the conservation of the species which are still living in our beautiful planet.

To start off, here some basics: all the Lepidoptera are herbivores, thus the larvae consume plant tissue. Nevertheless, they generally don’t kill the entire host plant. Are butterflies and moths predators or not? What does “predator” mean?

Predation is the consumption of a living organism (prey) by another organism (predator). Predation has consequences in the community structure of any ecosistems by leading to the natural selection, influencing the evolution of both prey and predator species.

How many kinds of predators are there?

Actually, there are 2 kinds of classifications:

  • Taxonomic type: the predators are classifed into carnivores (eat animal tissue), herbivores (eat plant tissue) and omnivores (eat both)
  • Funciontal type: true predators, herbivores, parasitoids, and parasites.

Butterfly, moths and other arthropods are herbivores. In this scenario they hardly lead to the death of their host plant, at least in the short term.

To sum up, moth larvae are predators. Ulike spiders, wasps, mantis and other arthropods that kill and eat other animals, Lepidoptera larvae can’t be considered true predators.

Antheraea pernyi larva
Antheraea pernyi larva consuming food

Insect behavior: Food preferences in butterflies and moths

Foremost, animals (in general) can be monophages, oligophages or polyphages.

Actually, the most of Lepidoptera larvae are monophages or oligophages animals since they eat exclusively few species of plants. Specialization is successful if:

  1. Prey (food plants) are abundant

There are plenty of host plants on which moth larvae can feed. It is hardly an exaggeration to say that plant-based food for caterpillars is abundant. This is the reason why the limiting factor for the herbivores is the quality of the food rather than the quantity of ingested food. So, it’s essential for the herbivores to find and eat high quality food, rich in nitrogen, tender and young (soft). Low-quality food is woody, fibrous and indigestible. Sometimes, Lepidoptera may choose a sub-optimal host as their evolutionary bet. This can be caused by a trade-off between host plant quality and other survival factors as interspecific competition or either selective predation.

2. Prey (food plants) are accessible and predictable

Lepidoptera and their host plants’ life cycles are coupled together in a way to ensure at the same time high-quality food for the larvae and protection from predators. For example; species with a winter diapause (Ex. Antheraea sp., some Samia sp., most of the European hawk moths) emerge during the spring and consequently lay their eggs. The young larvae are equipped with soft and not fully hardened mouthparts, they will feed on tender leaves in the spring. In addition, the caterpillars have all the summer to grow and thrive. Another important concept is that during the springtime, predators are still inactive (reptiles) or didn’t come yet (such as migratory birds). In the meantime, larvae are safe and free to develop successfully.

Contrarily to strictly specialized species (like Bombyx mori) that feed only on 2 species of host plants, there is plenty of polyphagous species. A nice exaple cames from the aboundant family Noctuidae. Insect behavior reflect on many human activities, as agricolture. Most of the polyphagous species are considered as potential pests.

Butterflies and moths that are commercially available

Species we usually breed eat a restricted number of plant species. Samia cynthia (available in 2022) will accept only Ailanthus, Magnolia and sometimes Prunus sp.. Acherontia is less specialized, being able to eat a wide variety of species in the family Oleaceae and Solanaceae.

Samia ricini is polyphagus a species, being able to eat a wide variety of plant species including Ligustrum, Prunus, Salix, Syringa and many more. However, the best results are obtained with Ricinus.

Samia ricini larva
Samia ricini larvae eating Ligustrum sp. during winter, a sub-optimal yet valuable host for the colder months

When a concrete butterfly or moth species does accept more than one host plant, is generally suggested to offer them more than one host plant species in the first day of their larval life. Doing this way you will ensure the caterpillar choose their favorite one. Pupal size and adult vitality can be influenced by the host plant choice. In the case of Samia ricini, the difference in pupal weight can be up to two fold when comparing Ricinus communis (the best) versus Ligustrum sp. (a sub-optimal host plant)

In conclusion, there is plenty of different strategies in the insect behavior world. Adaptation to different microhabitats influences most of the species of butterflies and moths to change their food prefences, over time, based on their natural distribution. This phenomenon is common in species with a wide geographic distribution as Antheraea polyphemus: in the northern part of their range, Antherea polyphemus is well adapted to eat the soft and tender birch leaves (Betula). Whereas, in the southern range, the poly moth eat the tick and woody leaves of the evergreen oak species.

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