Are there insects which are toxic as adults, but not as larvae?

Written by Joe Ballenger

I wanted to know if it is possible for the adult butterfly to be poisonous while the caterpillar of the same species is not poisonous?

I think this is a really insightful question, and I’ve got a soft spot for questions asked by kids.

I guess we should start by talking about why this question is really insightful.

Lepidopterans-butterflies and moths-have a huge chemical arsenal at their disposal which ranges from compounds which smell really bad, to venom that’s strong enough to kill humans. There’s a variety of ways they can introduce these compounds, but for the most part, they’re either stored in the body as larvae or they’re based on a developmental program that’s also part of the larva.

Blood feeding moths are going to be…kind of important here.

So it’s kind of an unusual situation where an adult would be toxic while a larva wouldn’t…but it does exist.

Kind of.

It’s complicated.

Chemistry

A lot of defensive compounds do get eaten as caterpillars, and end up in adults. Plants can’t poop, so it’s common for them to turn their waste into poisons instead. The caterpillars eat these poisons when they eat the plants, and instead of breaking them down they store these in their body in order to make them poisonous. A lot of butterflies and moths taste bad because the things they ate as caterpillars will survive the metamorphosis process.

The one notable exception to this I’ve seen are a group of chemicals called pyrrolizidine alkaloids. Don’t worry too much about pronouncing it, they’re just a specific group of chemical compounds which are different than the ones which monarchs and swallowtails use. It’s important to note that there are many species of caterpillars which pick these up as larvae and then sequester them into the adult stage.

These are pyrrolizidine alkaloids. The name isn’t super important, but they’re not sugars attached to cyanide…which is what monarchs use (AKA cyanogenic glycocides).

In some cases, there are butterflies/moths which feed on plants which don’t make these chemicals as larvae. Instead, the adults pick them up by feeding on the nectar or other secretions of plants which do make them.

Even though they’re picking up defensive compounds, it’s not really clear whether these are used for defense. Instead, these are usually boy insects insects usually use these as building blocks to turn these chemicals into perfumes which make them smell good to girls.

It really depends on what concentrations these chemicals end up in, and while they do likely have some defensive value, it’s probably more of a date-finding thing than it is a defensive thing.

Venom

First and foremost, this one might be stretching the definition of the word ‘venom’.

A lot of biologists define a ‘poison’ as something that’s eaten, while ‘venom’ is typically injected or otherwise delivered through trauma. There is some value in this, because it’s important to know whether a substance needs to be eaten in order to be active. There’s practical and evolutionary implications to this, which means the distinction is biologically important.

However, in the popular sphere, it’s often used to devalue the questions that people ask. Typically, when people ask if something’s ‘poisonous’, they just want to know if the critter is able to harm them. Correcting the language of the question purposely obscures the asker’s concerns for no reason other than to force a genuinely curious person into accepting the language of the field at the expense of the asker’s curiosity.

Beyond cultural reasons, there’s also some biological reasons to not like the distinction. Some animals, like cobras and assassin bugs, can spit the salivary secretions they use to subdue prey into sensitive tissues like eyes or nasal cavities. Despite the fact the substance was not introduced through trauma, these compounds can still cause a lot of harm to a potential predator. Sometimes, like in the case of spitting spiders, the same compounds can be used to subdue prey.

It’s important to note that both caterpillars and moths can be venomous. These are spines from the adults of a moth called Hylesia. Sometimes, caterpillars make venomous hairs which adults pick up from the coocoon. In this case, both caterpillars and adults make similar hairs which inject venom.

There’s also a lot of problems with what organisms we apply the terms to. Stinging nettles, for example, will inject skin irritants into anything which tries to eat them but are rarely considered to be venomous. Many insects will inject compounds into plants to kill them, in very much the same way snakes do to kill mice (for example), but these secretions are not considered to be venom despite the fact they’re using them to subdue a prey item. A lot of plant scientists use the term ‘predation’ to describe things eating plants, but a lot of people in other fields don’t accept the term.

So this is complicated and my take on this is a little controversial.

So, with all of that being said, I’d like to point out that the moth Calyptra thalictri fulfills the requirements that this person is asking.

Told you this little dude would be important. 🙂

Calyptra thalictri is a moth which feeds on blood. They’re closely related to moths which eat fruit juices, and their biology is a modified version of the equipment needed to suck juices from fruit. In Lepidopterans, it’s typically the males who feed on blood because they need salts to maintain their ability to mate. They have a wide host range, and will even feed on humans if they’re around.

Blood-feeding insects need to have a lot of adaptations to keep themselves from getting detected, and swatted, by the animals they feed on. These can range from stuff which shuts down nerves to stuff which keeps blood from clotting. The main difference between these proteins is that they act on a local level to prevent detection and keep blood flowing to the specific site, while snake venom tends to act more globally to kill the host. That being said, it is a substance which is injected to subdue prey.

The caterpillars of these moths feed on plants, which is also a risky affair for caterpillars. Plants, as mentioned earlier, have their own defenses, which are frequently turned on by the feeding of insects. The saliva of caterpillars usually have some kind of equipment which serves to shut down the capacity of plants to do this. Because of this, caterpillar saliva is also a compound introduced into the organism through trauma which shuts down the capacity of the prey to defend itself.

In short, the saliva of both the caterpillar and adults of Calyptra species both fulfill the requirements to be considered venom as per Nelsen et al (see Works Cited and note below). The interesting thing about this, however, is that each of these venoms is meant to work on a completely different suite of organisms. The caterpillar venom is meant to work on plants, and beyond subduing it’s prey, it has no function on potential vertebrate hosts. In the same vein (pun absolutely intended), the venom of the adult has no function on potential plant hosts. It’s a situation where the saliva of the caterpillar is meant to harm plants, and where the saliva of the adults harms mammalian hosts…including humans.

This is a naturally occurring transparent caterpillar! The white stuff are the trachea. The darker stuff behind them are the stomach contents. Anything which eats this caterpillar will also eat it’s last meal, which might consist of a poisonous plant. Photo: Jim Cordoba, Enio Cano

Thus, this is a situation where it’s only the venom of the adult which can have an effect on animals…which fits the answer to this question.

Unfortunately, blood-feeding Lepidopterans aren’t very well studied. They don’t spread disease (as far as anyone knows), so they’re more of a biological curiosity than something that attracts a lot of medical and economic attention. So we don’t know what those proteins are, what they do, or even if there are shared suites of salivary proteins between the other two. We can only make inferences from other parasitic insects which do the same thing. There are A LOT of interesting biological questions we should be asking about the differences between what the adult and larval saliva does in blood-feeding Lepidopterans.

The Bottom Line

At the end of the day, I think this is an open question for a few reasons.

First, it’s not really known whether the compounds that adult butterflies/moths ingest will provide them protective value. Caterpillars are basically all stomach, which means that compounds in food will provide some protective value through a phenomenon called gut-loading. Basically, the gut is loaded with food that’s potentially toxic, and while this stuff isn’t put into the caterpillar’s body, it’s still there and has some protective value. Even though butterflies/moths eat toxic stuff which is converted into a kind of perfume/cologne, it’s not actually known whether this is enough to provide them protection.

Second, it also depends on how we define the concept of a defensive compound. A lot of what we call ‘venom’ is meant to either subdue predators or potential food items. When it comes to larger prey, in the context of a host/parasite relationship, it’s a valid question as to whether this would even count as a venom.

Either way, here is how I would personally define a defensive compound:

A chemical substance, passed from one organism to another, the effects of which cause the target to become less able to defend against or resist the influence of another species.

It might seem like I’m mincing words here, and I assure you, that’s exactly what I’m doing. Understanding biology really relies on definitions, and when we start talking about defensive compounds, we can really run around for a long time talking about what does and doesn’t count as a venom or poison.

At the end of the day, though, caterpillars and butterflies have completely separate needs. A lot of these needs are met by storing and/or producing different suites of chemicals which are meant to serve different purposes.

As far as this question is concerned, I actually feel that it’s the blood-feeding moths which provide the most concrete answer to this question. Caterpillars need spit which keeps plants from producing poisons which hurt them. Moths need spit which keeps their hosts from swatting them or cutting off the blood flow to the area where they’re feeding.

Because the saliva of these organisms cater to two completely different kinds of organisms, I would argue that the defensive compounds of blood-feeding moths only defend against mammals…and therefore, it’s only the adults which are poisonous.

…or maybe the caterpillars and adults simply produce different kinds of poisons.

Sound off in the comments. 🙂

Works Cited

Greeney, H. F., Dyer, L. A., & Smilanich, A. M. (2012). Feeding by lepidopteran larvae is dangerous: a review of caterpillars’ chemical, physiological, morphological, and behavioral defenses against natural enemies. Invertebrate Survival Journal, 9(1), 7-34.

Hossler, E. W. (2010). Caterpillars and moths: Part II. Dermatologic manifestations of encounters with Lepidoptera. Journal of the American Academy of Dermatology, 62(1), 13-28.

Nelsen, D. R., Nisani, Z., Cooper, A. M., Fox, G. A., Gren, E. C., Corbit, A. G., & Hayes, W. K. (2014). Poisons, toxungens, and venoms: redefining and classifying toxic biological secretions and the organisms that employ them. Biological Reviews, 89(2), 450-465.

Nishida, R. (2002). Sequestration of defensive substances from plants by Lepidoptera. Annual review of entomology, 47(1), 57-92.

Rivera-Vega, L. J., Acevedo, F. E., & Felton, G. W. (2017). Genomics of Lepidoptera saliva reveals function in herbivory. Current opinion in insect science, 19, 61-69.

Trigo, J. R. (2000). The chemistry of antipredator defense by secondary compounds in neotropical Lepidoptera: facts, perspectives and caveats. Journal of the Brazilian Chemical Society, 11(6), 551-561.

Villas-Boas, I. M., Bonfá, G., & Tambourgi, D. V. (2018). Venomous caterpillars: From inoculation apparatus to venom composition and envenomation. Toxicon, 153, 39-52.

Zaspel, J. M., Scott, C. H., Hill, S. R., Ignell, R., Kononenko, V. S., & Weller, S. J. (2014). Geographic distribution, phylogeny, and genetic diversity of the fruit-and blood-feeding moth Calyptra thalictri Borkhausen (Insecta: Lepidoptera: Erebidae). The Journal of parasitology, 100(5), 583-591.

Note from Joe:

One question I anticipate as a follow-up from this post is: Do predatory caterpillars use venom?

I would argue the answer to this is NO.

There are quite a few predatory caterpillars, which need to eat many insects to reach their adult forms. For the most part, these are either ambush predators or grazers. The ambush predators eat smaller insects which they need to (somewhat) actively hunt down. The grazers eat scale insects which don’t really need to be subdued because they’re immobile. In both cases, they physically overpower their prey without any need for venoms or poisons to do so.

There are, however, a few species of caterpillar which are either parasitic or parasitoids. The parasitic caterpillars will feed on the spines of venomous caterpillars and weaken them without outright killing them. The parasitoid caterpillars (Epipyropidae, if you want to know more) will feed on leafhoppers, eventually consuming all of their insides.

The parasites might not have saliva which influences host biology at all, because they’re not really interacting with the host in a super-intimate way. I view this as similar to the grazers which eat scale insects, even though they’re only living off one prey item.

The parasitoid Epipyropids, on the other hand, are absolutely venomous because they need to keep their hosts in line. This is a group of insects which has recieved virtually zero study, so the potential activity of their saliva is inferred from their needs and how similar critters interact with their hosts.

This is another really complicated question, but it’s still related to this post. If you’d like to know more, please shoot us an email and we’ll be more than happy to write a detailed post. 🙂

Clausen, C. P. (1940). Entomophagous insects. McGraw-Hill book Company, Incorporated.

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1 Response to Are there insects which are toxic as adults, but not as larvae?

  1. jonrichfield says:

    Your standards of discussion on this site really are very impressive. Thank you.
    I find a lot of items new to me (not that that is surprising in any biological field,
    decidedly including entomology 😃👍

    Concerning this article, one item that I think deserves a remark in any discussion
    of insect venoms occurring only in particular stages would be genus Lonomia;
    I have not been able to find any reference to venom in the imago.
    It is not the only example among species with spiny larvae, but is surely
    the most dramatic.

    Like

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