What proportion of insects dying in captivity is considered ‘normal’?

Written by Joe Ballenger

Your Name: Caryl

Your Bug Question: I raise early season monarchs inside in New Zealand.(I bring the chrysalides inside because of our poor spring weather with heavy rains and gales).

Yesterday 12 eclosed but 3 fell soon after. Fortunately I was there to assist and lift them carefully on to a stem they could hang from and dry their wings. 3 perfect butterflies.

My question is that 25% to fall seems high, is this normal, would they be able to help themselves if they fell on to the ground in a natural habitat outside? It appears the wings are too wet for them to even try to move and climb.

Thank you, Caryl

A lot of people rear insects for fun, or keep them for pets, and it’s only natural to want to do it well. So if you see something that’s unexpected, like the situation above, it’s only natural to be curious about what’s going on.

Butterfly house

Pictured: Not what you’d see in natural conditions. Image credit: Nathan & Jenny, via Flikr License info: CC BY-NC-ND 2.0

Butterflies, when kept in captivity, are usually kept at relatively high densities. You might find one or two every ten square feet in nature, but in captivity you’re keeping dozens within a couple feet of one another. They might not act the way they act in nature, and things like falls have very different consequences in captivity.

Falls are probably common among butterflies in both nature and captivity, although no studies seem to have been done to look into this. If the wings are intact, I would imagine a fallen butterfly could recover pretty easily. It’s not uncommon to find a butterfly flying around, missing half it’s wing in nature. Butterflies are tougher than people give them credit for, and as long as they avoid predators I think they’d survive.

Insects can fall for a number of reasons, not all of which can be controlled by the keeper. It’s understandable to worry about diseases, but a fall doesn’t always mean something’s wrong. Like people, insects can sometimes be clumsy. Also like people, insects sometimes get sick.

For people who keep insects at home, I wouldn’t spend a whole lot of time worrying about disease. It can be tough to know if your bugs are sick. Diagnosing sick bugs like a doctor is challenging because, in some cases, diseased adults might just be weak and not visibly malformed. The University of Georgia has a pretty good website on one particular Monarch* parasite which causes deformities, Ophryocystis elektroscirrha. Sometimes adults are deformed, but sometimes they’re indistinguishable from healthy specimens. There’s some signs of infection you can look for on the pupae, but sometimes these signs can be hard to spot without a microscope.

Clorox_Bleach_products

There really isn’t a special laboratory bleach anyone uses. We use the stuff from the laundry aisle. Rubbing alcohol can work, too, but it’s not as strong as bleach. Image credit: Adina Firestone, via Wikimedia commons. License info: CC BY-SA 2.0

The best way to control pet insect diseases is sanitation: simply sterilize everything in the cage with a dilute bleach solution after every generation. Plant cuttings are how most diseases get into insect cultures, and sadly, there’s not a whole lot you can do about that. Unless you’re working with honeybees, there’s not really any medication for insects. Sanitation is really all you can do, so having the occasional diseased insect doesn’t mean you’re a bad keeper when you’re keeping insects at home.

To put Caryl’s 25% figure in perspective, some researchers estimate that only about 2- 10% of Monarchs* make it to adulthood in the wild. If you’re getting adults after feeding the insects on fresh plant cuttings from the proper host, and keeping them at reasonable temperatures, I think you’re doing just fine.

If you raise insects in captivity and are interested in knowing how many insects are just unable to grow properly…the number seems to be about 20-30%. The papers by Oberhauser and Altizer below report these numbers for Monarch* butterflies raised on leaf cuttings. Crop pests raised on artificial diets have about the same mortality, although I’m sure different groups have different success rates.

Assuming the butterflies are normal sized, not visibly deformed, and are able to fly when they come out, I think 3 out of 12 butterflies falling and surviving sounds somewhat normal. Unfortunately, I’m not there to inspect Caryl’s insects…but I wouldn’t assume anything was horrendously wrong unless they died before their wings dried.

The Bottom Line:

If we’re talking about rearing insects casually in your home, I think it pays to have a look into how it’s done on an industrial level:

 

If you’re rearing insects professionally (like Joe does), you’re going to have access to equipment that allows you to keep track of other metrics of success. Most scientists pay attention to mortality, but they also make sure the insects are a size that’s comparable to what you’d see in the wild. This requires specialized equipment, and statistics software. Most people wouldn’t have access to this-unless you want to drop $500 on a scale.

You don’t need any real fancy equipment to track the progress of your bugs. As long as you’re keeping the insects on the right hosts, keeping them at roughly room temperature, and getting adults which can fly away…you’re doing something right. You’re doing something you enjoy, and learning about the insect’s biology in the process.

Works Cited

Altizer, S. M., & Oberhauser, K. S. (1999). Effects of the protozoan parasite Ophryocystis elektroscirrha on the fitness of monarch butterflies (Danaus plexippus). Journal of invertebrate pathology, 74(1), 76-88.

Barrionuevo, M. J., Murúa, M. G., Goane, L., Meagher, R., & Navarro, F. (2012). Life table studies of Rachiplusia nu (Guenée) and Chrysodeixis (= Pseudoplusia) includens (Walker)(Lepidoptera: Noctuidae) on artificial diet. Florida Entomologist, 95(4), 944-951.

Oberhauser, K. S., & Solensky, M. J. (2004). Monarch butterfly biology & conservation. Cornell university press.

Oberhauser, K. S. (1997). Fecundity, lifespan and egg mass in butterflies: effects of male‐derived nutrients and female size. Functional Ecology, 11(2), 166-175.

*This information is for the American species Danaus plexippus, since I’m more familiar with this species. I do not know if OE, or an equivalent, is present in New Zealand Monarch species.

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