"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

August 7, 2018

Copidosoma floridanum

It's time for some student guest posts! One of the assessment I set for the students is for them to summarise a paper that they have read, and write it in the manner of a blog post. The best blog posts from the class are selected for re-posting (with their permission) here on the Parasite of the Day blog. I am pleased to be presenting these posts from the ZOOL329/529 class of 2018. To kick things off here's a tale of how what a caterpillar eats can affect the growing parasitoid brood within it, written by Deanna O’Leary.

Meet a cabbage looper caterpillar’s worst nightmare – Copidosoma floridanum. This parasitoid wasp cannot produce offspring without its caterpillar host, and the caterpillar, once parasitized is a terminal ticking time bomb. It kind of puts a new twist on the Harry Potter quote “neither can live while the other survives”, however the wasps have revamped the plot slightly. They do in fact need and allow the caterpillar to survive and grow in order for the wasps themselves to survive to adulthood. But once the caterpillar is ready to pupate – all bets are off for our herbivorous friend. This gruesome parasitoid life-cycle tale goes something like this…
Parasitized caterpillar filled with Copidosoma larvae.
Photo by Silvia Mecenaro from here

A female C. floridanum seeks out a moth egg from a Plusiinae moth. She then inserts her ovipositor (aka egg depositor) into the moth egg and lays 1-2 eggs of her own. Now here’s where things start to get insidious. These wasps are polyembryonic – meaning one egg can divide to produce more than one identical embryo. However, unlike the identical twins of humans, they produce an average of 1500 and up to 3000 clonal offspring! This larval legion need time and space to grow and the body cavity of the newly emerged caterpillar provides the perfect safe and nourishing abode.

You would think that having thousands of larvae living inside you – sucking off your energy supply - would mean you won't have long to live, but the wasps have another card up their tibia. They are gregarious koinobionts, meaning they regulate their host’s growth and immunity, allowing the caterpillar to continue to live, and most importantly eat, providing nutrients for all inside. This can result in caterpillars growing up to 50% larger than an unparasitized counterpart by their final stage. Once the caterpillar reaches this final stage - its fifth instar - it is completely eaten from the inside leaving a hollow casing known as a ‘mummy’. The wasps then emerge as adults from this tomb.

Caterpillars can have devastating  effects on plants and the cabbage looper, as its name suggests, is quite partial to a munch on cruciferous plants (think cabbage, broccoli, kale etc.). Cruciferous plants produce defensive chemicals called glucosinilates, designed to deter the feeding of a herbivorous insect generally by reducing the ‘well-being’ of that animal. Herbivores, in turn, have evolved mechanisms to aid in overcoming this obstacle, however as always, it is an evolutionary arms race between plant and insect. But what does this have to do with our parasitoid?

In general, parasitoids of pest herbivores are considered beneficial biological control agents, reducing the number of pests found on a plant population. However, there's been little research into how these parasitoids affect the insect host-plant interaction at a chemical level. Like the “dream within a dream” in Inception, is there an effect on the plants from the parasitoids through the caterpillar? The feature study of this post set out to answer this.

A group of researchers tested the expression of glucosinilates from four cabbage populations under three different conditions – (1) fed on by unparasitized caterpillars, (2) fed on by parasitized caterpillars, and (3) untouched (control) plants. They focused on two types of glucosinilates – indole and aliphatic. They not only wanted to measure the differences in plant chemical expression, but also the effect of these on both caterpillar and wasp growth, reproductive success, and survival. They found that plants fed on by parasitized caterpillars produced 1.5 times more indole glucosinilates than those fed on by unparasitized caterpillars, and 5 times more than the untouched plants! Because parasitized caterpillars needed to eat more to survive – the plants they fed on produced more defensive compounds in response.

An unexpected result of the study was that the effects of different glucosinilates have on the host and the parasitoids. Because of their close developmental ties, things that negatively affect the host can also affect the parasites inside them - but it depends on the specific compound. When feeding on plants producing higher levels of aliphatic glucosinilates, unparasitized caterpillars suffered reduced on growth and fertility, while parasitized caterpillars had decreased survival rates. In contrast high levels of indole glucosinilates resulted in negative brood size and developmental impacts on the parasitoid, with no effect on the caterpillars.

The researchers suggested that the reason for this could lie in the way the different compounds are broken down by the caterpillar during digestion. So, C. floridanum may have far reaching effects that not only impact their hosts, but also extend to other levels of the food web, and even possibly affecting the evolution of insect-plant interactions! Maybe these parasitoids are not such a hideous nightmare considering their beneficial traits – from a human perspective at least. But they’ll never be anything but a terror for the cabbage looper!

References
Ode, P. J., Harvey, J. A., Reichelt, M., Gershenzon, J., & Gols, R. (2016). Differential induction of plant chemical defenses by parasitized and unparasitized herbivores: consequences for reciprocal, multitrophic interactions. Oikos, 125(10), 1398-1407.

This post was written by Deanna O'Leary

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