Most people have a fairly good understanding of what a parasite is. Something that lives off its host, leaching (pun intended) the host’s nutrients to sustain itself. What people don’t often know is that parasites constitute a whopping 40 percent of the world’s organisms. So rather than thinking of them as icky bottom-dwellers (some, quite literally), perhaps we should embrace them as entrepreneurial! In loose terms, we could probably call ourselves parasites, draining away the earth's resources.
But today, I wanted to invite you into the lesser-known world of parasitoids.
Parasitism, like most things in life, is actually a spectrum. There are varying degrees from complete – or obligate – parasitism to facultative parasitism, which is more transient. Obligate parasites complete their entire life cycle on or in the host, whereas facultative parasites may only require a short time of vampiric activity, then enjoy the remainder of their lifecycle elsewhere.
Parasitoids fall somewhere in the middle but differ by one key factor. The aim of a parasite is to take what it needs from a host without killing it, because no host equals no meal or cosy house. Parasitoids are a more ruthless bunch who actually require the death of the host to complete their life cycle.
Flies (Diptera) and wasps (Hymenoptera) make up the majority of the extant parasitoids with some truly spectacular examples. And plenty of parasitoids call the Illawarra home.
Their lifecycle is wonderfully gruesome: a mature and mated female ready to lay eggs (a term known as gravid) seeks out a host. Once she finds a suitable host, she deposits her eggs on the host or injects them inside the host. The eggs hatch and slowly consume the host from the inside out until it dies and the parasitoid young pupate inside or near the dead host.
The host is both a development incubator and nutrition source, both a womb and a meal. Some poor hosts fall prey to multiple or many parasitoid eggs being laid and carry a heavy load of ravenous young within them.
For movie buffs, this grisly life history may ring some bells. That’s because Ridley Scott used parasitoid wasps as the inspiration for the human-devouring Xenomorphs in the Alien film franchise. Delightfully, since then, a genus of extinct parasitoid wasps has been described and named Xenomorphia after the film’s alien star, along with a living species of parasitoid wasp named Dolichogenidea xenomorph.
You may be surprised to know that these organisms are not your enemy in the garden. Quite the opposite, in fact.
One of the favoured womb-meals of a parasitoid is none other than a nice juicy caterpillar munching its way through your cavolo nero. One such hero are the Tachinid flies, which to anyone without a microscope probably look like normal house flies. But these flies don’t tend to move into your house to spend their lives trying to either fly into your nostril or escape through the window’s invisible force field. No, they tend to hang out on and around flowers since the adults feed exclusively on the nectar and they are reasonable pollinators.
The larvae however, have a carnivorous diet: the non-essential fluids of a caterpillar.
I should explain: The host must remain alive long enough for the larvae to grow to maturity before pupation. So, the young parasitoids first feast on the non-essential tissue of the host insect before turning to the more vital bits. You can see this visually in a parasitised host as they begin to wither and ‘deflate’ as the larvae grow until they are nothing more than a bruised empty husk.
These flies are important in Australian cotton production because they parasitise the number one enemy of Australian cotton growers: Helicoverpa spp.
Helicoverpa are moths whose larvae (caterpillars) terrorise the cotton industry. Much money and time is spent navigating their control. In recent decades, increased facilitation of biological control with natural predators and parasitoids has been an important strategy for pest control in light of consumer concerns regarding pesticide usage.
Thus, cotton farms all contain refuge islands of crops such as pigeon pea that are favoured by Helicoverpa, attracting in more parasitoids and providing a relatively undisturbed environment.
So, next time you’re picking white cabbage moth larvae off your broccoli in the garden, take a closer look. And if one or two appear a little saggy, leave them be – they may just sprout several more caterpillar assassins.
