Almost everybody could explain you more or less accurately what both parasites and predators are. But could everybody say you what a parasitoid is?
Animals (and especially insects) set up a lot of different symbiotic relationships, but often we find organisms whose relationship is somewhere between one and another (this is not a matter not of black or white!). In the case of parasitoid insects, we talk about organisms that establish a symbiotic relationship with traits of both predator-prey relationships and a parasitic ones.
Read this article to find out what parasitoid insects are, which is their origin and which kind of parasitoid insects exist. They are more useful than they seem to be!
Parasites, parasitoids and predators
Parasitoids are not exclusively insects, but the greater part of parasitoids belong to the subphyllum Hexapoda. For this reason, I will focus my explanation on parasitoid insects.
Before giving you further explanations, we must make the differences between parasitoids, parasites and predators clear.
In a parasitic relationship, parasites benefit at the expense of other organisms, the hosts, which are damaged in result. But despite of hurting it, parasites try to keep their hosts alive as long as possible in order to keep on benefiting from them, so parasites rarely kill their hosts.
In a predator-prey relationship, predators feed on a lot of organisms (the prey) throughout their life cycle in order to keep on developing. Unlike parasite organisms, predators don’t try to keep their prey alive so long, because the purpose of preying on other organisms is to obtain energy as faster as possible (for example, mantids, dragonflies…).
Finally, between parasitism and predation we find parasitoid organisms: insects with a parasitic larval stage that develop by feeding on a single host, which is usually another insect or arthropod. In contrast with parasites, parasitoids larvae kill their hosts to complete their life cycle; so, in which sense are they different from predators? The answer is that parasitic larvae only need to feed on a single host to reach adulthood. While parasitoid larvae are a parasitic life form, parasitoid adults tend to be herbivores or predators.
Origin and diversity of parasitoids
Parasitoid insects are present in many insect orders (Coleoptera, Diptera..), but the greater part of them is located in the Hymenoptera order (bees, wasps and ants). Because of that, in this section I will focus on talking only about the origin and diversity of hymenopteran parasitoids.
The most important and also evolved group of hymenopterans is the suborder Apocrita, which includes wasps, bees and ants. In turn, the suborder Apocrita is divided in two artificial groups:
- Aculeata: they don’t have a parasitic larval stage. The ovopositor (an organ that females use to lay their eggs) has been transformed into a sting that inoculates venom (organisms of this group are also called “stinging wasps and bees”).
- “Parasitica”: they have a parasitic larval stage. Adult females of the group Parasitica have a long and sharp ovopositor they stab into different surfaces (wood, another insect…) so they can lay their eggs inside. In contrast with Aculeata, Parasitic hymenopterans don’t sting (they’re not venomous).
About 77% (66.000 species more or less) of parasitoid insects known nowadays belong to the Parasitica group, and most of them are wasps.
Origin of hymenopteran parasitoids
To understand the origin of certain morphological, anatomical or conductual traits of an organism, we often have to study the traits of a “sister taxon” or “sister group”, i.e. a group more closely related to the group in question than any other group (they share the most recent common ancestor).
The sister group of Apocrita is the family Orussidae (from the Symphyta suborder), which is also considered the most ancient groups of hymenopterans.
It’s believed that the common ancestor of Apocrita and Orussidae had first developed a parasitic life form among hymenopterans. This conclusion is based on the studies about ecologic traits of current Orussidae specimens: some of these organisms establish a positive relationship with some symbiotic xylophagus fungi (i.e. fungi that feed primarily on wood); these fungi usually develop inside a sort of tiny baskets located over the surface of ovopositors, so they can be inoculated inside the wood the Orussidae feed on when laying. Thus, fungi process wood to obtain a product that can be digested by Orussidae. However, there exist Orussidae specimens which don’t establish this kind of symbiotic relationship and parasite other specimens instead (especially the ones that possess symbiotic fungi). Thus, these parasitic Orussidae obtain nutrients by feeding on other Orussidae members and obtain more energy in result.
So, this being an ancient group it’s believed that the observed behavior in some current Orussidae members could be a reflection of the ancient origin of parasitism and parasitoids among the Hymenoptera order.
Types of parasitoids
Even if there are many ways to classify parasitoids, we can divide these organisms mainly into two groups: the ones that stop host’s development when laying inside it and the ones that don’t stop host’s development. Let’s talk about these two groups:
Idiobiont parasitoids paralyze or prevent further development of hosts when laying, so parasitoid larvae could have a reliable and immobile source of food at their birth.
Usually, idiobionts attack hosts that are concealed in plant tissues (for example, wood) or exposed hosts that possess other kinds of physical protections, so female parasitoids have developed long and sharp ovopositors that allow them to pierce these barriers.
Idiobiont parasitoids can be both ectoparasitoids and endoparasitoids (i.e. if larvae attack hosts from outside or inside host’s body), although mostly are ectoparasitoids. Moreover, parasitoid larvae feed on hosts only on the last development stages until the moment they reach adulthood.
Ectoparasitoid idiobiont females first inject venom into the host, to induce temporary or permanent paralysis, and then ovoposits on or near the immobilized host. In some cases, females that have just layed their eggs stay near the lay to protect it and also to prevent host to be eaten by other organisms.
Generally, idiobiont adult females don’t have any preference when looking for a place to proceed on egg laying, so larvae feed on a wide variety of organisms.
Most of parasitoid insects (and especially hymenopterans, dipterans and coleopterans) are koinobionts.
Unlike idiobionts, almost all koinobionts are endoparasitic and lay their eggs directly inside the host, which can be both exposed and concealed. However, the trait that truly differentiates koinobiont parasitoids from idiobiont parasitoids is the fact that koinobionts allow the host to continue its development while feeding on it. Thus, the parasitic larvae feed on the host while growing inside host’s body without causing it any damage…until the moment larvae reach the adulthood, when they emerge from the body of the host, causing its death.
Once the parasitic larvae are inside host’s body begin to grow to reach the pupal stage. Until this moment, larvae use different mechanisms to avoid or block the immune response of the host (for example, by placing eggs in hosts tissues where immune system doesn’t work). So, larvae can develop by feeding on host’s nutrients until the moment they metamorphose, when adult parasitoids emerge from inside the body of the host, killing it consequently.
Due to the close relationship established by parasitoids and hosts, koinobiont parasitoids tend to be less generalist than idiobionts when looking for a suitable host.
Ecological function of parasitoids
Parasitoids, like predators or parasites, perform an important ecological role because they act as natural regulators of other organisms populations. So, parasitic larvae kill a lot of organisms that could damage the environment or even other organisms if their populations grow excessively. Thus, the disappearance of parasitoids (just like predators or parasites) could entail an excessive increase of some animal populations (especially other insects populations).
For that reason, parasitoids are considered as a great biological control agent against different plagues in gardens and crops.
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- Notes from the subject “Biology and Biodiversity of Arthropods” taken during my Biology studies at Universitat Autònoma de Barcelona (UAB).
- Timothy M. Goater, Cameron P. Goater, Gerald W. Esch (2013). Parasitism: The Diversity and Ecology of Animal Parasites. Ed. 2. Cambridge University Press.
- Vincent H. Resh, Ring T. Cardé (2009). Encyclopedia of Insects. Ed.2. Academic Press.
- Donald L. J. Quicke (2014). The Braconid and Ichneumonid Parasitoid Wasps: Biology, Systematics, Evolution and Ecology. John Wiley & Sons.
Main image by Ton Rulkens (Flickr, CC).