This was originally written as a final paper for my entomology course. You may or may not notice some parallels between the contents here and some of my other works, namely the focus on fascinating evolutionary transitions. For those of you who don't like the idea of tiny larvae eating other insects from the inside out, perhaps look at one of my pieces about elephants.
Parasitism is a widespread life-strategy across all forms of life, and is found in a grand diversity of taxa. Darwin, in a letter to Asa Gray, stated, “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars [...]” (Darwin, 1860), indicative of the ubiquity of parasitoid in the annals of natural history. The term “parasitoid” has historically been used to describe insects whose larvae live and feed parasitically upon or within a host until adulthood, often to the exclusion of any other animals that utilize such a lifestyle (Eggleton and Gaston, 1990). Of the insect parasitoids, consisting of approximately 10% of all insect species (Eggletown and Belshaw, 992), approximately three-quarters of these are Hymenoptera (Belshaw et al., 2003). Of these wasps of the superfamily Ichneumonoidea are the most familiar to the layman, with the incredibly diverse and ubiquitous Ichneumonids being far more conspicuous than the less large and species rich Braconids. Both families parasitize hosts across the full range of possible available arthropod hosts, as well as life stages of those hosts; some members are plant parasitoids, although this lifestyle is more common in Cynipidae, the gall wasps (Ronquist and Liljeblad, 2001).
The free-living adults of parasitoids generally differ little morphologically from their nonparasitoid relatives. Mature females locate hosts, whereupon she lays her eggs either on or adjacent to it. Hymenopteran parasitoids have highly specialized ovipositors for both egg manipulation and the stinging of the host. The sting can permanent paralyze the host (typically in the idiobiont lifestyle), or the host may recover and continue feeding (koinobiosis). Cutting ridges at the end of the ovipositor allow wasps to chisel through plant tissue and even wood to access hosts concealed within leafy materials, galls, or beneath bark. Adult parasitoids may feed from flowers, sap fluxes, and other energy sources, and many also feed on potential hosts.
In some cases, the adult female does not lay her eggs on the hsot but on the host’s food plant. There are also some parasitoids that lay their eggs away from the host, whose active free-living first instar larvae responsible for host location. The hosts of parasitoids are almost exclusively insects, although some other arthropod groups are also parasitized. Juvenile stages are most frequently hosts, although a few groups use adult insects. The parasitoids can be identified as egg, larval, pupal, or adult parasitoids depending on the life stage used. Many lay eggs in one stage which endure and feed until a later stage. In hemimetabolous insects there is less distinction made by the parasitoids as regards life-stages.
Life-styles can divided into a number of dichotomies, beginning with the location of the parasitoids while they feed; Endoparasitoids or ectoparasitoids. This dichotomy holds fairly strong, although some species begin as one before transitioning to the other after a few instars. Another dichotomy exists in whether they are present singly or in numbers in/on a host, with numbers ranging multiple orders of magnitude in the gregarious sort. Many complicated elaborations of course exist, allowing for multiple bouts of parasitoidy in a single host, hyperparasitism of the parasitoids themselves by other parasites.
Another significant lifestyle dichotomy is whether the host is allowed to continue to feed and grow after parasitoids become active on them (koinobiosis), or where the host fails to continue developing and are rapidly depleted by the parasitoids (idiobiosis). Idiobonts are found to grow quickly and then have elongated adult stages, which mirrors the shortened lifespan of the stagnated hosts, while koinobonts tend to have prolonged larval stages as they feed on the still growing larvae, and comparatively shorter adult lifespans (Mayhew, 2016). It is typical that idiobonts are egg, pupal, or adult parasitoids, or feed on permanently paralyzed larval prey. Koinobonts, conversely, usually are targeted to egg-larva, larval-pupal, or active larval hosts. Koinobionts success is dependent on their selectively feeding on nonessential organs of their hosts to allow it to continue to feed, providing more resources for the growing parasitoids.
In spite of all this, these dichotomies are often highly variable, and so are more useful for general life-history descriptions than meaningful taxonomic organization.
Koinobiosis and endoparasitism are commonly associated lifestyle traits in opposition to idiobiosis and ectoparasitism, part of the so-called dichotomous hypothesis (Godfray, 1988), and while this is a general trend, as mentioned prior there are numerous exceptions with all combinations found throughout Ichneumonoidea. In the Ichneumonoidea, Braconids are most commonly idiobionts, with the ichneumonids having a broader distribution of the two main side of the dichotomy, with a notably richer diversity of koinobiont endoparasites. Godfray (1988) proposed a number of evolutionary histories for the various forms of parasitoid in both families, and the general consensus seems to support that parasitoids evolved from phytophagous species, who would incidentally consume present insect material, and slowly transitioned to selectively finding and parasitizing these food items (Rasnitsyn, 1980). From here, there are countless proposed evolutionary trajectories to explain the various parasitoid strategies, and within both orders endoparasitism and koinobiosis seem to have occurred multiple times convergently (quicke and Belshaw 1998, more detail below). Braconids have been well-established as a monophyletic group consisting primarily of idiobionts, and much work has been done to explore their phylogeny using both morphological and molecular data (Quicke and Belshaw,1999; Sharanowski et al., 2011) in order to better understand the evolution of their life histories. In Ichneumonids, however, there remains much to be done to properly explore their interrelations, their life histories vary far more, and the monophyly of the group is still unclear.
The means by which parasitoid wasps overcome the natural defenses of their hosts are numerous, and include a range of weapons including symbiotic polyDNA viruses specific to each family (Braco- and Ichnovirus) (Cook and Stoltz, 1983), venom, teratocytes (special extraembryonic cells), and secretions of the larvae themselves. The polyDNA proviruses exist as endogenous viral elements in the wasp genome, and are delivered virally from egg coatings into the host’s cells, whereupon they are expressed and aid in suppressing host immune systems (Stoltz, 1990; Fleming and Summers, 1991). This viral domestication originates from baculovirus and related nudiviruses, both found as pathogens exclusively in insect systems, and the initial infection of Bracovirus’s ancestral nudivirus predates 100 Mya (Whitfield, 1997; Bezier et al., 2009), while Ichnovirus and other associated Ichnumonid viruses are less established (Herniou et al., 2013). Some wasp genes in turn have entered the viral genome, and unburden the metabolic cost of producing venom components onto the hosts (Webb and Summers, 1990). Bracovirus has been studied rather extensively (Whitfield, 2002; Burke and Strand, 2014; Herniou et al., 2013), and it’s diversification corresponds directly with Braconid evolution (Cook and Stoltz, 1983). What has occurred is a distinct symbiosis between wasp and virus, wherein the expression of the virus aids in the success of the parasitoids, while the parasitoids themselves carry forward the viral genetic line.
In terms of their parasitoid lifestyle, the most conspicuously distinct element that varies between the two families is adult body length and host body length, with Ichneumonids typically being large wasps with large hosts, and Braconids small with small (Traynor 2004). Below the family level, a number of different traits vary greatly, with traits including Ecto/endoparasitoidy, Koino/Idiobiosis, pupation locations (internal v external), and a number of other varying elements of life history (summarized in Mayhew, 2016). What is seen is that at broader taxonomic levels there are fewer distinct differences between the groups, and various elaborations and specializations occur more significantly in more narrow branches of the phylogeny (Mayhew, 2016).
Morphological traits often converge with lifestyle in Ichneumonoids (Quicke and Belshaw, 199), and this incongruence makes appreciating the evolutionary transitions involved in parasitoidy difficult to distinguish. Even a cursory examination of the host specialization of various ichneumonids within a single genus reveals a huge amount of diversity, and frequent specialization nodes with limited clustering of host specificity (Tschopp et al., 2013, see figures 1 and 2, below). Within the genu Ichneumon, which parasitizes Lepidopterans, there is ample scattering of the families of the hosts, and this, within the generic level, is indicative of the degree of variation in wasp/host interrelations. While all of these species are idiobionts on larvae, the adaptation to host pupation sites (above v below ground) seem to have arisen multiple times, and so the ecology of the hosts influences a number of characters of the corresponding parasite, creating multiple convergent events as species shift their hosts.
It has been shown that parasitism has evolved but once in Hymenoptera, with the various elaborations therein such as endoparasitism, and from thence, ectoparasitism, having occuring convergently multiple times (Whitfield, 1998; Quicke and Belshaw, 1999, Downton and Austin, 2001) in both Ichneumonids and Braconids (Quicke, 2009). This in conjunction with the baculo/nudivirus derived PolyDNAviruses which are ubiquitous in Ichneumonoids makes the mapping of the evolution of specific parasite stratagems challenging. While individual elements can be analyzed, building a cohesive picture is fraught with difficulties that remain to be addressed adequately.
What evidence there is suggest that the common evolutionary thread, as briefly summarized above, was a transition of plant-feeding adults facultatively making use of the food resource that was any present boring larvae, transitioning to other insect hosts that may be present in such burrows, and then moving outward to hosts exterior to the plant or fungal sources, obscured pupae, or a combination/reiteration of both. The development of koinobiosis from idiobiosis, ectoparasitism from endoparasitism, and the various life-stages of hosts specialized upon seem to unsurprisingly follow rather sequential patterns. However, the ample abundance of homoplasy in parasitoid behavior makes it difficult to find easily observable gross patterns over the broader framework of Ichneumonoid phylogeny.