Before getting to the encyrtids themselves, it’s worth looking at where they fit within the insects. They are chalcidoid wasps, so they belong to the Hymenoptera. Hymenopterans are the third most species-rich order of insect (behind the beetles and butterflies) – although if yet-undescribed species are taken into account, they may come out as the most speciose insects. They can be colloquially split between stinging hymenoptera and parasitic wasps. Going into the details and arguments of hymenopteran phylogeny is beyond the scope of this post.
The Chalcidoidea belong to the parasitoid wasps, where they arguably form the most species-rich group (although the Ichneumonidae may outumber them). They parasitise all insect orders at any stage of their life cycle, from embryos to adults. The smallest insect known is a chalcidoid (Dicopomorpha echmepterygis, 0.13 mm); some chalcidoids are so derived that it is difficult even for experienced entomologists to recognise them as hymenopterans. Again, it would be overkill to go into chalcidoid relationships; suffice it to say that the chalcidoids are monophyletic, encyrtids are nested within them and are also monophyletic.
There are ~470 genera and over 3500 species of encyrtid wasp. They are all parasites, ranging in size from tiny 25 µm to regular 4 mm body length. They live in tropical and subtropical areas, including rainforests. Biogeographically, they are more or less cosmopolitan, found on all relevant continents, reaching their highest diversity in neotropical areas and Australia.
One of their distinguishing morphological characters includes having less than 13 antennal segments (as opposed to the ancestral 13-segmented antenna of the chalcidoids), however this is also a derived state found in some other chalcidoid groups; the antennas are really distinguished by having 1 anellus and between 2 and 7 funicular segments. The other apomorphies for the encyrtids are as follows:
- The cerci are placed more anteriorly than in usual chalcidoids
- A projecting mesoscutal process
- The middle coxa is articulated anteriorly to the mesosternal midline.
- The wings have a very short marginal vein.
For their number of species, they are relatively diverse in their biology; this has led to considerable difficulties in classification and there has not yet been a single reliable taxonomic system for the encyrtids. Even regional descriptions simply treat them as a homogeneous group without daring to break them down to into tribes or other smaller, more manageable groups. As a result, any field guides with encyrtids included use completely artificial keys, without reference to potential phylogenetic characters, and as such are rather unreliable (no fault of their own though, it’s a simple matter of lack of research).
Some encyrtids are polyembryonic egg-larval endoparasitoids. We’ll use the example of Copidosoma floridanum here, which infects plusiine moths. The wasp lays a couple of eggs onto a moth egg, regardless of the developmental stage the moth egg is in. The wasp egg then divides, forming a morula surrounded by a syncytium. It is this ball of cells that infects the embryo. Compare this to regular egg-larval parasitoids, where an actual larva physically bores into the host egg.
So how does a mere embryo manage to be parasitic? It doesn’t cause any physical damage to the host egg (by boring or piercing it to enter); this is the remarkable thing and is not seen in any other parasitic animal group, from other wasps to trematodes to protozoans. Instead, this parasitism is much more reminiscent of standard cell-cell interactions, where the cells recognise each other because of certain molecular markers and form gap junctions or connect with villi to allow chemical transfers between them (cellular communication). In fact, it’s not only reminiscent of it: that’s exactly what happens! Of course, this means that one species can only infect one species specifically.
Once the morula establishes this connection, it leeches nutrients. The host embryo extends microvilli that envelop the morula – it doesn’t recognise it as an enemy – allowing it to become integrated within the embryo as a cyst. The wasp can then take its time developing in there, remaining until its fully grown (the moth may be in the adult stage by then) and breaking out when the need arises (killing the moth in the process).
That was just one specific example chosen for coolness, but encyrtid parasitism is very important in agricultural pest control, being especially used for controlling mealybug populations in rice, citrus, vine, mango, cacao, coffee and cotton plantations (among others). Encyrtids have also been known to target true bugs (Heteroptera), beetles, roaches, grasshoppers (and other orthopterans), flies, lacewings; the list goes on – they can be viewed as universal insect parasites.
As for their fossil record, some stem-group encyrtids are known from Baltic amber. The related Ukrainian Rovno amber contains true encyrtids.
And since this was a very light post – no fault of mine, there’s simply not so much to say about these guys! – here’s a couple of drawings, from Noyes & Hanson (1996).
As I said, there’s not much to read specifically about them. A book about hymenopterans is sure to have a paragraph mentioning them. The sources of the pictures are:
Noyes, J. S. 1980. A review of the genera of Neotropical Encyrtidae (Hymenoptera: Chalcidoidea). Bulletin of the British Museum of Natural History (Entomology) 41, 107-253.
Noyes, J. S. & Hanson, P. 1996. Encyrtidae (Hymenoptera: Chalcidoidea) of Costa Rica: the genera and species associated with jumping plant-lice (Homoptera: Psylloidea). Bulletin of the British Museum of Natural History (Entomology) 65, 105-164.