Parasitoid - Types of Parasitoids

Types of Parasitoids

The parasitoidal type of relationship seems to occur largely in organisms that have fast reproduction rates, such as insects or (perhaps more rarely) mites or nematodes. Workers in this field have pointed out that parasitoids often are closely coevolved with their hosts, which is inarguably true. To maintain a sound perspective of the matter though, one must remember that coevolution might reasonably be expected to develop to even higher degrees of sophistication in the more intimate classes of parasitic relationships. In fact advanced degrees of coevolution occur in the complex interplay between simultaneously extant predator-prey relationships as well.

In using the term parasitoid it is common to think in terms of parasitoidal insects such as Tachinidae and Pompilidae. Some writers recognise or discuss no other classes of parasitoidy. More realistically however, the life histories of several other groups of organisms are equally parasitoidal. In general there is no logical basis for excluding wider use of the term. For example, among the so-called worms, many Nematoda are important parasitoids of insects, snails and similar commercially important pest organisms. Under favourable circumstances they commonly multiply in the host until the carcase is a shell overflowing with a pullulating mass of worms.

Other organisms that might merit the term include certain Pteromalid gall wasps that abort host plant inflorescences, seed weevils, certain plants largely regarded as parasitic, and certain bacteria and viruses (e.g., bacteriophages), in relationships where the beneficiaries obligately destroy their hosts.

Not all such organisms regularly behave quite so parasitoidally of course; for example some bacteriophages establish complex life cycles in which phage particles do get released catastrophically, but only at intervals of many generations of the host, whereas other bacterial viruses emerge intermittently but fairly harmlessly in small numbers at a time.

The clearest cases of fully functional parasitoidy are the likes of many parasitoidal wasps and flies that consume their hosts as completely as any spider or hawk that summarily eats its prey. However, there also are many species of parasitoid that frequently or even routinely kill their "host" or "prey" without consuming much of it. This apparently wasteful strategy sometimes might have the effect of reducing the risk that the prey could escape or offer resistance. In other cases the residue of the victim simply might be difficult to eat or not very nutritious. For example various Phorid flies such as Apocephalus species, are parasitoids of particular species of ants. Various species attack ant genera such as "big-headed ants", "Fire ants" or Solenopsis, Paraponera, and leaf-cutter ants. However, the larvae of most such Phoridae eat mainly the contents of the ants' "head capsules" abandoning the rest of the carcases when pupating. In laying her eggs, the parent fly selects the largest ant workers, which have just the size of head to produce an adequate adult phorid. Presumably the large head-capsule contains the most concentrated nutritious muscle and brain tissue. One also could think of the Rabies virus in similar terms, or the aforementioned soft ticks. Both commonly or invariably cause the death of the host, after consuming at most a trivial fraction of the host's resources.