TACHINID PARASITOIDS: ARE THEY KOINOBIONTS?

Parasitoids have been divided by Askew and Shaw (1986) into koinobionts and idiobionts, on the basis of whether they allow their host to be mobile and continue developing after oviposition (koinobionts) or paralyze or kill the host before the egg hatches (idiobionts). As no tachind kills or paralyzes the host when first entering it, all of them should be considered as koinobionts. However, as stated by Belshaw (1994), tachinids don’t fit well into the koinobiont/idiobiont dichotomy. In fact, koinobiont parasitoids display complex physiological interaction with their hosts, but, in the case of tachinids, only some species [for example Pseudogonia rufifrons (Wiedemann)] actually show a high degree of physiological adaptation to the host and present a developmental synchrony. The larvae of other species [e.g. Exorista larvarum (L.)] grow quickly following attack, kill the host within a few days and develop independently of host physiology, thus behaving more as idiobionts. The in vitro rearing technique provides further evidence for the inadequacy of the koinobiont/idiobiont dichotomy for tachinids, as koinobiont parasitoids are known to be difficult to culture on artificial media, but some tachinids displaying simple host-parasitoid interactions (including E. larvarum) have been successfully reared in vitro on media composed on crude components. It could be therefore more advisable not to use the term “koinobionts” for tachinids, but rather divide them on the the basis of the presence or absence of developmental synchrony with their hosts. References Askew R.R.& Shaw M.R., 1986. In “Insect parasitoids” (J. Waage & D. Greathead eds.), pp. 225-264. Academic Press, London Belshaw R., 1994. In “Parasitoid community ecology” (B.A.Hawkins & W. Sheehan eds.), pp. 145–162.Oxford University Press, New York.