Reznik S.Ya., Umarova T.Ya, Voinovich N.D. 2003. Egg retention in Trichogramma (Hymenoptera: Chalcidoidea: Trichogrammatidae): learning or diapause? // Acta Soc. Zool. Bohem. Vol. 67, N1. P. 25-33.

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It is well known that insects are capable of modifying oviposition behavior and oogenesis in a response to environmental conditions. These modifications form a continuum of steps - from learning to reproductive diapause. The object of our study, Trichogramma, is a minute egg parasitoid. We have demonstrated that Sitotroga cerealella eggs were accepted for oviposition by certain Trichogramma principium  females, while other wasps of the same strain delay parasitization or totally refuse to parasitize the same host. These ‘refusing’ females carried a high number of mature ovarian eggs, they moved actively, and even might incidentally contact the host, but the usual sequence of behavioral reactions resulting in parasitization was interrupted at the stage of arrestment and host recognition, suggesting that refusal to oviposit may be considered as egg retention. Less than a half of ovipositing females started oviposition during the first two days of the experiment, while the rest of ovipositing females showed a delay of parasitization ranging from 2 to 10 days of contact with the host. Thus, parasitization by a group of simultaneously emerged wasps was almost uniformly distributed over 8-10 days which suggests that the refusal to oviposit in an immediately available host may benefit risk-spreading.

As for Trichogramma females that accepted S. cerealella eggs, they appeared to maintain the tendency to parasitize when sequentially offered new portions of the host eggs. This ‘parasitization state’ was stable both in the presence of a host and under host deprivation extended up to 8 days, and even when preferred and usually rejected host ages or species were sequentially offered. As a result, T. principium females more often accepted less preferred host eggs when previously offered a more preferred host. In most studied cases of learning, experience with a given host species either does not influence the level of affinity for other hosts or acceptance of a low-quality host decreases if a high-quality (preferred) host was previously encountered. This suggests that the stability of the parasitization state is based on endocrine mechanisms. Then, refusal to oviposit may be considered as a specific state when not oogenesis, but oviposition behavior is subject to neurohormonal regulation. 

Thus, the studied phenomenon shows certain properties peculiar to learning: it is manifested in behavior (not in oogenesis); it is determined by the immediate environment (not by anticipatory token stimuli); neither reduced activity nor increased longevity nor resistance to environmental extremes were revealed. On the other hand, separation of females into ovipositing and refusing is of ‘all-or-none’ type, similar to that into active and diapaused, and ‘parasitization state’ is practically irreversible (while learning usually induced reversible gradual changes).