A study aimed at determining the effect of host size on the pattern of sex allocation by an aphid parasitoid Lysiphlebia mirzai Shuja‐Uddin (Hymenoptera: Braconidae: Aphidiinae) was undertaken to test the validity of the following three hypotheses: (1) larger hosts will yield a greater proportion of daughters, (2) differential mortality of the sexes of the parasitoid during development is a function of host‐size, and (3) daughters emerging from larger hosts are more fecund. The results suggested a propensity in the parasitoid to deposit fertilised (diploid) eggs in large hosts (third instar nymph) and unfertilised (haploid) eggs in small hosts (first and second instar nymphs). Unpreferred fourth instar nymphs and apterous aphid adults also received more haploid eggs despite being larger in size than the preferred third instar nymphs. However, the perception of host size by the mother was dependent on the extent of temporal variation in the host size distribution and on her previous experience of host size. Developmental period, longevity, mating potential, fecundity and progeny sex ratio of L. mirzai emerging from small versus large hosts indicated that the host size affected the fitness of the daughters more than that of sons. No differential mortality of the sexes during development of L. mirzai was observed in small versus large hosts. This shows that L. mirzai, while ovipositing in growing stages of the host, adjusts progeny sex ratio according to the host size and by doing so she tends to contribute maximally to progeny fitness without knowing about the future host quality.

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Trioxys (Binodoxys) indicus parasitises mostly Aphis craccivora and A. gossypii and does not show any significant affinity for others. The parasitoid, at first, attracted towards certain plants that harbour the acceptable host species and she does it irrespective of the presence of host. The observation confirms that it is the chemical clue — allomone — that helps the parasitoid in search for host‐habitat. The parasitoid is attracted towards hosts (aphids) by sensing their kairomonal secretions. The olfactory receptors are located in the antennae of the parasitoid. Gustation of the honeydew excreted by the aphid helps the parasitoid by reducing the hostapproaching period. Sight is helpful in proper orientation with respect to the host after she has recognised them. Host‐acceptance has largely been attributed to a number of factors. The third instar nymph of A. craccivora is most acceptable for parasitisation. Aphid‐movement and coloured light have nothing to do in host acceptance by T. indicus. The aphid ist not paralysed during parasitisation. Laterals of the body of the aphid are most preferred sites for pricking. Abdomen of the host is maximally pricked and also received the maximum number of eggs of the parasitoid. Zusammenfassung Zur Bionomie von Trioxys (Binodoxys) indicus Subba Rao & Sharma, einem Parasitoiden der Blattlaus Aphis craccivora Koch. XIII. Wirtswahl des Parasitoiden Trioxys indicus paraisitiert vor allem Aphis craccivora und A. gossypii und zeigt keine deutliche Bindung an andere Wirte. Der Parasitoid wird erst durch die Wirtspflanzen, auch ohne Anwesenheit der Wirte, angelockt. Es zeigte sich, daß chemische Faktoren, Allomone, dem Parasitoiden bei seiner Suche nach dem Wirt‐Habitat helfen. Duftausscheidungen, Kairomone, vom Wirt ausgeschieden, führen den Parasitoiden zu diesem hin. Die Geruchsrezeptoren liegen in den Antennen. Die Ernährung mit dem von den Blattläusen ausgeschiedenen Honigtau hilft dem Parasitoiden die Annäherungszeit an den Wirt zu vermindern. Sobald der Wirt wahrgenommen ist, setzt bei dem Parasitoiden die Sichtorientierung ein. Die Wirtsannahme ist von einer Anzahl Faktoren abhängig. Am geeignetsten zur Parasitierung sind die L3 der Blattläuse. Bewegung der Blattläuse und farbiges Licht spielen bei der Wirtsannahme durch T. indicus keine Rolle. Die Blattlaus wird durch die Parasitierung nicht paralysiert. Die Körperseiten der Blattlaus sind die bevorzugten Stellen für den Anstich des Parasitoiden. Die größte Zahl an Anstichen und abgelegten Eiern befanden sich im Abdomen des Wirtes.

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