Gordon Gordh

Anatomy of the mouthparts and digestive tract during feeding in larvae of the parasitoid wasp Trichogramma australicum Girault (Hymenoptera : Trichogrammatidae)

Abstract To aid in the development of artificial diets for mass rearing parasitioids, we investigated the anatomical changes in the digestive tract during feeding behaviour of larval Trichogramma australicum (Hymenoptera : Trichogrammatidae). Larvae begin to feed immediately upon eclosion and feed continuously for 4 h until replete. Feeding is characterised by rhythmic muscle contractions (ca 1 per s) of the pharynx. Contractions of the pharyngeal dilator muscles lift the roof of the lobe-shaped pharynx away from the floor of the chamber, opening the mouth and pumping food into the pharyngeal cavity. Another muscle contraction occurs about 0.5 s later, forcing the bolus of food through the oesophagus and into the midgut. The junction of fore- and midgut is marked by a cardiac valve. The midgut occupies most of the body cavity and is lined with highly vacuolated, flattened cells and a dispersed layer of muscle cells. In the centre of the midgut, food has the appearance of host egg contents. Food near the midgut epithelial cells has a finer, more homogeneous appearance. This change in the physical properties of the gut contents is indicative of the digestion process. In the prepupa, where digestion is complete, the entire gut contents have this appearance. After eclosion, the vitelline membrane remains attached to the posterior end of the larva. We believe this attachment to be adaptive in two ways: (1) to anchor the larva against the movements of its anterior portion, thereby increasing the efficiency of foraging within the egg; and (2) to prevent a free-floating membrane from clogging the mouthparts during ingestion.

Influence of rearing hosts on host size acceptance by Trichogramma australicum

We investigated the acceptance of different sized host models by Trichogramma australicum in the laboratory. We used isoline cultures of T. australicum reared in hosts of different sizes. Helicoverpa armigera represent relatively large hosts, and those of Sitotroga cerealella are small, termed the Ha and Sc biotypes, respectively. Five sizes of glass beads were tested for acceptance: diameter 0.5, 0.75, 1.0, 1.5 and 2.0 mm. The acceptance of a host model was determined by persistent attempted drilling of a glass bead by a female. The relationship between host egg size and number of eggs laid by a female was also investigated. We used three sizes of artificial egg (diameter 0.75, 1.00, and 1.50 mm of hemispherical cupules) each containing artificial diet. Ha biotype wasps accepted host models in the range 0.75--1.50 mm dia. (vol. 0.166--1.325 μl), whereas those of the Sc biotype accepted host models in the range 0.50--1.00 mm dia. (vol. 0.05--0.393 μl). This evidence suggests a lineal (possibly genetic) influence to host size acceptance for T. australicum, dependent on the size of the host in which the wasp has been reared. Also T. australicum lay fewer eggs in smaller artificial eggs than in larger ones. The role of host size in host acceptance and number of eggs delivered, and its implications, is discussed.

Development of Trichogramma australicum (Hym.: Trichogrammatidae) at low and high population density in artificial diet

We compared the development ofTrichogramma australicum Girault in artificial-diet filled artificial “eggs” containing 10–15 (low density) or 35–50 (high density) immature parasitoids. The size of low- and high-densityT. australicum eggs at 1, 15, 30 and 36 h was not significantly different. Also, incubation time of low- and high-density eggs in artificial diet was similar. Appearance of urate bodies in larvae developing low and high density occurred at the same time (100–108 h). Size of immatureT. australicum in low density significantly increased after the urate bodies had appeared. Black deposits appeared on the surface ofT. australicum prepupae soon after all diet had been consumed (at 108–120 and 144–168 h in low and high density, respectively). We concluded that the appearance of the black deposits was an indication of the onset of the prepupal stage. The pupal size was significantly larger at low density than high density. Mortality of prepupae and pupae was significantly higher at low density than at high density. Proportion of larvae at high density developed to abnormal and normal wasps. Wasps did not emerge from larvae reared at low density.SummaryNous avons comparé le développement sur milieu artificiel deTrichogramma australicum Girault dans des “œufs” artificiels contenant soit 10–15 (conditions de faible densité) soit 35–50 (conditions de forte densité) parasitoïdes immatures. La taille des oeufs deT. australicum après 1, 15, 30 et 36 heures de développement en conditions de faible ou de forte densité n’a pas été significativement différente. De même, le temps d’incubation des œufs a été similaire dans les deux conditions de densité. Dans les deux cas, l’apparition des corps d’urates chez les larves s’est produite au même moment (apres 100–108 heures de développement). En condition de faible densité, la taille des stades immatures deT. australicum a significativement augmenté bien que les corps d’urates soient apparus. Des dépôts noirs ont été observés à la surface des prénymphes deT. australicum une fois le milieu entièrement consommé (après 108–120 heures et 144–168 heures de développement en conditions, respectivement, de faible et de forte densité). L’apparition de ces dépôts noirs semble donc indiquer l’imminence du stade prénymphal. En conditions de faible densité, les nymphes étaient de taille significativement plus grande et la mortalité aux stades prénymphal et nymphal a été significativement plus élevée qu’en conditions de forte densité. Dans le cas de forte densité de population, une partie des larves s’est développée en adultes anormaux et normaux, alors qu’aucun adulte n’a été obtenu à partir de larves élevée en conditions de faible densité.