Étude en Laboratoire du Développement Larvaire de Oryctes Monoceros et O. Rhinoceros [Col. Scarabaeidae](1)

Summary

Laboratory investigations about the larval development of Oryctes monoceros 01 and O. rhinoceros L.

With the use of a mixture made of 2/3 decaying wood and 1/3 cow dung, the breeding of O. monoceros and O. rhinoceros during several successive generations is possible with considerable limitation of the incidence of Meturrhizium anisopliae mycosis, which, until now, had prevented the study of these insects in the laboratory.

After having recalled the systematics characters for distinguishing the imagos and the larvae in these two species, the features of the larval development are presented, either in individual breeding or in collective breeding, at the temperatures of 28-30 °C.

There are three larval stages, from which the third is by far the longest, particularly by O. rhinoceros, for which it lasts about 6 months (2 months in average by O. monoceros). This stage ends by a period of about 8 days, the so-called prepupal stage, during which the pupal process are preparing. At the end of this period, the pupal moult brings out the pupa, and the imago will emerge about two weeks later for the two Oryctes species considered, so as adults of O. monoceros emerge in average 14 weeks after the egg hatching, when O. rhinoceros needs 30 weeks.

The effect of the composition and structure of the breeding substratum results especially in the amount of mortality by mycosis. Feces of domestic animals are absolutely necessary to avoid fungal epizootics, but what is added is equally important: the decaying wood, for instance, gives better results than leaf-mould or sawdust.

In the practice, the mixture decaying wood + cowdung is the best and the easiest. The middle yield is then of 60 to 80 imagos for every 100 larvae. In large container (500 to 700 liters) the combination sawdust + cowdung is also satisfactory, probably because of the fermentations that bring the whole to a temperature letal for the Metarrhizium spores.

But the physiological state of the larvae in connection with the breeding medium probably plays a more important part than the direct effect of this medium on the survival and the aggressiveness of the fungus.