Efficiency of host utilisation by coleopteran parasitoid

In insect larvae, optimising food utilisation with respect to available meals and time is essential for achieving maximum adult body size, which is a relevant proxy of fitness. We studied the efficiency of food conversion, body size, mortality, and development time in a solitary idiobiont ectoparasitoid, Brachinus explodens (Coleoptera: Carabidae), reared in the laboratory on the pupae of another carabid genus, Amara. The efficiency of conversion index (ECI – ratio of ingested to assimilated food) was, on average, 54.1 ± 1.1% (n = 76), with a minimum of 26.9% and a maximum of 81.6%. The rate of increase in biomass gained (Wgained) with biomass of the host was constant in females, but it decreased in males over the range of host body mass. Females, therefore, grew heavier from hosts of the same mass compared to males. Body length increased with the host mass and was correlated with Wgained identically for both sexes. Mortality was unaffected by the host mass, but it significantly increased below 20 °C. In contrast, the development time of the feeding phase of the larva increased with the host mass at 20.3 and 23.7 °C, but it remained unaffected at 26.9 °C and in all three temperatures considering pupal development. Wgained increased with development time up to ca. 8 days of larval feeding at 23.7 °C. To our knowledge, our data are the first on food utilisation in solitary idiobiont coleopteran ectoparasitoids, and they present the highest values of ECI in insects.