Poulton (1906), Timon-David (1953), Adamovic (1963a, b, 1966, 1972) and Musso (1970) have previously described the prey of D. diadema. The results of this study agree with their results. D. diadema is distinctly stenophagic, more than 70 percent of the prey being Hymenoptera. The species is also a significant predator of Apis mellifera.
Apis mellifera was rare in Darmstadt during the study period, the abundant Hymenoptera being species of Bombus. That is possibly the reason why, out of 308 prey records, 84 percent were Bombus spp. and 16 percent were species of Andrena, Megachile, Paravespula and Apis mellifera. Females were observed more frequently than males with prey (178 of 308 records). Feeding usually took place from a hanging position and manipulation was frequent, usually five or six times during which the point of insertion of the hypopharynx changed (thorax - abdomen - head - thorax and so on).
The duration of the sucking period was 44 min. on average for a Bombus spp. (minimum: 28, maximum: 187 min.). Abdominal pumping was observed during feeding, but also during and after high activity (flight, at times of highest temperature, or oviposition). D. diadema is excellently well-adapted for struggling with Aculeata species with its long and thin legs, strong spur at the apex of the fore tibia and long proboscis. These features are important adaptations to the dangerous prey.