Efficiency of rearing Dastarcus helophoroides using Monochamus alternatus pupae

Dastarcus helophoroides (Coleoptera: Bothrideridae) is a significant parasitic natural enemy of Monochamus alternatus (Coleoptera: Cerambycidae) that carry the Bursaphelenchus xylophilus. This study aims to investigate the rearing effect of D. helophoroides by inoculating different numbers of its larvae onto M. alternatus pupae and observing developmental parameters such as cocoon count, eclosion number, and offspring quality. The results showed that under laboratory conditions, the parasitism rates of D. helophoroides larvae on M. alternatus pupae were found to be above 80% across five inoculation densities. The larval and pupal durations of D. helophoroides varied significantly. When four larvae were inoculated onto one host, both the larval durations (11.0 days) and pupal durations (30.1 days) were longest compared to other treatments (6, 8, 10, and 12 larvae per host) where the larval duration shortened to 8~9 days, and the pupal duration gradually decreased to a minimum of 22.1 d (8 d shorter than at 4 larvae/host) as the number of inoculated larvae increased. Cocoon count, eclosion number, cocooning rate, eclosion rate and reproductive success rate all exhibited a trend of initially increasing and then decreasing with the addition of inoculated numbers. Specifically, The highest values were observed at 8 larvae/host, with averages of 7.3 cocoons, 7.2 eclosed adults, 91.4% cocooning rate, 99.1% eclosion rate, and 90.6% reproductive success rate. At 12 larvae/host, the cocooning rate dropped to 56.6%, while the lowest eclosion rate (81.3%) and reproductive success rate (50.0%) occurred at 4 larvae/host. Significant differences in the total weight of eclosed adults among the five inoculation densities were also noted; specifically, when six or eight larvae were inoculated, their total adult weights (0.108 g/tube and 0.102 g/tube respectively) exceeded those of other treatments. Conversely, the total adult weight was lowest at an inoculation density of 12 larvae per host (0.050 g/tube, a 50% reduction). The individual adult weight decreased as inoculation density increased, peaking at 0.031 g/adult with an inoculation density of four larvae per host but dropped to 0.014 g/adult with eight larvae per host before reaching its lowest point at 0.007 g/adult with twelve larvae per host, less than one-quarter of the maximum. In general, an optimal inoculation density for breeding high-quality D. helophoroides adults using M. alternatus is four larvae per host when prioritizing individual weight. For field release aimed at pest control, eight larvae per host is recommended based on eclosion number and reproductive success rate.