Abstract: The tachinid fly, Nemorilla maculosa Meigen (Diptera: Tachinidae) is an important parasitoid of the beet webworm, Loxostege sticticalis L. (Lepidoptera: Pyralidae) in mainland China. Females of N. maculosa lay heavy-shelled macrotype eggs on the last-stadium host larvae of L. sticticalis and other lepidopterous larvae. Little has been studied and understood so far in the host species and instar preference, and parasitic position of N. maculosa. Studies were therefore designed to investigate and understand the host preference of this parasitoid under laboratory conditions (22℃, L16∶D8). The results indicated that N. maculosa could parasitize the beet webworm L. sticticalis, the beet armyworm Spodoptera exigua Hübner (Lepidoptera:Noctuidae), and the oriental armyworm Mythimna separata (Walker) (Lepidoptera: Noctuidae) when the final instar of the three host larvae were presented simultaneously. However, the parasitism rate for L. sticticalis was significantly higher than that for the beet armyworm, which in turn was significantly higher than that for the oriental armyworm. Besides, the number of tachinid eggs per host larva received also followed the trend of parasitism rate in these three species, indicating that N. maculosa was more likely to select L. sticticalis larvae as its host. The parasitoid mainly selected the last instar larvae as its host when 3rd, 4th, and 5th instar larvae of L. sticticalis were mixed together. The parasitism rates for these three instars of larvae were increased as the larval instar increased. The tachinid eggs were mainly located on the thorax (66.4%), followed by on the head (23.4%) and the abdomen (10.9%) of the host larvae. The number of tachinid eggs on the thorax was significantly greater than that on the head and abdomen of the host larvae, while the egg numbers between the head and abdomen were insignificantly different. Mechanisms underlying the host instar, position and species preference of N. maculosa were finally discussed.

Key words: Nemorilla maculosa, Loxostege sticticalis, tachinid parasitoid, host species, larval instar, parasitized position, parasitization preference