Density Effect And Selective Parasitism Of Nemorilla Maculosa Meigen (Diptera: Tachinidae) On Different Instar Larvae Of Loxostege Sticticalis L. (Lepidoptera: Pyralidae)

Loxostege sticticalis L. (Lepidoptera: Pyralidae) is one of the most important pests in China, which periodically occurred and resulted in serious damage to crops and pasturage during the three outbreaks since the foundation of P. R. China. Tachinid flies play an important role in suppressing the population of L. sticticalis as the predominant parasitic natural enemies of meadow moth and Nemorilla maculosa is just one of them. N. maculosa which has widely distribution and occurs suitably to different environment could parasitize many kinds of hosts. It was considered as one of the very important natural enemies of serious pest insects in agriculture.There were a few studies on the tachinid fly, however, the efficient factors of tachinid fly parasitizing L. sticticalis have not been clear. Some field investigations showed that parasite rates of N. maculosa on meadow moth increased quickly with the increase of the host lava population density How to explain this phenomenon? Does the high host density stimulate tachinid fly's oviposition, or the large host population attracts more natural enemies, or any other factors influence tachinid fly parasitizing L. sticticalis? Moreover, the tachinid fly lay eggs on the body of its host larva. Does the period of different larval instar stage affect the parasite rates of the tachinid fly?In this paper we gave at first an introduction of rearing technique for L. sticticalis as well as the judging method of each instar of meadow moth. We developed techniques in artificial rearing of the tachinid fly, studied its developmental threshold temperature and effective accumulated temperature, and the effect of different density ratios (tachinid fly : host) on the tachinid fly's parasitism to L. sticticalis. The result showed 1) in the experimental temperatures (18-30℃), there was a significant positive relationship between the developmental period of the tachinid fly's egg and temperatures. The developmental threshold temperature for the egg of N. maculosa was 11.9℃. By comparing the developmental period of tachinid fly's egg with those of its host larvae, we made it clear that the different period of the larval instars were the key factor which affected the parasite rates. The fifth-instar larvae of L. sticticalis was the main stage which could be parasitized. 2)the parasitized number of meadow moth increased, while the parasitized ratios decreased with host density increasing as the ratios of tachinid fly number to host larvae one varied from 1∶5 to 1∶25. 3) At the same ratios of tachinid fly to its host, more larvae of meadow moth would be parasitized at the ratio of 2 : 20 than those of 1:10, 3:30, 4:40 and 5:50.The role of N. maculosa in suppressing meadow moth population was also discussed in this paper. It is important to coordinate the relationship of tachinid fly and its host to control the damage made by 5th instar of meadow moth. Meanwhile, we should actively take actions to find other natural enemies, such as predators and parasitoids to control the low instars of meadow moth. Meadow moth should be much more effectively controlled by combining different natural enemies which suppress its different developmental stages.