Effects Of Raining On The Pupation And Emergence Of Cotton Bollworm And Its Occurrence

Experiments have been carried out to find how raining affects the pupation and emergence of Helicoverpa armigera (CBM). The results show that this effect depends on the raining time, rain mount, flooding time and soil type. Raining before the larva entered soil with unsaturated humidity has little effect with the emergence rate being over 90% in general. However, too dry or too damp soil destroys imago wing outspreading and emergence from soil. The difference of the pupation rate and the emergence rate between different sorts of soil was also remarkable. When raining and keeping the soil wet after the larva of CBW entered soil, the emergence rate dependent on not only the soil moisture but the tightness of the pupa-cell as well. The pupa-cell in loose soil was easily struck to collapse by heavy rain because of its low tightness, and the emergence rate was also low. On the contrary, the emergence rate would increase in relatively tight soil. The rain flooding had great effect on the larvae, which had entered soil and would pupate, and led to these larvae almost all to death. Around the 6th day after the larvae entered soil the flooding had lest effect on the pupae. Anyway, if the soil was flooded for over 1 day the mortality rate of pupae and of imagoes would increase remarkable. The emergence rate in air-dry or sun curing soil which was formerly soaked with rainwater was visibly lower than in relatively damp soil. Because so hard and solid the surface of dry soil had become. The earlier it rained after the larvae entered soil, the harder the soil afterward would become, and the lower the emergence rate would be. Among different sorts of soil, the emergence rate of CBW in solonchak was the highest, in damp soil the moderate, and in rice soil the lowest. The distribution of soil of different sorts related close to the occurrence degree of CBW. The CBW occurred heavily in the area where there was high proportion of sandy soil, contrarily, the CBW occurred lightly in clay soil area. Of lots of factors related to the occurrence of CBW, three factors as volume weight, sand content and clay content reacted main effectiveness on CBW. The cluster analysis of the three above factors appeared to separate Tongzhou into three areas where CBW occurred, and the result of division was in accordance with the actual occurrence of CBW.