Observations were carried out for exploring the factors causing field shedding of reproductive organs of cotton at square boll stage in Northern China cotton areas through a successive five years period. The results show that more than 90% of the squares and young bolls fallen off from cotton plants in this stage were due to physiological shedding, and only less than 5% of the shedding was caused by the damage of 3rd generation cotton bollworm (CBW) Helicoverpa armigera (Hubner). Based on the researche...