The authors found problems caused by some methods in the literature applied to the research of economic thresholds(ET's)of the cotton bollworm, Helicoverpa armigera (Hubner),in North China.The problems were as follows:①the ET would be over estimated by taking the damage amount by a part of instars in stead of that by the whole of instars of the larvae;②the ET would be over estimated or under estimated by taking the mean damage amount per larva in the first or second half of the time duration of a gener...