Abstract
Infection rates of the nuclear polyhedrosis virus (NPY) in the cotton leafworm, Spodoptera littoralis larvae were assessed from 1985 through 1986 in clover fields at El-Behiera, El-Fayoum and El-Gharbia governorates. Infection rates of the pathogen were highest near the end of spring when the cotton leafworm larval populations were increasing. For the different governorates, the percentages of virus infection was almost higher in 1985-season than in 1986. El-Fayoum-larval population was the most infected by NPV followed by El-Gharbia and El-Behiera-populations, respectively. There was a positive correlation between infection rates of NPV on one hand, and density of larva and ambient temperature on the other.