The point location of insect is randomly and independently infected by the infectious pathogens. After the multiple-infection has been taken into account in total point locations

the infection threshold(TI) of the pathogen can be calculated. The results shown that the TI of nuclear polyhedrosis virus of Spodoptera litura F. (SINPV) was positively related to temperature and larval instars. Under the temperature of 20℃ to 40℃

the TI for 3rd instar were between 9 090. 9 PIBs and 666 666. 7 PIBs per larvae

and for 5th instar were between 131 578. 9 PIBs and 714 285. 7 PIBs per larvae. The TI differences between different instars were gradually reduced with the rising temperature. In some inoculum numbers

the infection of SINPV have certain self-antagonism and selfmutualism. As the indices describing infection threshold

pathogeneicity

and disease-resistance of insect

the TI is superior to LD50 and LC50.