An attempt has been made here to predict the appearance and development of aphids on mustard crop using data collected from a field experiment conducted during rabi seasons of 2001-2005 with c.v. Varuna and 10 dates of sowing. Minimum temperature and maximum temperature showed significant negative correlation whereas morning RH and rainfall showed positive correlation with aphid population. The afternoon relative humidity did not show any association with aphids. Aphid population build up, decline and thermal time were found to be non-linearly related. Functional relations with different lead periods involving maximum, minimum and mean temperatures were developed which may be utilized in the development of DSS for eastern plains of Rajasthan.