Finite rates of population increase (lambda) of a pesticide susceptible strain of Oryzaephilus surinamensis (L.) living in kibbled wheat were determined at 20, 22.5, 25, 30, 32.5 and 35-degrees-C at 30, 50 and 70% relative humidity (r.h.) using survivorship and fertility schedules, i.e. observed numbers of adult offspring produced by cohorts of insects over a physiological time frame of twice the immature development period at each condition. Population growth rate increased with temperature up to 30-degrees-C at 30% r.h. and up to 32.5% at 50 and 70% r.h. Populations subjected to drier conditions grew slower than those subjected to moister conditions. The greatest value of lambda (2.4122) was observed at 32.5-degrees-C/70% r.h. The rate of population increase declined rapidly after a maximum value was reached and at 35-degrees-C/30% r.h. a negative rate of increase was observed. The value of lambda at low temperatures was little affected by relative humidity with a minimum theoretical population growth threshold varying from 19.0 to 19.4-degrees-C between 30 and 70% r.h. Egg to adult development and immature age-specific mortality were examined in detail at 20, 30 and 35-degrees-C at 30, 50 and 70% r.h. Minimum mortality occurred at 30 and 35-degrees-C at 70% r.h. and maximum mortality occurred at 20 and 35-degrees-C at 30% r.h. Maximum mortality accounted for almost complete extinction of the immature population and occurred mainly during the egg stage. At increasingly suboptimum conditions, a larger proportion of surviving immatures required an extra instar before pupation. Demographic data for O. surinamensis were compared with other available data for the species, as well as with similar data for Sitophilus oryzae and Rhyzopertha dominica. Concerns were raised about relating estimates of population growth of O. surinamensis to wet bulb temperature. It was considered that relating population growth to dry bulb temperature and relative humidity or equilibrium grain moisture content, in the conventional manner, was more appropriate for this insect.
