Estimating the abundance of adults and immatures

At the single field or farm level, pest managers who consider how Heliothis spp. affect their crops need current information on Heliothis numbers on which to base management decisions and assess the efficacy of past control tactics (Wilson and Room, 1982). At the farming district or regional level, population researchers who consider how crops and control tactics affect Heliothis must assess population densities of Heliothis developmental stages continually. From these data, calculation of current population numbers may be used to estimate, for example, birth and survival rates or assess the worth of alternative control strategies. In all but the smallest areas of interest (for example, a leaf) we can never know exactly how many Heliothis are present, and we must estimate thelr density by sampling. Such estimates may be either absolute, expressing Heliothis densities as numbers per unit area; or relative, measuring population in "unknown" units that allow only temporal and/or spatial comparisons. The latter are either catch per unit effort or trapping methods, the results of which can depend on factors other than population size (Southwood, 1978). The distinction between absolute and relative methods is not always clear, as absolute estimates are usually less than 100% correct al).d relative estimates can be very accurate or adjusted to give absolute estimates (Southwood, 1978).