Intraseasonal variation in a population of Fountainea ryphea (Cramer) (Lepidoptera, Nymphalidae)

Abstract

ABSTRACT. A different approach was used for the key-factor method in a population study of the tropical butterfly FOlilltaillea ryphea (Cramer, [1776» (Lepidoptera, Nymphalidae), marking 20 artiticial cohorts to identify the mOl1ality levels and associated instars responsible for the variation in numbers within the season of occurrence, when generations overlap broadly. Highest mortality was detected during first instar in 13 cohorts; during second instar in three cohorts; third and fourth inslars suffered hi ghest m0l1ality twice. Results showed that first instar mOl1ality due to rainfall and predation, and parasitism on fourth instar could be the main factors promoting differences in number between cohorts throughout the season, although no density-dependent processes could be identified . KEY WORDS. Bracoll , k-factor, parasitism, predation Among the methods developed for the analysis of factors acting on populations, life tables are especially useful to evaluate mortality in each life stage for a constantly reproductive population (ROYAMA 1981). Three types of results are important: I) the determination of mortality acting on each life stage and their sources; 2) the relative importance of these sources in the density variations between generations (key-factors) and 3) the ro le of these sources in the natural regulation of density (regulation processes). Recent reviews (STILlNG 1988; HASSEL et af. 1989) show that most studies using life tables involve temperate insects, mainly Lepidoptera. They analyze both agricu ltural (BANERJEE 1979;BARLOW et al. 1986) and natural systems (BAUER 1985 ;WARREN et al. 1986), the former being dominant in number. Some of them include only immature stages due to difficulties in estimating adult populations.The main reason for the dominance of temperate insects in thi s kind of study is the fact that, in their populations, one can more easily identify generations and treat them separately . The construction of one life table for each generation is necessary for the identification of the key-factor acting on that particular species. In many tropical insects it is difficult to do the same, because generations usually overlap and even the number of generations per season is hard to be accurately identified. This restricts the use of li fe tables in these populations.A different approach was used for the key-factor method in a population study of the tropical butterfly Fountainea ,yphea (Cramer, [1776]) (Lepidoptera, Nymphalidae). The method has been originally used to identify the main factor responsible for differences in numbers between generations (MORR1S