Abstract:
Following outbreaks of Papilio demodocus populations on citrus plantations and the consequent use of expensive control methods in southern Africa the Project aim was to evaluate the natural regulating factors regulating the populations thus contributing to a more efficient pest control strategy in the Umbeluzi region of Mozambique. Field studies were carried out during the period March 1998 to November 1999 in the sites INIA, University Campus and LOMACO citrus orchards < 2 years old in Umbeluzi while laboratory research was conducted at the Faculty of Agronomy and Forest Engineering of the Eduardo Mondlane University at Maputo. The population distribution studies carried out in treated citrus plantation showed that P. demodocus egg population distribution was aggregated and that the coefficients a and b of Taylor’s Power Law were respectively 0.28 and 1.12. Peak P. demodocus activity occurred between October and March, the rainy season. Generation time was 52 days, evolutive cycle lasted 41 days with a sex ratio of 1:1. During the rainy season the population net reproductive rate and capacity for increase were respectively 0.855 and – 0.031. During the dry season these values were 1.467 and 0.005. Fecundity differed between seasons. 30 cohort life tables were produced from data collected in pesticide free orchards using trials established at INIA, University Campus and LOMACO 1° Maio field. The citrus plants utilized were C. sinensis cv. Valencia, C. paradise cv. Marsh and cv. Star Ruby. The survival curves only differed between the dry and rainy seasons (P<0.05) but not between cultivars or sites. Larval disappearance at the second and fourth larval instars was the most important mortality factor but larval disappearance was not density-dependent. In general, mortality factors were not related to environmental factors. The effect of the initial population size on its changes between generations indicated density dependent processes between generations. Predators and parasitism under natural field conditions were seldom observed.
