Aspects of the Regulation of Size of the Jamaican Population of Sceliphron assimile Dahlbom (Hymenoptera: Sphecidae)

Abstract

The mud-nesting sphecid S. assimile Dahlbom was studied in Jamaica by estimating the density of its disused cells at 286 sites and dissecting samples of them at 116 of these in order to assess the stage at death and its cause. Mean mortality during development varied from 11.4% at sites with the least density of cells (< 1/m2) to 55.6% at those with the greatest (> 100/m2). Such mortality for Jamaica as a whole was 38.88% and of this, 73.5% was due to the parasitoid Melittobia sp. (hawaiiensis complex). These values are similar to those in a preliminary study (Freeman 1973). They do not provide a mechanism whereby the observed densities could be regulated. Harsh physical factors (high altitude, high annual precipitation) were associated with areas where low densities of Sceliphron occurred but also with low developmental mortality. Minimum values for emigration and/or mortality of adults were higher in low-density areas (82.4%) than in high-density areas (33.8%). Minimum fecundity (approximately equal to the number of cells per nest) varied between only 6.15% and 7.45% in the areas where different densities occurred. There is circumstantial evidence that the easterly trade and daytime on-shore winds systematically affect parasitization of Sceliphron by Melittobia: percentage parasitization was greater than expected away from the coast and in the west. The presence of inquiline aculeate species in the disused cells of Sceliphron did not seem to increase the level of parasitization by Melittobia on the host species itself, although they themselves suffered high mortality from this parasitoid. A modified k value budget (Varley and Gradwell 1968) is used to show that over many generations pre-reproductive mortality of the females and the loss of fecundity are quantitatively the most important factors in the population dynamics of the species, and that they have fundamental implications in its life-style. It is possible to reconcile some of the tenets of Nicholson and of Andrewartha and Birch.