Distribución de los ataques del parasitoide Trichopoda giacomellii (Diptera: Tachinidae) sobre una población de Nezara viridula (Hemiptera: Pentatomidae)

Abstract

In the northeast of the Buenos Aires province, parasitism by Trichopoda giacomellii is one of the most important mortality factors for N. viridula. Different relative densities and difterent susceptibility to parasitism among hosts, and a clumped .spatial distribution of hosts and/or parasitoids are mechanisms that could account for the observed persistence and stability of the system, since they would generate a skewed distribution of attacks among hosts. In order to analyze the distribution of attacks among hosts as a function of both mechanisms this work studied (1) the proportion of parasitoid eggs stuck on hosts with different susceptibility to parasitism (grouped as nymphs, adult females, adult males) and (2) the distribution of eggs among hosts of each group. In the first case, the results showed that a greater proportion of attacks were concentrated on adults, rather than other developmental stages, and moles rather than females. In the second case, showed that the distribution of the number of eggs/host fitted a negative binomial distribution. The aggregative parameter k was ¡n general < 1, indicating that while the majority oj’hosts were not parasitized, only a small number was attacked 8 or more turres. The relationship between k and host density also suggest that the attacks tend to be more randomly distributed when host density increases. Despite the intense parasitism by Tricopoda giacomellii, both mechanisms will produce a well skewed distribution of attacks among hosts acting as a partial refuge for hosts with a lesser preference by the parasitoid or those located in patches of low density. In this way, these hosts would be attacked sufficiently late as to oviposit at least once, this would explain the persistence of the system.