The effect of varying Bemisia argentifolii and Eretmocerus mundus ratios on parasitism

We investigated the effects of different host: parasitoid ratios on the efficacy of the parasitoid Eretmocerus mundus attacking the silverleaf whitefly, Bemisia argentifolii. When host density was held constant (100 second instars) and parasitoid density was decreased from 15 to 1 females, the percentage of total host mortality was significantly lower at low parasitoid densities. The number of host nymphs killed, and the number of female parasitoid progeny per female, increased 3.6 and 20.4 times, respectively. The emergence rate, sex ratio, longevity, and body lengths of progeny were significantly larger at the lowest parasitoid density while developmental time was significantly shorter. When the number of hosts was increased from 5 to 250 and parasitoid density was held constant (5 females), the percentage of nymphal mortality decreased 1.6 times. The percentage of desiccated nymphs was significantly highest (65.7%) at the lowest host density, while percentage parasitism (34.3%) was significantly lowest at the lowest host density. The data could be described using a Type I functional response curve. We propose a generalized index of efficacy (GIE) to summarize and compare the total effects of parasitoid-host ratios. This index showed that the most efficient ratio was one parasitoid female per ten second instar host nymphs.