Reproduction now or later: optimal host‐handling strategies in the whitefly parasitoidEncarsia formosa

Abstract

We developed a dynamic state variable model for studying optimal host-handling strategies in the whitefly parasitoid Encarsia formosa Gahan (Hymenoptera: Aphelinidae). We assumed that (a) the function of host feeding is to gain nutrients that can be matured into eggs, (b) oögenesis is continuous and egg load dependent, (c) parasitoid survival is exponentially distributed and (d) parasitoids encounter hosts randomly, are autogenous and have unlimited access to non-host food sources to obtain energy for maintenance and activity. The most important prediction of the model is that host feeding is maladaptive under field conditions of low host density (0.015 cm (2 ) and short parasitoid life expectancy (maximum reproductive period of 7 days). Nutrients from the immature stage that can be matured into eggs are sufficient to prevent egg limitation. Both host density and parasitoid life expectancy have a positive effect on the optimal host-feeding ratio. Parasitoids that make random decisions gain on average only 35% (0.015 hosts cm (2 ) to 60% (1.5 hosts cm (2 ) of the lifetime reproductive success of parasitoids that make optimal decisions, independent of their life expectancy. Parameters that have a large impact on lifetime reproductive success and therefore drive natural selection are parasitoid life expectancy and the survival probability of deposited eggs (independent of host density), the number of host encounters per day (when host density is low) and the egg maturation rate and number of host types (when host density is high). Explaining the evolution of host-feeding behaviour under field conditions requires field data showing that life expectancy in the field is not as short as we assumed, or may require incorporation of variation in host density. Incorporating variation in walking speed, parasitised host types or egg resorption is not expected to provide an explanation for the evolution of host-feeding behaviour under field conditions. Natural selection favours organisms that behave in a way that maximises their lifetime reproductive output. Insect parasitoids are good model systems to study evolution of behaviour because of the direct link between behaviour and reproductive output (Stephens and Krebs 1986, Godfray 1994). Host feeding behaviour is the consumption of host body fluids by the adult female parasitoid (Flanders 1942). It provides nutrients that allow parasitoids to mature eggs or to increase longevity (Jervis and

Whitefly control in cut gerbera: is it possible to control Trialeurodes vaporariorum with Encarsia formosa?

“…3). This paradox is likely due to host feeding behavior of adult E. formosa, whereby females ingest hemolymph that exudes from wounds created by piercing the host with her ovipositor (Flanders 1942;Jervis and Kidd 1986;Heimpel and Collier 1996;Hoddle et al 1998;Burger et al 2004). Host feeding provides nutrients that enhance egg maturation and/or longevity of female parasitoids and directly contributes to a reduction in the number of whiteflies (Jervis and Kidd 1986;Heimpel and Collier 1996).…”

Natural history of whitefly in Costa Rica: an evolutionary starting point

“…(2001) found that adult T. vaporariorum aggregated in patches of 12.5 m. Whereas most of these studies were done to develop sampling schemes for monitoring biological control programmes (Ekbom & Xu, 1990), these data provide insight in the natural environment and help to improve understanding of both biological control and evolutionary ecology. Further work will predict (Burger et al ., in press) and test (Burger et al. , in prep.)…”

“…Even extremely synovigenic species that have limited egg storage capacity like E. formosa (Jervis et al ., 2001) are unlikely to become egg limited at this host encounter probability. Investment in future reproduction through host feeding therefore does not seem adaptive (Burger et al ., in press). Some optimisation models have indeed predicted that the critical egg load at which parasitoids should host feed decreases towards very low host encounter rates (Jervis & Kidd, 1986; Chan & Godfray, 1993; Collier, 1995).…”

Temporal variation and the evolution of a parasitoid foraging cue

“…Although this model was inspired by experimental studies on the duration of learned foraging odors, the decision‐making process of parasitoids may be far more complex, involving a constant tracking and calculation of foraging quality and quantity. For example, dynamic optimization models have been used to successfully model the foraging decisions of the parasitoid Trichogramma thalense on variable quality moths (Keasar et al 2001) and less successfully to model the evolution of traits in Encarsia formosa (Burger et al 2004). In addition, parasitoid survival may differ between the host populations.…”