How to behave? : evolution of host-handling behaviour in the whitefly parasitoid Encarsia formosa

J.M.S. Burger

Research output: Thesisinternal PhD, WU

Abstract

<p>The main aim of evolutionary ecology is to explain the adaptation of form, function and behaviour of organisms to their environment. In this thesis, I studied host-handling behaviour of the whitefly parasitoid <em>Encarsia formosa</em> form such an evolutionary point of view. This parasitoid is applied as biological control agent against whitefly pests in greenhouses. Previous studies revealed mechanistic explanations for the parasitoid's behaviour. Here I focussed on functional explanations for the parasitoid's decision to reject, feed upon or parasitise a host. Since host feeding yields nutrients for egg maturation but destroys an opportunity to oviposit, the decision between host feeding and oviposition reflects a life-history trade-off between current and future reproduction.</p><p>First, manipulation experiments were conducted to seek for evolutionary benefits of destructive host feeding over non-destructive feeding on host-derived honeydew. The first experiments suggested that honeydew could be an advantageous alternative to host feeding. Honeydew did have a positive effect on egg load and estimated number of eggs matured within 20 and 48 hours, whereas a host feeding did not. Host feeding did not positively affect survival when honeydew was supplied. In the second experiments the observation period was extended. Even in the presence of honeydew, host feeding did have a positive effect on the number of ovipositions per hour of foraging per host-feeding attempt, without affecting parasitoid survival or egg volume.</p><p>To understand the conditions under which <em>E. formosa</em> has evolved, fieldwork was carried out in Costa Rica to quantify natural whitefly densities and distributions. The number of hosts on the lower side of a leaflet of an average plant within an average spot along an average transect could be described by a Poisson distribution with mean and variance equal to 0.241. This mean was most variable at plant level. Spatial dependence between numbers of whiteflies on leaves was detected within individual plants and within a 100-m transect. Thus, host density in the field was low compared with pest densities, but aggregation occurred at several spatial scales.</p><p>A dynamic state-variable model was developed to predict optimal host-handling decisions, which maximise lifetime reproductive success, in relation to host density and parasitoid's life expectancy. Random decisions resulted in only 35 to 60 % of the lifetime reproductive success from optimal decisions. Host feeding was predicted to be maladaptive at presumable field conditions of low average host density and short parasitoid's life expectancy. Nutrients from the immature stage should be sufficient to prevent egg limitation. Both host density and parasitoid's life expectancy had a positive effect on the optimal host-feeding ratio. Explaining evolution of host-feeding behaviour under natural conditions may require incorporation of variation in host density, incorporation of parasitised host types or field data showing that parasitoid's life expectancy in the field is longer than assumed.</p><p>In a semi-field set-up, parasitoids were allowed to forage at field host density in either a uniform or an aggregated host distribution. Contrary to predictions by the optimal-foraging model, parasitoids did host feed, i.e. upon about 11 % of the accepted hosts, even when host aggregation was ignored. In the model, host encounter rate was underestimated from host density. Nevertheless, there was also a host distribution by time interaction on host-feeding tendency. Host encounter decreased the leaving tendency on an average leaflet when time since latest host encounter was short, but increased the leaving tendency when time since latest host encounter was long, independent of host distribution. This suggested that parasitoids can forage efficiently at different host distributions.</p><p>The value of future reproduction and the decision whether to host feed or parasitise strongly depends on the parasitoid's life expectancy. I hypothesised that a parasitoid's life expectancy in the laboratory is an overestimate of a parasitoid's life expectancy in the field. Several age determination methods were tested, using pteridine, wing fray and relative residual longevity. Unfortunately, none proved a useful technique to estimate the age of a field-caught specimen.</p><p>The main conclusion is that host-handling behaviour in the whitefly parasitoid <em>Encarsia formosa</em> may have evolved as an adaptation to a spatially and possibly temporally heterogeneous environment. To truly understand the evolution of host-handling behaviour, future studies should focus on physiological, biochemical and genetic mechanisms; the effect of spatial and temporal heterogeneity in host availability on the risk of egg limitation; and the parasitoid's life expectancy in the field.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • van Lenteren, Joop, Promotor
  • Vet, Louise, Promotor
  • Hemerik, Lia, Promotor
Award date10 Dec 2002
Place of PublicationS.l.
Print ISBNs9789058087430
Publication statusPublished - 2002

Keywords

  • encarsia formosa
  • biological control agents
  • insect pests
  • aleyrodidae
  • parasitoids
  • host-seeking behaviour
  • feeding behaviour
  • evolution

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