Patterns of parasitism by insect parasitoids in patchy environments

Abstract. 1. This paper shows how the different spatial patterns of per cent parasitism in patches of different host density can be explained within a single model framework that takes into account the parasitoid's aggregative response, and the factors limiting the degree of host exploitation within patches.
2. Two contrasting laboratory examples are presented in which the distribution of searching parasitoids and the resulting levels of parasitism in different patches are both known for a range of parasitoid densities.
3. A model is described predicting the number of hosts parasitized per patch, in which the number of parasitoids searching is determined from a simple expression allowing different degrees of aggregation.
4. The model generates patterns of parasitism encompassing the two laboratory examples and a wide range of examples from the field.
5. The importance of density dependent spatial distributions of parasitism to population stability is briefly discussed.     

Host–parasitoid dynamics in a fragmented landscape: Holly trees,holly leaf miners and their parasitoids

Many species inhabit fragmented landscapes, where units of resource have a patchy spatial distribution. While numerous studies have investigated how the incidence and dynamics of individual species are affected by the spatial configuration and landscape context of habitat patches, fewer studies have investigated the dynamics of multiple interacting resource and consumer species in patchy landscapes. We describe a model system for investigating host–parasitoid dynamics in a patchy landscape: a network of 166 holly trees, a specialised herbivore of holly (the leaf miner, Phytomyza ilicis (Curtis, 1948)), and its suite of parasitoids. We documented patch occupancy by P. ilicis, its density within patches, and levels of parasitism over a 6-year period, and manipulated patch occupancy by creating artificially vacant habitat patches. Essentially all patches were occupied by the herbivore in each year, suggesting that metapopulation dynamics are unlikely to occur in this system. The main determinants of densities for P. ilicis and its parasitoids were resource availability (patch size and host density, respectively). While P. ilicis is apparently not restricted by the spatial distribution of resources, densities of its parasitoids showed a weaker positive relationship with host density in more isolated patches. In patches where local extinctions were generated experimentally, P. ilicis densities and levels of parasitism recovered to pre-manipulation levels within a single generation. Furthermore, patch isolation did not significantly affect re-colonisation by hosts or parasitoids. Analysing the data at a variety of spatial scales indicates that the balance between local demography and dispersal may vary depending on the scale at which patches are defined. Taken together, our results suggest that the host and its parasitoids have dispersal abilities that exceed typical inter-patch distances. Patch dynamics are thus largely governed by dispersal rather than within-patch demography, although the role of demography is higher in larger patches.     

Coevolution of Contrary Choices in Host-Parasitoid Systems

We investigate patch selection strategies of hosts and parasitoids in heterogeneous environments. Previous theoretical work showed that when host traits vary among patches, coevolved populations of hosts and parasitoids make congruent choices (i.e., hosts and parasitoids preferentially select the same patches) and exhibit direct density dependence in the distribution of percent parasitism. However, host-parasitoid systems in the field show a range of patterns in percent parasitism, while behavioral studies indicate that hosts and parasitoids can exhibit contrary choices (i.e., hosts avoid patches favored by the parasitoid). We extend previous theory by permitting life-history traits of the parasitoid as well as the host to vary among patches. Our analysis implies that in coevolutionarily stable populations, hosts preferentially select patches that intrinsically support higher host equilibrium numbers (i.e., the equilibrium number achieved by hosts when both populations are confined to a single patch) and that parasitoids preferentially select patches that intrinsically support higher parasitoid equilibrium numbers (i.e., the equilibrium number achieved by the parasitoids when both populations are confined to a patch). Using this result, we show how variation in life-history traits among patches leads to contrary or congruent choices or leads to direct density dependence, inverse density dependence, or density independence in the distribution of percent parasitism. In addition, we determine when populations playing the coevolutionarily stable strategies are ecologically stable. Our analysis shows that heterogeneous environments containing patches where the intrinsic rate of growth of the host and the survivorship rate of the parasitoid are low result in the coevolved populations exhibiting contrary choices and, as a result, promote ecological stability.     

农业景观格局与麦蚜密度对其初寄生蜂与重寄生蜂种群及寄生率的影响

农业景观格局与过程能够强烈影响寄生蜂对寄主的寻找及寄生作用,寄主密度亦是影响寄生蜂分布的重要因素,然而农业景观的格局和寄主密度对寄生蜂寄生率的相互影响是一项值得研究的工作.在简单与复杂2种麦田农业景观结构下,调查了麦蚜的分布格局与2种寄主密度下麦蚜的初寄生率与重寄生率,分析了景观结构对麦蚜密度的影响、景观格局与麦蚜密度对寄生蜂寄生率与重寄生率的影响及交互作用.结果表明:景观结构的复杂性对麦蚜分布和寄生蜂初寄生率与重寄生率的影响均不明显,但寄主密度与景观结构的复杂性对寄生蜂的影响存在着明显的交互作用,寄主密度与寄生率呈正相关,寄主密度较低时烟蚜茧蜂为优势种,寄主密度较高时燕麦蚜茧蜂为优势种.麦蚜初寄生蜂与重寄生蜂对寄主密度的反应与其形态学、体型大小以及生活史特征相关,初寄生蜂与重寄生蜂的群落组成显著影响其对麦蚜的寄生率,而与景观结构的复杂性关系不大.     

Associational resistance mediated by natural enemies

Abstract.  1. Associational resistance theory suggests that the association of herbivore-susceptible plant species with herbivore-resistant plant species can reduce herbivore density on the susceptible plant species. Several casual mechanisms are possible but none has so far invoked natural enemies. Associational resistance mediated by natural enemies was tested for by examining densities of a gall fly, Asphondylia borrichiae (Diptera: Cecidomyiidae), and levels of parasitism on two closely related seaside plants, Borrichia frutescens and Iva frutescens , when alone and when co-occurring.
2. Both Borrichia and Iva grow alone or together on small offshore islands in Florida. Each host plant species has its own associated race of fly, but both races of fly are attacked by the same four species of parasitoids. Borrichia normally has a higher density of galls than Iva , and galls are larger on Borrichia than on Iva .
3. Gall size, gall abundance, parasitism levels, and parasitoid community composition were quantified on both Borrichia and Iva on islands where each species grew alone or together. Some islands were then manipulated by adding Borrichia to islands supporting only Iva , and by adding Iva to islands supporting only Borrichia . Subsequent gall densities and gall parasitism levels on the original native species were then examined.
4. On both natural and experimentally manipulated islands, gall densities on Iva were significantly lowered by the presence of Borrichia . This is because bigger parasitoid species that were common on Borrichia galls, which are bigger, spilled over and attacked the smaller Iva galls. Thus, parasitism rates on Iva were higher on islands where Borrichia co-occurred than on islands where Borrichia were absent. Most parasitoids from Iva were too small to successfully attack the large Borrichia galls and so gall density on Borrichia was unaffected by the presence of Iva .     

Optimal use of patches by parasitoids with a limited fecundity

1. a mathematical model is presented which predicts the expected optimal-patch-use strategy for solitary parasitoids with a limited fecundity.
2. The model predicts that the quality of the patches is determined by the proportion of unparasitized hosts and not by the density of those hosts, and that throughout the searching period the parasitoids should maintain the level of parasitism equal in all the patches irrespective of the host density per patch.
3. The spatial pattern of parasitism among field patches by a parasitoid with a low fecundity, Praestochrysis shanghaiensis, was in agreement with the prediction of the model, i.e., a similar level of parasitism in different patches was observed when the ratio of female parasitoids to hosts in the whole study area exceeded 0.07. When the ratio was less than 0.05, however, the level of parasitism per patch showed an inverse relation to the host density, and was positively correlated with the female parasitoid-host ratio.
4. The model assumes that the parasitoids move between patches without cost and have perfect information about patch quality. Consideration of the cost of moving and sampling bridges the gap between the observed and predicted rates of parasitism found when the female parasitoid-host ratio in the whole study area was low

     

Patterns of parasitism by Trybliographa rapae, a cynipid parasitoid of the cabbage root fly, under laboratory and field conditions

ABSTRACT.

• 1 The spatial patterns of parasitism of the cabbage root fly caused by the cynipid parasitoid Trybliographa rapae (Westw.) have been studied in a laboratory system, within field cages and in a natural situation.
• 2 Continuous observations during the laboratory experiments showed the parasitoids to spend proportionately more time on the patches of high host density. This resulted in the per cent parasitism per patch being directly density dependent.
• 3 Similar patterns of parasitism were found from the field cage system, and also from experiments using the natural parasitoid population and either manipulated or natural host densities.
• 4 While mutual interference was marked in the laboratory experiments, there was little or no sign of it within the larger field cages.

     

“Searching time aggregation” and density dependent parasitism in a laboratory host-parasitoid interaction

Summary Assuming random search by parasitoids within host-containing patches, and a constant search rate, current host-parasitoid models suggest that searching time aggregation by parasitoids in patches of high host density should tend to produce spatially density dependent parasitism at the patch level. It is not clear, however, that statistically significant searching time aggregation necessarily implies that significant density dependent parasitism will occur. In actual host-parasitoid systems the amounts of searching time allocated to patches of equal host density may vary a great deal from patch to patch. Such behavioral variability may be capable of obscuring an underlying density dependent trend, producing density independent parasitism at the patch level despite significant searching time aggregation in patches of high host density. This possibility is tested using data from an earlier laboratory study (Morrison and Lewis; Ent. exp. et appl. 30:31–39 [1981]) of the foraging behavior of Trichogramma pretiosum Riley, a generalist parasitoid of lepidopteran eggs. Searching time allocation is found to be highly variable among patches of equal host density, and significant density dependent parasitism does not occur despite significant searching time aggregation in patches of high host density. This suggests that in cases in which density independent or density vague patterns of parasitism are observed in field samples, direct field measurements of searching time allocation in patches of different host density may be necessary to demonstrate the presence or absence of significant searching time aggregation by foraging parasitoids.     

Mating and host density affect host feeding and parasitism in two species of whitefly parasitoids

Abstract The parasitoids in the genera of Encarsia and Eretmocerus (Hymenoptera: Aphelinidae) are important biological control agents of whiteflies, and some of them not only parasitize hosts but also kill them with strong host‐feeding capacity. Two whitefly parasitoid species, Encarsia sophia and Eretmocerus melanoscutus were examined to determine if mating and host density affected their host feeding and parasitism. The whitefly host, Bemisia tabaci, was presented to these two wasp species in densities of 10, 20, 30, 40, 50 and 60 third‐instar nymphs per clip cage. Mated whitefly parasitoid females fed on more hosts than unmated females under a range of host densities (under all six host densities for En. sophia; under the densities of 40 nymphs or more for Er. melanoscutus). Meanwhile, mated females parasitized more whitefly nymphs than unmated females under all host densities for both species. With increase of host density, mated or unmated Er. melanoscutus females killed more hosts by host feeding and parasitism. Mated En. sophia females killed more hosts by host feeding with increase of host density, whereas unmated females did not parasitze whitefly nymphs at all. Our results suggest that only mated female parasitoids with host‐feeding behavior should be released in crop systems to increase their bio‐control efficiency.     

Inverse density dependent parasitism in a patchy environment: a laboratory system

Abstract. 1. Two species of parasitoids (Anisopteromalus calandrae (Howard) and Heterospilus prosopidis Vier) attacking the bruchid beetle, Callosobruchus chinensis (L.), show marked inverse density dependent relationships between per cent parasitism and host density per patch.
2. These patterns are well described quantitatively using data on the spatial distribution of searching time by the parasitoids and their attack rates on patches of different host density.
3. A model of optimal foraging predicts just the opposite (i.e. density dependent) patterns of parasitism.
4. Both density dependent and inversely density dependent spatial patterns of parasitism can be explained mechanistically in terms of (a) the allocation of searching time in patches of different host density and (b) the maximum attack rate per parasitoid that constrains the extent of host exploitation within a patch.     

Landscape structure and agricultural intensification are weak predictors of host range and parasitism rate of cereal aphids

Proportions of specialist and generalist primary parasitoids have been described by the resource breadth and the trade-off hypothesis. These alternative hypotheses predict either decreased or increased, respectively, parasitism rate of shared aphid species by specialist parasitoids. We tested both hypotheses and the confounding effects of landscape structure and agricultural intensification (AI) using extensive samplings of aphids and their parasitoids in Polish agricultural landscapes. Abundances, species composition of aphids, primary parasitoids, and parasitism rate of aphids by specialists and generalist parasitoids were analysed. Contrary to our expectations we found equally decreased parasitism rates by both types of primary parasitoids at higher aphid densities and thus proportion of specialists to generalists did not change with increasing host density. In line with the resource breadth hypothesis, specialist parasitoids had always lower abundances and parasitism rates than generalist parasitoids. Landscape diversity and agricultural intensification did not influence the host-parasitoid population dynamics. We speculate that these contrasting results could be caused by the additional density effects of secondary parasitoids. We conclude that simplistic two-trophic-level population models are not able to fully describe the complex dynamics of trophic networks. We also argue that agricultural intensification has lower effects on abundance and effectiveness of parasitoids than predicted by respective predator–prey models and empirical studies performed in controlled and artificial conditions.     

Impact of cocoon predation and parasitism on endemic populations of the common pine sawfly, Diprion pini (L.) (Hymenoptera, Diprionidae) in different forest types

1 Predation and parasitism on litter‐buried cocoons of the common pine sawfly Diprion pini (L.) were compared in different forest types with endemic sawfly populations by field exposure of laboratory‐reared cocoons during three consecutive years (1993–1995). 2 The impact of cocoon predation was dependent on season and forest type. The highest predation (up to 95%) was found during autumn in forest stands with a dense understory vegetation. 3 Cocoon parasitism varied between year, season and forest type. The highest parasitoid attack was observed in pure pine forests with more or less barren soils, but did not exceed 24% of exposed cocoons. 4 Cocoons were exposed in small patches. Predators tended to exploit all cocoons of a patch, whereas parasitoids only attacked a few cocoons of a patch. Predation was similar on cocoons placed in the litter and those buried more deeply in the soil, whereas parasitism of soil‐buried cocoons was rare. 5 These results indicate that predators can have a remarkable potential for limiting endemic sawfly densities, if habitat conditions in a forest maintain their population and support their foraging behaviour. A notable effect of parasitoids on sawfly cocoons deposited in the litter is obviously restricted to typical pure and barren pine forests, but may play there a similar role as predation.     

Cultivar‐specific plant odour preferences of a generalist aphid parasitoid Aphidius colemani and a possible mechanism for maternal priming of resistance to toxic plant chemistry

Aphidius colemani Viereck, emerging from Myzus persicae (Sulzer) mummies on the Brussels sprout cultivar ‘Bedford Winter Harvest’ (BWH), responds positively in the olfactometer to the odour of that cultivar in comparison with air. Responses to the odours of other sprout cultivars, cabbage and broad bean could be explained by the humidity from plant leaves. In a choice between BWH and other sprout cultivars, the BWH odour is preferred, or that of cv. ‘Red Delicious’ (RD) if the parasitoids are reared on RD. This confirms previous work showing that the secondary chemistry of a cultivar is learnt from the mummy cuticle during emergence. Adults emerging from pupae excised from the mummy show a similar but less pronounced preference. Parasitoids developing in aphids on an artificial diet do not discriminate between the odours of BWH and RD, unless allowed contact with a mummy from the same cultivar that the mother develops on. This suggests a cultivar‐specific maternal cue. This cue is speculated to consist of a small amount of the secondary chemistry (probably glucosinolates in the present study) that are left in or on the egg at oviposition, which subsequently induces enzymes that detoxify plant‐derived toxins in the aphid host. Indeed, when parasitoids emerging from diet‐reared aphids are released on aphid‐infested sprout plants, fewer mummies are produced than by parasitoids emerging from mummies of plant‐reared aphids or from excised pupae. Only parasitoids that emerge from mummies of plant‐reared aphids prefer the cultivar of origin as shown by the number of mummified hosts.     

Phenological diversity in the interactions between winter moth (Operophtera brumata) larvae and parasitoid wasps in sub-arctic mountain birch forest

Population cycles of the winter moth (Operophtera brumata) in sub-arctic coastal birch forests show high spatiotemporal variation in amplitude. Peak larval densities range from levels causing little foliage damage to outbreaks causing spatially extensive defoliation. Moreover, outbreaks typically occur at or near the altitudinal treeline. It has been hypothesized that spatiotemporal variation in O. brumata cycle amplitude results from climate-induced variation in the degree of phenological matching between trophic levels, possibly between moth larvae and parasitoids. The likelihood of mismatching phenologies between larvae and parasitoids is expected to depend on how specialized parasitoids are, both as individual species and as a guild, to attacking specific larval developmental stages (i.e. instars). To investigate the larval instar-specificity of parasitoids, we studied the timing of parasitoid attacks relative to larval phenology. We employed an observational study design, with sequential sampling over the larval period, along an altitudinal gradient harbouring a pronounced treeline outbreak of O. brumata. Within the larval parasitoid guild, containing seven species groups, the timing of attack by different groups followed a successional sequence throughout the moth's larval period and each group attacked 1-2 instars. Such phenological diversity within parasitoid guilds may lower the likelihood of climate-induced trophic mismatches between victim populations and many/all of their enemies. Parasitism rates declined with increasing altitude for most parasitoid groups and for the parasitoid guild as a whole. However, the observed spatiotemporal parasitism patterns provided no clear evidence for or against altitudinal mismatch between larval and parasitoid phenology.     

Indirect competitive effects of stemborers on a gall community

Interspecific competition between phytophagous insects using the same host plant occurs frequently and can strongly affect population densities of competing species. Competition between gallmakers and stemborers could be especially intense because both types of herbivore are unable to avoid competition by relocation during their immature stages. For apical meristem gallmakers the main result of competition is likely to be the interruption of resources to the gall by the stemborers' devouring of stem contents. The proximate effect of such competition could be to reduce gall size, thereby increasing the number of chambers per gall unit volume, and reducing the size and potential reproductive output of the gallformer. In addition, smaller galls may be more susceptible to attack from size‐limited parasitoids, resulting in a second indirect effect of competition. Using a community of galling and stemboring insects on the saltmarsh shrub Iva frutescens L. (Asteraceae), we measured for indirect effects of competition. We examined the primary indirect effect of competition on gall midge crowding and the secondary effects on parasitism rates and parasitoid guild composition. Results indicated that galls co‐occurring with stemborers were smaller, crowding of gall inhabitants was 22% greater, and the composition of the parasitoid guild was altered relative to galls on unbored stems. The overall parasitism rate was not different between galls on bored vs. unbored stems. These results show that competition resulting from the presence of stemborers has the potential to affect the gall midge Asphondylia borrichiae Rossi & Strong (Diptera: Cecidomyiidae) and secondarily to affect its guild of hymenopteran parasitoids.     

Changes in function and temporal variation in a guild of gall‐parasitoids across a temperature gradient in Australian subtropical rainforest

Parasitoids play an important role in ecosystem functioning through their influence on herbivorous insect populations. Theoretical and experimental evidence suggest that increased species richness can enhance and stabilize ecosystem function. It is important to understand how richness‐driven functional relationships change across environmental gradients. We investigated how temperature affected the relationship between parasitoid richness and parasitism rate in a guild of gall‐parasitoids along an elevational gradient. We collected galls at 15 sites along five elevational gradients (between 762 m and 1145 m asl) on six occasions over a year. A total of 1902 insects, including 1593 parasitoids, were reared from 12 402 galls. Parasitism rate increased significantly with temperature on all sampling occasions, except December and February. We found a significant, positive richness–parasitism relationship. This relationship, however, was weaker at higher elevations which may be linked to decreased functional efficiency of parasitoids at lower temperatures. Temporal variability in parasitism rate and parasitoid richness were significantly related, regardless of temperature. A stable functional guild of this kind may provide a more reliable ecosystem service under environmental changes.     

The effects of plant dispersion and prey density on parasitism rates in a naturally patchy habitat

Despite extensive research on parasitoid-prey interactions and especially the effects of heterogeneity in parasitism on stability, sources of heterogeneity other than prey density have been little investigated. This research examines parasitism rates by three parasitoid species in relationship to prey density and habitat spatial pattern. The herbivore Itame andersoni (Geometridae) inhabits a subdivided habitat created by patches of its host plant, Dryas drummondii, in the Wrangell Mountains of Alaska. Dryas colonizes glacial moraines and spreads clonally to form distinct patches. Habitat subdivision occurs both on the patch scale and on the larger spatial scale of sites due to patchy successional patterns. Itame is attacked by three parasitoids: an ichneumonid wasp (Campoletis sp.), a braconid wasp (Aleiodes n. sp.), and the tachinid fly (Phyrxe pecosensis). I performed a large survey study at five distinct sites and censused Itame density and parasitism rates in 206 plant patches for 1–3 years. Parasitism rates varied with both plant patch size and isolation and also between sites, and the highest rates of overall parasitism were in the smallest patches. However, the effects of both small- and large-scale heterogeneity on parasitism differed for the three parasitoid species. There was weak evidence that Itame density was positively correlated with parasitism for the braconid and tachinid at the patch scale, but density effects differed for different patch sizes, patch isolations, and sites. At the site scale, there was no evidence of positive, but some indication of negative density-dependent parasitism. These patterns do not appear to be driven by negative interactions between the three parasitoid species, but reflect, rather, individual differences in habitat use and response to prey density. Finally, there was no evidence that parasitism strongly impacted the population dynamics of Itame. These results demonstrate the importance of considering habitat pattern when examining spatial heterogeneity of parasitism and the impacts of parasitoids. Received: 3 June 1999 / Accepted: 4 October 1999     

Non-random distribution among a guild of parasitoids: implications for community structure and host survival

Abstract 1. Immature stages of the gall midge, Asphondylia borrichiae, are attacked by four species of parasitoids, which vary in size and relative abundance within patches of the gall midge’s primary host plant, sea oxeye daisy (Borrichia frutescens). 2. In the current study, a bagging experiment found that the smallest wasp, Galeopsomyia haemon, was most abundant in galls exposed to natural enemies early in the experiment, when gall diameter is smallest, while the wasp with the longest ovipositor, Torymus umbilicatus, dominated the parasitoid community in galls that were not exposed until the 5th and 6th weeks when gall diameter is maximal. 3. Moreover, the mean number of parasitoids captured using large artificial galls were 70% and 150% higher compared with medium and small galls respectively, while stem height of artificial galls significantly affected parasitoid distribution. Galls that were level with the top of the sea oxeye canopy captured 60% more parasitoids compared with those below the canopy and 50% more than galls higher than the plant canopy. 4. These non‐random patterns were driven primarily by the differential distribution of the largest parasitoid, T. umbilicatus, which was found significantly more often than expected on large galls and the smallest parasitoid of the guild, G. haemon, which tended to be more common on stems level with the top of the plant canopy. 5. Large Asphondylia galls, especially those located near the top of the Borrichia canopy, were more likely to be discovered by searching parasitoids. Results using artificial galls were consistent with rates of parasitism of Asphondylia galls in native patches of sea oxeye daisy. Gall diameter was 19% greater and the rate of parasitism was reduced by almost 50% on short stems; as a result, gall abundance was 24% higher on short stems compared with ones located near the top of the plant canopy. 6. These results suggest that parasitoid community composition within galls is regulated by both interspecific differences in ovipositor length and preferences for specific gall size and/or stem length classes.     

Learning pays off: influence of experience on host finding and parasitism in Lariophagus distinguendus

Abstract. 1. The influence of experience on egg maturation, parasitism rate, and behaviour during host searching was investigated for Lariophagus distinguendus (Först.) (Hymenoptera: Pteromalidae) parasitizing larvae of the granary weevil Sitophilus granarius (L.) in grains of wheat Triticum aestivum L.
2. Dissection of female parasitoids and parasitism bioassays at high host density revealed that experience with hosts (e.g. by oviposition or by host feeding) is not required either for triggering oogenesis or for oviposition.
3. In parasitism experiments at low host density, when single host-infested grains were offered within a bulk of healthy grains, host finding and parasitism rate were increased by experience.
4. Behavioural observations revealed that searching time required for finding an infested grain was shorter for experienced parasitoids than for naive parasitoids, because travel time from grain to grain is shorter for experienced parasitoids, and because experienced parasitoids spend less time than naive parasitoids on non-infested grains.
5. In conclusion, experience due to host exposure increases parasitism and thereby the fitness of the parasitoids. It is discussed that this increase is more likely due to learning than to different egg load dynamics of experienced parasitoids.     

Shifting preference between oviposition vs. host-feeding under changing host densities in two aphelinid parasitoids

Destructive host-feeding is common in hymenopteran parasitoids. Such feeding may be restricted to host stages not preferred for oviposition. However, whether this is a fixed strategy or can vary according to resource levels or parasitoid needs is less clear. We tested the trade-off between host feeding and oviposition on two whitefly parasitoids under varying host densities. Females of two aphelinid parasitoids, Eretmocerus hayati and Encarsia sophia were exposed to nine different densities of their whitefly host, Bemisia tabaci, in single-instar tests to identify their functional response. Mixed-instar host choice tests were also conducted by exposing whiteflies at four densities to the parasitoids. We hypothesized that the parasitoid females can detect different host densities, and decide on oviposition vs. host-feeding accordingly. The results showed that both Er. hayati and En. sophia females tended to increase both oviposition and host-feeding with increased host density within a certain range. Oviposition reached a plateau at lower host density than host-feeding in Er. hayati, while En. sophia reached its oviposition plateau at higher densities. At low densities, Er. hayati parasitized most on first and second (the optimal ones), and fed most on third nymphal instars (the suboptimal one) of the whitefly host as theory predicts, while at high densities, both parasitism and host-feeding occurred on first and second instars which are preferred for oviposition. En. sophia parasitized most on third and fourth (the optimal ones), while fed on first instars (the suboptimal one) at low densities, and utilized third and fourth instars for both at high densities. In conclusion, oviposition vs. host-feeding strategy of parasitoid females was found to vary at different host densities. The balance between reserving optimal hosts for oviposition or using them for host-feeding depended on parasitoid life history and the availability of host resources.