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.     

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.     

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     

A hierarchical multi‐scale analysis of the spatial relationship between parasitism and host density in urban habitats

Studies on spatial density dependence in parasitism have paid scarce attention to how changes in host density at different hierarchical scales could influence parasitism in an herbivore at a particular scale. Here, we evaluated if rates of parasitism per leaf (by the whole parasitic complex and by dominant species) of the specialist leaf miner Liriomyza commelinae (Diptera: Agromyzidae) respond to variations in host density at the leaf, plant patch and site levels in an urban setting. We used multi‐level Bayesian models that incorporate the spatial hierarchy occurring in this system, as well as habitat factors previously found to have an effect on the L. commelinae parasitoid community in an urban context (patch size, patch isolation and urbanization level). According to the fitted model, overall parasitism rates decreased with increasing number of mines per leaf, being independent of host‐density variations at patch and site level. Patch structure was found to have a strong effect on parasitism rates per leaf. The analysis of parasitism by parasitoid species separately showed consistent results with the response at community level. These results suggest that parasitism of the parasitoid community here studied would be sensitive to hierarchical cues related to the host at the leaf level and to the host habitat at the patch level.     

Superparasitism as an ESS: to reject or not to reject, that is the question

A stochastic model is formulated to determine the optimal strategy for a solitary parasitoid which has discovered an already parasitized host. The model assumes that the parasitoid can count both the number of eggs already present in a host and the number of conspecifics searching in the same patch. The survival probability of an egg is assumed to depend on the total number of eggs in a host. The decision to (super)parasitize depends both on the degree to which the discovered host already is parasitized and on the number of conspecific females searching in the same patch. We consider both the case that egg laying does not involve any costs for the parasitoid and the case that it involves some marginal costs. Uniform behaviour of all the conspecific parasitoids in a patch, i.e. laying one additional egg in all encountered larvae containing a particular number of eggs, appears to be a pure evolutionary stable strategy (ESS). If either the probability that a parasitoid emerges from a host decreases with an increasing degree of parasitism, at least from a particular number of eggs onwards, or if parasitism involves marginal costs, the maximum number of eggs for which it is still profitable to superparasitize a host once more is limited. This number increases with the number of conspecifics searching in the patch. Large marginal costs (i.e. the expected gain of not parasitizing now) decrease the profit of superparasitism. For newly emerged parasitoids the rejection of an already parasitized host is not advantageous as long as the marginal costs of parasitism are small, because the host can never contain an egg of its own.     

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.     

Nectar provisioning close to host patches increases parasitoid recruitment,retention and host parasitism

In the adult stage, many parasitoids require hosts for their offspring growth and plant-derived food for their survival and metabolic needs. In agricultural fields, nectar provisioning can enhance biological control by increasing the longevity and fecundity of many species of parasitoids. Provided in a host patch, nectar can also increase patch quality for parasitoids and affect their foraging decisions, patch time residence, patch preference or offspring allocation. The aim of this study was to investigate the impact of extrafloral nectar (EFN) provisioning close to hosts on parasitoid aggregation in patches. The aphid parasitoid Diaeretiella rapae (M’Intosh) was released inside or outside patches containing Brassica napus L. infested by Brevicoryne brassicae L. aphids and Vicia faba L. with or without EFN. When parasitoids were released outside patches, more parasitoids were observed in patches with EFN than in patches deprived of EFN. This higher recruitment could be linked to a higher attraction of a combination of host and food stimuli or a learning process. A release–recapture experiment of labeled parasitoids released within patches showed the higher retention of parasitoids in patches providing EFN and hosts, suggesting that food close to the host patch affects patch residence time. Both attractiveness and patch retention could be involved in the higher number of parasitoids foraging in host patches surrounded by nectar and for the higher parasitism recorded. Nectar provisioning in host patches also affected female offspring allocation inside the patch.     

Parasitoids of grass-feeding chalcid wasps: a comparison of German and British communities

We compared the parasitoid communities associated with grass-feeding herbivores in Germany and Britain to examine geographical consistency in community composition and to test ecological characteristics of the plants and host insects that may explain variability in parasitoid community structure. The parasitoid communities of 16 chalcid wasps feeding on ten grass species were sampled between 1986 and 1989 at 4-11 sites per grass species in southwest Germany. The data were compared to published data from Great Britain, comprising 18 chalcid hosts on ten grass species sampled between 1980 and 1992 at 24 sites in Wales and England. Results showed that many conclusions drawn from patterns in Britain did not hold for Germany, emphasizing the need to repeat analyses in different geographical regions. The parasitoid communities of the Tetramesa hosts included on average 8.1 parasitoid species in Germany, while the British hosts supported only 4.1 parasitoids. The number of monophagous parasitoid species was similar in both areas (2.4 vs 3.2), but German host populations supported many more polyphagous species (5.1 vs 0.9). This difference reinforces the earlier conclusion that parasitoid communities in Britain are highly undersaturated. Increased numbers of parasitoid species in Germany did not result in increased parasitism rates, so the closer species packing was paralleled by reduced impact of each species. In Germany, percent parasitism (range: 5-74%) was closely correlated with log host density, explaining 90% of the variance, while in Great Britain, percent parasitism was less variable (range: 36-76%) and was not related to host density or other host or host plant characteristics. Gallers and non-gallers supported equal numbers of parasitoids in both Germany and Britain, offering support for neither the enemy hypothesis of the adaptive nature of plant galls nor for the finding that galls are often more susceptible to enemy attack than their non-galling relatives. Furthermore, gregarious Tetramesa hosts were not attacked by more parasitoid species than solitary hosts.     

Spatial ecology of host–parasitoid interactions: a threatened butterfly and its specialised parasitoid

Habitat conservation for threatened temperate insect species is often guided by one of two paradigms: a metapopulation approach focusing on patch area, isolation and number; or a habitat approach focusing on maintaining high quality habitat for the focal species. Recent research has identified the additive and interacting importance of both approaches for maintaining populations of threatened butterflies. For specialised host-parasitoid interactions, understanding the consequences of habitat characteristics for the interacting species is important, because (1) specialised parasitoids are particularly vulnerable to the consequences of fragmentation, and (2) altered interaction frequencies resulting from changes to habitat management or the spatial configuration of habitat are likely to have consequences for host dynamics. The spatial ecology of Cotesia bignellii, a specialist parasitoid of the threatened butterfly Euphydryas aurinia, was investigated at two spatial scales: within habitat patches (at the scale of individual aggregations of larvae, or ‘webs’) and among habitat patches (the scale of local populations). Parasitism rates were investigated in relation to larval web size, vegetation sward height and host density. Within patches, the probability of a larval webs being parasitized increased significantly with increasing number of larvae in the web, and parasitism rates increased significantly with increasing web isolation. The proportion of webs parasitized was significantly and negatively correlated with cluster density. Among habitat patches the proportion of parasitized webs decreased as cluster density increased. Clusters with a high proportion of larval webs parasitized tended to have lower parasitism rates per larval web. These results support the call for relatively large and continuous habitat patches to maintain stable parasitoid and host populations. Conservation efforts directed towards maintenance of high host plant density could allow E. aurinia to reduce parasitism risk, while providing C. bignellii with sufficient larval webs to allow population persistence.     

Community dynamics of leafminers (Diptera: Agromyzidae) and their parasitoids (Hymenoptera) in a natural habitat from Central Argentina

We examined the temporal community dynamics of leafminers (Agromyzidae: Diptera) and their mainly polyphagous parasitoids in a natural habitat from Central Argentina. Changes in community composition are shown by changes in dominant species and similarity coefficients. Abundance of leafminers was highest in winter, and was not related to their species richness. Abundance and species richness were correlated in the parasitoid community. Temporal variation in parasitoid diversity and abundance were positively correlated with species number and density in the host community. Apparent parasitism was greater when parasitoid species richness was high and parasitoid community dominance was low.     

Introduction of parasitoids has maintained a stable population of arrowhead scales at extremely low levels

We previously reported the drastic decline of the arrowhead scale, Unaspis yanonensis Kuwana (Hemiptera: Diaspididae) following the introduction of two parasitoid species, Aphytis yanonensis DeBach et Rosen and Coccobius fulvus Compere et Annecke (Hymenoptera: Aphelinidae), which were used as biological control agents in a Japanese grove of Satsuma mandarin oranges, Citrus unshiu Marc. (Rutaceae). In this study, we examined whether the parasitoids regulated the scale population at lower levels after its initial decline. Specifically, we monitored the population dynamics of the scale and the rates of parasitism by the two parasitoids three times per year for 16 years following the introduction of the wasps. The two parasitoid species maintained a U. yanonensis density at 1/200 of the density prior to their introduction. When we excluded the wasps, the scale population grew at a rate that was more than fivefold that of a control (parasitoid‐infested) group. Although the rates of parasitism by C. fulvus fluctuated, they remained at relatively high levels, whereas those of A. yanonensis were 0% over the last 6 years. A repeated‐measures ANOVA indicated that scale density remained stable subsequent to its rapid decline. This showed that the parasitoids stabilized the scale population at a lower level than host plant limitations would have dictated, and strongly suggests that C. fulvus alone regulates the scale population density at an extremely low level. The latter finding contradicts other studies which have suggested that the two parasitoid species complement each other in regulating scale density. We discuss whether a behavioral refuge used by the scale against parasitoids, which we have demonstrated in an earlier study, might contribute to the observed stable host–parasitoid system at low densities.     

Exploitative competition and coexistence in a parasitoid assemblage

Most insect populations are exploited by a complex of different parasitoid species, providing ample opportunities for competitive interactions among the latter. Despite this, resource-mediated competition (i.e., exploitative competition) among insect parasitoids remains poorly documented in natural systems. Here we propose a novel way to infer the presence of competitive interactions from covariance patterns in parasitism levels, and illustrate the use of this approach on a relatively well-defined and simple host–parasitoid system. The parasitism levels caused by three parasitoid species on a shared host showed a highly consistent negative covariance among samples. With the levels of parasitism by one species increasing, the levels of parasitism attributable to the two others decreased. Importantly, negative covariance between parasitism levels by different species appeared at high abundance, but not at low abundance of the phenologically earlier parasitoid species. This as well as several other lines of evidence indicates the importance of competitive interactions in this system. Feeding biology and phenology of the parasitoids suggest that competition in this parasitoid assemblage is primarily resource-mediated rather than occurring through direct interference. The species attacking earlier stages of the host are competitively superior to those attacking their host later in the season. Better dispersal ability and use of alternative host species by the inferior species could contribute to the coexistence of these competing parasitoids.     

The response of aphid secondary parasitoids to different patch densities of their host

Aphids are attacked by a large guild of natural enemies including many primary parasitoids which mummify their hosts. These mummies are themselves attacked by a guild of mummy parasitoids which are potentially important in regulating primary parasitoids at densities below which they can exert biological control. The response of mummy parasitoids to mummy densities was investigated in an experiment in which mummy densities of the pea aphid (Acyrthosiphon pisum) attacked by the parasitoid Aphidius ervi were manipulated across host plant patches. Overall, the risk of parasitism was density independent, though with very high inter-patch variability which may allow probabilistic refuges from secondary parasitism. Six species of four genera of mummy parasitoids were recorded. Of the responses of the individual genera, Coruna were reared most frequently from patches of high host density while amongst patches from which Syrphophagus was reared parasitism was inversely density dependent.     

Effects of parasitoids on a mycophagous drosophilid community in northern Japan and an evaluation of the disproportionate parasitism hypothesis

In a host–parasitoid system comprising mycophagous drosophilids and their parasitoids, the drosophilid and parasitoid species assemblages, host use, and the prevalence of parasitism were assessed, and the “disproportionate parasitism hypothesis” was examined with consideration given to yearly variations. The mycophagous drosophilids, their fungal food resources and parasitoids were studied by carrying out an intensive census throughout the activity seasons of 4 years (2000–2003) in Hokkaido, northern Japan. Five hymenopterous parasitoid species, four braconids and one eucoilid, were found. Parasitoids of mycophagous drosophilids are reported for the first time from Asia. Most parasitism (99.2%) was by braconids, in contrast to the dominance of eucoilids in Europe. Parasitism was restricted to the summer, and the rate was high from early July to early August every year. There was considerable yearly variation in the composition of abundant fungus, drosophilid and parasitoid species, especially between 2000 and 2001. The alternation of dominant host species was coupled with the alternation of dominant parasitoid species that differed in host use. Despite the yearly variation in the system, the most dominant host species suffered disproportionately heavy parasitism by the correspondingly dominant parasitoid species every year. The parasitism rate was positively correlated with the relative host abundance. This thus indicates that the disproportionate parasitism mechanism may operate, via which species coexistence is promoted by a higher rate of parasitism of the dominant species.     

Aggregation of parasitism risk in an aphid-parasitoid system: Effects of plant patch size and aphid density

Few studies have linked density dependence of parasitism and the tritrophic environment within which a parasitoid forages. In the non-crop plant-aphid, Centaurea nigra–Uroleucon jaceae system, mixed patterns of density-dependent parasitism by the parasitoids Aphidius funebris and Trioxys centaureae were observed in a survey of a natural population. Breakdown of density-dependent parasitism revealed that density dependence was inverse in smaller colonies but direct in larger colonies (>20 aphids), suggesting there is a threshold effect in parasitoid response to aphid density. The CV² of searching parasitoids was estimated from parasitism data using a hierarchical generalized linear model, and CV²>1 for A. funebris between plant patches, while for T. centaureae CV²>1 within plant patches. In both cases, density independent heterogeneity was more important than density-dependent heterogeneity in parasitism. Parasitism by T. centaureae increased with increasing plant patch size. Manipulation of aphid colony size and plant patch size revealed that parasitism by A. funebris was directly density dependent at the range of colony sizes tested (50–200 initial aphids), and had a strong positive relationship with plant patch size. The effects of plant patch size detected for both species indicate that the tritrophic environment provides a source of host density independent heterogeneity in parasitism, and can modify density-dependent responses.     

Population dynamics of Thecodiplosis japonensis (Diptera: Cecidomyiidae) under influence of parasitism by Inostemma matsutama and Inostemma seoulis (Hymenoptera: Platygastridae)

To understand influence of two species of parasitoids on host population dynamics, adult population dynamics of pine needle gall midge (PNGM), Thecodiplosis japonensis and two species of parasitoids, Inostemma matsutama and Inostemma seoulis were observed using emergence traps from 1986 to 2005. Density of PNGM decreased after outbreaks in 1986 and 1987 and showed density-dependent regulation. Relationships between density of PNGM and its parasitoids were linear except the period of outbreak regardless of parasitoids species. Relationships between host density and parasitism of I. matsutama and I. seoulis were density-independent and inverse density-dependent, respectively. I. seoulis was the dominant parasitoid against PNGM. Interspecific competition between two parasitoids was not strong and temporal niche segregation between two parasitoids was a possible mechanism for coexistence of two parasitoids. The parasitoid complex responded to changes in host density more sensitively than single parasitoid species. These results suggested that two parasitoid can stabilize PNGM population density without strong negative effects on each species of parasitoids.     

Spatial patterns of Trybliographa rapae parasitism of Delia radicum larvae in oilseed rape and cauliflower

Trybliographa rapae (Westwood) is an important parasitoid of Delia radicum (L.). Parasitism of D. radicum larvae by T. rapae in relation to host density on canola (oilseed rape) and cauliflower roots was examined at 10 field sites in Germany and Switzerland. For roots with host larvae, the proportion of roots with one or more parasitized hosts increased with increasing host density. However, for these infested roots, the parasitism of individual larvae was not consistently related to host density. When considering only roots on which there were parasitized larvae and the opportunity for multiple attacks, the proportion of larvae that were parasitized decreased with increasing host density in the field locations, and in a cage study under controlled conditions. A model of patch‐finding and number of attacks by female parasitoids suggests that patch‐finding is density‐dependent, but that low attack rate and interference effects limit numbers of attacks to three or less per visit to a host patch; the reduced number of attacks per visit leads to the inverse relationship of larval parasitism with host density in the host patches visited. The interplay of the density‐dependent and inversely density‐dependent processes appears to be responsible for the inconsistency of density dependence of overall larval parasitism in this and previous studies. In the laboratory, adult female T. rapae parasitized hosts at ≤4 cm deep in soil, but not at 6 cm deep. From the depth distribution of larval feeding sites in the field, we infer that between 4% and 20% of Delia larvae may be in a physical refuge from T. rapae parasitism, which may have a stabilizing influence on the host–parasitoid interaction.     

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.     

Consequences for a host–parasitoid interaction of host-plant aggregation, isolation, and phenology

Abstract.  1. Spatial habitat structure can influence the likelihood of patch colonisation by dispersing individuals, and this likelihood may differ according to trophic position, potentially leading to a refuge from parasitism for hosts.
2. Whether habitat patch size, isolation, and host-plant heterogeneity differentially affected host and parasitoid abundance, and parasitism rates was tested using a tri-trophic thistle–herbivore–parasitoid system.
3.  Cirsium palustre thistles ( n = 240) were transplanted in 24 blocks replicated in two sites, creating a range of habitat patch sizes at increasing distance from a pre-existing source population. Plant architecture and phenological stage were measured for each plant and the numbers of the herbivore Tephritis conura and parasitoid Pteromalus elevatus recorded.
4. Mean herbivore numbers per plant increased with host-plant density per patch, but parasitoid numbers and parasitism rates were unaffected. Patch distance from the source population did not influence insect abundance or parasitism rates. Parasitoid abundance was positively correlated with host insect number, and parasitism rates were negatively density dependent. Host-plant phenological stage was positively correlated with herbivore and parasitoid abundance, and parasitism rates at both patch and host-plant scales.
5. The differential response between herbivore and parasitoid to host-plant density did not lead to a spatial refuge but may have contributed to the observed parasitism rates being negatively density dependent. Heterogeneity in patch quality, mediated by variation in host-plant phenology, was more important than spatial habitat structure for both the herbivore and parasitoid populations, and for parasitism rates.     

Parasitoid Encapsulation as a Defense Mechanism in the Coccoidea (Homoptera) and Its Importance in Biological Control

Encapsulation is a common defense mechanism exerted by a host insect in response to invasion by a metazoan parasitoid or other foreign organisms. In the process of encapsulation, the host forms a capsule around the parasitoid egg or larva, which is usually composed of host blood cells and the pigment melanin. The capsule may kill the parasitoid and thus prevent successful parasitism. Encapsulation may adversely affect the degree of biological control effected by parasitoids as it may either prevent the establishment of exotic parasitoids in new regions or reduce parasitoid efficacy. A high incidence of encapsulation may also cause difficulties in mass rearing of parasitoids. In the Coccoidea (Homoptera), parasitoid encapsulation has so far been recorded in three families: Coccidae (soft scale insects), Diaspididae (armored scale insects), and Pseudococcidae (mealybugs). Important factors that affect the frequency of parasitoid encapsulation in the Coccoidea include: Host and parasitoid species, the host's physiological age and physiological condition, the host origin (or strain), superparasitism, the rearing and/or ambient temperature, and the host plant. The effects of these factors on the incidence of parasitoid encapsulation in the Coccidae, Diaspididae, and Pseudococcidae are described.