Parasitoid Wasp Species Research Articles

A test of temporal and spatial density dependence in the parasitism rates of introduced parasitoids on host, the arrowhead scale (Unaspis yanonensis) in stable host–parasitoids system

Abstract: In 1980, two species of parasitoid wasps (Aphytis yanonensis DeBach et Rosen and Coccobius fulvus Compere et Annecke) were introduced to Japan from China as biological control agents to combat the arrowhead scale (Unaspis yanonensis Kuwana). These introductions represent one of the most successful projects in the history of biological control in Japan. To examine whether density dependent parasitism was inevitable for success of biological control, we tried to detect temporal and spatial density dependence in parasitism rates using time‐series data of scale density, as well as parasitism, over a 16‐year period. The work was conducted in a Satsuma mandarin orange (Citrus unshiu Marc.) orchard in which we previously demonstrated that the system appeared to have stabilized after a decline in scale density following the introduction of the parasitoids. Earlier work also indicated that C. fulvus contributes most to the reduction in, and the stability of, scale density. In this study, we examined: (1) the relationship, on a whole‐orchard basis, between scale density and the rates of parasitism by A. yanonensis, C. fulvus, and a combination of the two species; (2) whether parasitism was positively correlated to scale density on a single‐tree basis among generations and (3) whether spatial density dependence was detectable within generations on an individual‐tree basis. Parasitism by A. yanonensis was temporally density‐dependent on scale population density at the whole‐orchard level, while parasitism by C. fulvus was not. Parasitism by A. yanonensis or by C. fulvus was rarely positively correlated to scale density at the single‐tree level, and spatial density‐dependence was hardly detected at all at this level. Most analyses of combined parasitism rates were similar to rates of parasitism by C. fulvus alone. Contrary to conventional wisdom of biological control theory, this study demonstrates that density dependence is not necessarily detected, even in a system in which a natural enemy has long held pest density stable at low levels.

Genetic variation can promote system persistence in an experimental host‐parasitoid system

AbstractWe tested experimentally the effects of genetic variation in host population on the host‐parasitoid system persistence. The experimental systems consisted of one parasitoid wasp species (Heterospilus prosopidis), one bean weevil species (Callosobruchus chinensis), and one bean species, of which only the host species (bean weevil) was genetically manipulated. As control treatments with low genetic heterogeneity in the host population, we used two bean weevil strains (Kyoto and Niigata strains) which have several contrasting ecological traits. For the high genetic heterogeneity treatment, hybrid bean weevils which were generated by crossbreeding the strains Kyoto and Niigata were used. In the multiple‐generation experiments, all three treatments had different patterns of extinction. The control treatment with the Niigata strain was very prone to system extinction, whereas the treatment with hybrids showed coexistence of constituent species in almost all replicates and also showed stabilized population dynamics. The other control treatment, using the Kyoto strain, showed intermediate proneness. To interpret the results of multiple‐generation experiments, we conducted several short‐term experiments. The different persistence patterns between the two control treatments were explained by the shapes of the host‐finding abilities of wasps and the growth rate of the bean weevils. The mean values of many ecological traits of hybrid lines were not different from those of the Kyoto strain, but their variability increased. These outcomes corresponded well to the prediction of models by Doebeli [J Theor Biol (1997) 188:109–120]. We discuss the mechanisms in which the variability in host species population was effective for the prolonged system persistence.

Rapid change in the settling behavior of the arrowhead scaleUnaspis yanonensisas an avoidance mechanism against introduced parasitoids,Aphytis yanonensis and Coccobius fulvus

AbstractParasitoids are thought to exert immense selection pressures that shape the traits of herbivores. We examined whether two species of parasitoid wasps,Aphytis yanonensisDeBach et Rosen andCoccobius fulvusCompere et Annecke (Hymenoptera: Aphelinidae), affect the settling behavior of the arrowhead scaleUnaspis yanonensisKuwana (Hemiptera: Diaspididae), in order to demonstrate the evolution of antiparasitism behavior of herbivores using parasitoids in the field. We used the following five methods: a comparison of parasitism risk in different settling behaviors‐parasitoid introduction into a parasitoid‐free population; a comparison of the settling behavior between parasitoid‐present and parasitoid‐free populations; a common garden experiment, in which scales were transferred from parasitoid‐present and parasitoid‐free populations into the same garden; and a laboratory observation of the settling behavior of the first instars derived from the two population categories. Both parasitoids were introduced into a parasitoid‐free population in Wakayama in 1987, and the settling mode of the scales was examined in 1987, 1994, and 1995. The introduction of parasitoids modified the scale‐settling mode so that more crawlers settled under another scale (called burrowing), which was consistent with the results observed in parasitoid‐present (including South China) and parasitoid‐free populations. Moreover, only the burrowing scale exhibited a lower parasitism rate compared to scales settling singly and being burrowed. The common garden experiment demonstrated that scales introduced from the parasitoid‐present population had a greater proportion of burrowers than the parasitoid‐free population, even in the same field cage under parasitoid‐free conditions. Laboratory observations demonstrated that the population difference in parasitism rate was principally due to intrinsic differences in the settling behavior of nymphs; some first‐instar nymphs derived from the parasitoid‐present population burrowed under another scale settled. These results strongly suggest that the natural selection pressure imposed by the parasitoids modified the settling behavior of the arrowhead scale.

Three-dimensional organization of the glomeruli in the antennal lobe of the parasitoid wasps Cotesia glomerata and C. rubecula.

Two closely related parasitoid wasp species, Cotesia glomerata and C. rubecula, differ in their use of associative learning. To investigate the neural basis underlying these differences, it is necessary to describe the olfactory pathway of both wasp species. This paper focuses on the organization of the glomeruli in the antennal lobe. Glomeruli were stained by retrograde axon tracing of all axons in the antennal nerve and observed by confocal laser scanning microscopy. Stacks of optical sections were processed with AMIRA software, and 3D digital models of the glomeruli were produced. The combined use of 2D images and 3D surface models of the antennal lobes enabled the identification of a set of corresponding glomeruli in both wasp species. This offers unique opportunities for the study of subtle differences involved in synaptic plasticity that may occur at the glomerular level and factors regulating this plasticity.

GC-EAG-analysis of volatiles from Brussels sprouts plants damaged by two species of Pieris caterpillars: olfactory receptive range of a specialist and a generalist parasitoid wasp species

Feeding by Pieris brassicae or P. rapae caterpillars on Brussels sprouts plants induces the emission of synomones that attract natural enemies of the caterpillars, Cotesia glomerata, a generalist parasitoid, and C. rubecula, a specialist on P. rapae. Previous research on this tritrophic system has identified a large number of volatiles in the headspace of herbivore-damaged Brussels sprouts plants, and this paper addresses the question which of these volatiles are perceived by the two parasitoid species. Headspace odors from both P. brassicae- and P. rapae-damaged Brussels sprouts plants were analyzed by coupled gas chromatography electro- antennogram (GC-EAG) detection. Twenty volatiles evoked consistent EAG reactions in the antennae of both species and nineteen of these volatiles could be identified with GC-MS. One component that could not be identified due to its low concentration, evoked EAG responses in antennae of C. rubecula only. Possible consequences for searching behavior of the two parasitoid species are discussed.

Influence of temperature on pea aphid Acyrthosiphon pisum (Hemiptera: Aphididae) resistance to natural enemy attack.

The ability to resist or avoid natural enemy attack is a critically important insect life history trait, yet little is understood of how these traits may be affected by temperature. This study investigated how different genotypes of the pea aphid Acyrthosiphon pisum Harris, a pest of leguminous crops, varied in resistance to three different natural enemies (a fungal pathogen, two species of parasitoid wasp and a coccinellid beetle), and whether expression of resistance was influenced by temperature. Substantial clonal variation in resistance to the three natural enemies was found. Temperature influenced the number of aphids succumbing to the fungal pathogen Erynia neoaphidis Remaudière & Hennebert, with resistance increasing at higher temperatures (18 vs. 28 degrees C). A temperature difference of 5 degrees C (18 vs. 23 degrees C) did not affect the ability of A. pisum to resist attack by the parasitoids Aphidius ervi Haliday and A. eadyi Starý, González & Hall. Escape behaviour from foraging coccinellid beetles (Hippodamia convergens Guerin-Meneville) was not directly influenced by aphid clone or temperature (16 vs. 21 degrees C). However, there were significant interactions between clone and temperature (while most clones did not respond to temperature, one was less likely to escape at 16 degrees C), and between aphid clone and ladybird presence (some clones showed greater changes in escape behaviour in response to the presence of foraging coccinellids than others). Therefore, while larger temperature differences may alter interactions between Acyrthosiphon pisum and an entomopathogen, there is little evidence to suggest that smaller changes in temperature will alter pea aphid-natural enemy interactions.

Polydnavirus replication: the EP1 segment of the parasitoid wasp Cotesia congregata is amplified within a larger precursor molecule

Polydnaviruses are unique viruses: they are essential for successful parasitism by tens of thousands of species of parasitoid wasps. These viruses are obligatorily associated with the wasps and are injected into the host during oviposition. Molecular analyses have shown that each virus sequence in the segmented polydnavirus genome is present in the wasp DNA in two forms: a circular form found in the virus particles and an integrated form found in the wasp chromosomes. Recent studies performed on polydnaviruses from braconid wasps suggested that the circular forms were excised from the chromosome. The different forms of the EP1 circle of Cotesia congregata polydnavirus during the pupal-adult development of the parasitoid wasp were analysed. Unexpectedly, an off-size fragment formerly used to diagnose the integration of the EP1 sequence into wasp genomic DNA was found to be amplified in female wasps undergoing virus replication. The EP1 sequence is amplified within a larger molecule comprising at least two virus segments. The amplified molecule is different from the EP1 chromosomally integrated form and is not encapsidated into virus particles. These findings shed light on a new step towards EP1 circle production: the amplification of virus sequences preceding individual circle excision.

Review of the <I>Apanteles</I> Species (Hymenoptera: Braconidae) Attacking Lepidoptera in <I>Bombus</I> (<I>Fervidobombus</I>) (Hymenoptera: Apidae) Colonies in the New World, with Description of a New Species from South America

Field investigations of the nest architecture of the Amazonian bumble bee, Bombus transversalis (Olivier), revealed the presence of a new species of parasitoid wasp belonging to the microgastrine braconid genus Apanteles Foerster, reared from case-bearing tineid moth caterpillars feeding within the nests. The only other two species of Apanteles Foerster known to attack moth larvae within bumble bee colonies in the New World are here reviewed and compared taxonomically and biologically. An illustrated key to the three species recognized in this paper [Apanteles galleriae Wilkinson, A. nephoptericis (Packard) and A. nidophilus Whitfield & Cameron, n. sp.] is presented, and the latter two species are fully described and illustrated for the first time.

Mexican Species of Parasitoid Wasps of the Genus Marietta (Hymenoptera: Aphelinidae)

A key to females and notes on 5 species of the Mexican fauna of the genus Marietta are given. Marietta montana n. sp. from the Biosphere Reserve El Cielo, State of Tamaulipas, Mexico, is described and illustrated.

Selfish element wars in a wasp battleground

Bacteria of the genus Wolbachia are masters in the art of sex-determination manipulation. By exploiting the haplodiploid system of sex determination (in which haploid eggs develop into males and diploid eggs develop into females) in certain Hymenoptera, Wolbachia are able to convert unfertilized eggs, which would normally develop as males, into females. As a consequence, a sexually reproducing wasp population might, on infection with Wolbachia, become parthenogenetic. Furthermore, it is a well-established fact that the life style of many parasitoid wasp species favours a female-biased sex ratio. Therefore, one would expect in Trichogramma kaykai, a minute wasp that parasitizes the eggs of a lycaenid butterfly and which has a typical sex ratio consisting of 22–29% males, that infection with Wolbachia would mean just one thing: switch to parthenogenesis. Alas, this is not the case, with Wolbachia infection levels being stably maintained in frequencies of between 5% and 25%. In a recent paper, Stouthamer et al. 1xSelfish element maintains sex in natural populations of a parasitoid wasp. Stouthamer, R. et al. Proc. R. Soc. London B Biol. Sci. 2001; 268: 617–622Crossref | Scopus (53)See all References1 exploited this system for some elegant experimental and theoretical work.Three hypotheses could explain such infection levels; inefficient transmission of Wolbachia; presence of a suppressor gene that either kills Wolbachia or balances its sex-ratio-distorting effect; or another non-Mendelian balancing factor acting in the opposite direction from Wolbachia (i.e. favouring a male-biased sex ratio). Plotting Wolbachia transmission frequencies versus a range of infection costs is enough to show that, for the range of infection frequencies observed in field populations of T. kaykai, the two possible outcomes are either the fixation or loss of the Wolbachia infection (rather than a maintenance of a stable equilibrium of about 5–25%). Adding to this, the recent demonstration (by the same research group) that Wolbachia might be horizontally transmitted at an appreciable frequency (37%) and inefficient transmission of Wolbachia as the sole explanation for the low levels of infection becomes unlikely.Alternatively, low infection levels of Wolbachia could be explained by the presence of suppressor genes that could either kill the bacteria or eliminate their effect on the host. Modelling carried out by Stouthamer et al., which takes into account the peculiarities of the T. kaykai system, shows that the presence of a suppressor gene would be a plausible theoretical outcome; however, detailed crossing protocols failed to demonstrate its presence.A third scenario invokes the presence of a sex-ratio factor acting in the opposite direction from Wolbachia, favouring male production. Such elements are collectively called paternal sex ratio (PSR) factors and have been identified in previous studies on parasitic wasps. PSRs are paternally transmitted and eggs fertilized with sperm from PSR-bearing males become male instead of female (exactly the opposite effect of that of Wolbachia). By crossing experiments and chromosome karyotypes, Stouthamer et al. present compelling evidence for the major effect of a PSR factor keeping Wolbachia infection levels low.This study is an elegant example of the occurrence of genomic conflict in a natural population. As our knowledge of the characterization and capabilities of these amazing reproductive parasites increases, ever more exciting cases of genomic parasitism will appear.

Life‐history strategies in parasitoid wasps: a comparative analysis of ‘ovigeny’

Summary Ecologists concerned with life‐history strategies of parasitoid wasps have recently focused on interspecific variation in the fraction of the maximum potential lifetime egg complement that is mature when the female emerges into the environment. Species that have all of this complement mature upon emergence are termed ‘pro‐ovigenic’, while those that do not are termed ‘synovigenic’. We document and quantify the diversity of egg maturation patterns among 638 species of parasitoid wasps from 28 families. We test a series of hypotheses concerning variation in ‘ovigeny’ and likely life‐history correlates by devising a quantitative index – the proportion of the maximum potential lifetime complement that is mature upon female emergence. Synovigeny, which we define as emerging with at least some immature eggs, was found to be by far the predominant egg maturation pattern (98·12% of species). Even allowing for some taxonomic bias in our sample of species, pro‐ovigeny is rare among parasitoid wasps. There is strong evidence for a predicted continuum in ovigeny index among parasitoid wasps, from pro‐ovigenic (ovigeny index = 1) to extremely synovigenic species (ovigeny index = 0). As predicted, synovigenic species are longer‐lived than pro‐ovigenic ones, and ovigeny index and life span are negatively correlated across parasitoid taxa, suggesting a life span cost of concentrating reproductive effort early in adult life. There is equivocal evidence that host feeding (i.e. consumption of host haemolymph and/or tissues by adult wasps) is confined to synovigenic parasitoid wasps. It is also not certain from our analyses whether host feeding is associated with a relatively low ovigeny index. As predicted, egg resorption capability is concentrated among producers of yolk‐rich eggs. Also, the hypothesis that it is associated with a tendency towards a low ovigeny index is supported. Parasitoid species that produce yolk‐rich eggs also exhibit a lower ovigeny index than species that produce yolk‐deficient eggs. Ovigeny index appears to be linked to parasitoid development mode (koinobiosis–idiobiosis). We conclude that ‘ovigeny’ is a concept applicable to insects generally.

Venom gland of the digger wasp Liris niger: morphology, ultrastructure, age-related changes and biochemical aspects.

Females of the parasitoid digger wasp species Liris niger hunt crickets as food for their future brood. The wasps paralyse the prey by injecting their venom directly into the CNS. The venom is produced in a gland consisting of two ramified glandular tubules terminating in a common reservoir. The reservoir contents enter the sting bulb via a ductus venatus. Secretory units of dermal gland type III line the two free gland tubules, the afferent ducts to the reservoir and the cap region within the reservoir. Secretion products of tubules reach the reservoir through the cuticle-lined central funnel. Secretory cells in the distal and middle parts of the tubules contain extensive rough endoplasmic reticulum and numerous electron-dense vesicles, whereas secretory cells of the afferent ducts and the cap region of the reservoir lack electron-dense vesicles and the endoplasmic reticulum is poorly developed. The secretory apparatus undergoes age-related changes. The secretory units in the venom gland tubules and inside the reservoir complete differentiation 1 day after imaginal ecdysis. After 30 days, massive autolytic processes occur in the secretory cells and in the epithelial cells of the reservoir. Analysis of the polypeptide composition demonstrates that the venom reservoir contains numerous proteins ranging from 3.4 to 200 kDa. A dominant component is a glycoprotein of about 90 kDa. In contrast the polypeptide composition of Dufour's gland is completely different and contains no glycoproteins. Comparison of the venom reservoir contents with the polypeptide pattern of venom droplets reveals that all of the major proteinaceous constituents become secreted. Thus the secreted venom contains exclusively proteins present in the soluble contents of the venom gland.

Insect immunity: a genetic factor (hrtp) is essential for antibacterial peptide expression in Drosophila after infection by parasitoid wasps.

We have used a parasitoid wasp Drosophila melanogaster system to investigate the relationship between the humoral and cellular immune responses in insects. Expression of the gene encoding diptericin, an antibacterial peptide in various D. melanogaster strains parasitized by several species of parasitoid wasps, was studied by Northern blot. These strains have the capacity to encapsulate parasitoid eggs. Two strains appeared to produce diptericin mRNA after parasitoid challenge, regardless of their cellular immune reaction to the wasp species. This suggests that a specific genetic factor, or factors, here designated humoral response to parasitoid (hrtp), is present in these two strains of D. melanogaster and is implicated in the expression of the antibacterial gene after parasite infection. This hrtp genetic factor is recessively expressed and located on the second chromosome, suggesting that it is monofactorial. The transgenic strain Dipt.2.2-lacZ:1, in which the transgene is present on the first chromosome, is normally susceptible to the parasitoid wasp. The chromosome bearing the hrtp factor was transferred to this transgenic strain, which then became reactive when triggered by wasp infection. The hrtp factor appears necessary for the activation of diptericin by the parasitoid wasp. No correlation between the cellular immune capacity and the humoral response was observed, suggesting that the two components of insect immunity are regulated independently. Arch.

ASSOCIATION BETWEEN FEEDING RATE AND PARASITOID RESISTANCE IN DROSOPHILA MELANOGASTER.

Replicate lines of Drosophila melanogaster have been selected for increased resistance against one of two species of parasitoid wasp, Asobara tabida and Leptopilina boulardi. In both cases, it has been shown that an improved ability to mount an immunological defense against the parasitoid's egg is associated with reduced survival when the larvae are reared under conditions of low resource availability and thus high competition. We show here that in both sets of selected lines, lower competitive ability is associated with reduced rates of larval feeding, as measured by the frequency of retractions of the cephalopharyngeal skeleton. This suggests that the same or similar physiological processes are involved in the trade-off between competition and resistance against either parasitoid and shows how the interaction between adaptations for competition and natural enemy resistance may be mediated.

Egg Parasitism of the Jumping Spider, Phidippus clarus (Salticidae) by the Parasitoid Wasp, Idris saitidis (Scelionidae)

A female jumping spider, Phidippus clarus Keyserling (Araneae; Salticidae) was discovered guarding an eggsac on herbaceous vegetation on 23 October 1997 at Clinton Lake, Douglas County, Kansas. Laboratory examination of the eggsac revealed the presence of many unhatched pupal cases of a species of tiny parasitoid wasp, later identified as Idris saitidis (Howard)(Hymenoptera; Scelionidae). The adult wasps were observed emerging from their pupal cases later the same evening. Immediately upon emergence, several matings occurred within the spider eggsac. Ten of these individuals were placed in a vial containing a recently produced eggsac of the orbweaver, Neoscona arabesca (Walckenaer) (Argiopidae). A second group of six wasps were placed in a vial with an eggsac of a funnelweb spider (Agelenopsis sp.) (Agelenidae). Oviposition was observed in neither group, and eventually spiderlings emerged from the eggsacs.

Responses of two parasitoid wasps to nectar odors as a function of experience

AbstractAdult parasitoid wasps can learn to recognize specific resource‐based cues, making them adept at locating essential resources within their native habitats. However, relatively little is known about their ability to recognize the odors emitted by flowers and extra‐floral nectar glands. A novel test arena mimicking the distribution of nectaries within an umbelliferous inflorescence was developed to measure and compare the responses of two eulophid wasps, Edovum puttleri and Pediobius foveolatus, to nectar‐based odors. Although both parasitoid species were able to associate nectar location with the odors emitted by both real nectaries (dill flowers and snap bean stipules) and artificial nectars (1 M sucrose solution scented with banana or lemon food flavoring), the responses of inexperienced wasps to nectary odors differed. While inexperienced E. puttleri displayed little attraction to the odor of either dill flowers or snap bean stipules, inexperienced P. foveolatus displayed a strong attraction to odors of both types of nectaries. However, once it had experienced foraging on either dill or snap bean nectar, E. puttleri responded to those nectary odors as strongly as did P. foveolatus. The responsiveness of both wasp species to the odor of artificial nectar greatly increased after they foraged on sugar solution scented with either banana or lemon odor. That parasitoid wasp species can differ in their ability to recognize food‐based cues is of fundamental and applied importance.

Effect of Parental Age at Coition on Reproduction of Lysiphlebia mirzai (Hymenoptera: Braconidae)

Third instar nymphs [L 3 ] of the aphid species Rhopalosiphum maidis (Fitch 1856) were exposed individually each to a female [F] of the parasitoid wasp species Lysiphlebia mirzai Shuja-Uddin 1975. These were of variable age-groups (0-12 h, 48-60 h, 96-108 h old) reciprocally inseminated by males [MM] of variable age-groups (0-12 h, 48-60 h, 96-108 h old). The results indicated: (a) With the increase of parental age, the total progeny yield by the F (6-day observation period) decreases considerably from 154.6±21.1 SD progeny (0-12 h old) to 57.2±7.5 SD progeny (96-108 h old), and the variation in the maternal age causes greater variance in the data than the paternal age; (b) the progeny sex ratio [SR] (measured as proportion of MM in the population) is minimum (SR = 0.36±0.05 SD), when parents mate just after emergence (0-12 h old), but increases about 50% (SR = 0.53±0.05 SD), when the parents are inseminated after 96-108 h of emergence and the FF are allowed to oviposit. The analysis of variance shows that the maternal age influences the total progeny yield; also the SR and the generalised reproductive index is much greater than the paternal age. The implications of these results are discussed with reference to the maximisation of yield of total progeny and particularly with respect to FF for mass propagation and field release in biological control.

The evolution of gregariousness in parasitoid wasps

Data are assembled on the clutch-size strategies adopted by extant species of parasitoid wasp. These data are used to reconstruct the history of clutch-size evolution in the group using a series of plausible evolutionary assumptions. Extant families are either entirely solitary, both solitary and gregarious, or else clutch size is unknown. Parsimony analysis suggests that the ancestors of most families were solitary, a result which is robust to different phylogenetic relationships and likely data inadequacies. This implies that solitariness was ubiquitous throughout the initial radiation of the group, and that transitions to gregariousness have subsequently occurred a minimum of 43 times in several, but not all lineages. Current data suggest that species-rich and small-bodied lineages are more likely to have evolved gregariousness, and contain more species with small gregarious brood sizes. I discuss the implications of these data for clutch-size theory.

Interaction of Resistant Plants and Wasp Parasitoids of Tobacco Budworm (Lepidoptera: Noctuidae)

The interactions of 2 species of parasitoid wasps, their host, Heliothis virescens (F.), and transgenic tobacco producing a low concentration of Bacillus thuringiensis Berliner toxin were investigated in field tests. Campoletis sonorensis (Cameron) and toxic plants each decreased survival of larvae during the first 6 d on plants. C. sonorensis and toxic plants acted synergistically in combination, decreasing larval survival beyond the level expected for an additive interaction. Cardiochiles nigriceps Viereck did not significantly reduce 6-d survival of host larvae and did not interact with plant toxicity. Parasitism of H. virescens exposed for 1 d to natural populations of the same parasitoids was highly variable from day to day, and also varied between years. In 1990, C. sonorensis was the more prevalent species; in 1991, C. nigriceps was dominant. In 1990, single-day parasitism of neonates by C. sonorensis was lower on toxic plants than nontoxic plants. This result, which contradicted previous findings of increased parasitism among larvae exposed for several days on toxic plants, probably was caused by differences in initial behavior of larvae on toxic versus nontoxic plants. In 1991, position of larvae on plants did not affect parasitism by C. sonorensis ; however, parasitism by C. nigriceps was significantly higher among larvae collected from exposed positions on leaves compared with apical buds. Patterns of daily parasitism of 1- to 8-d-old larvae were extremely different for the 2 parasitoids: vulnerability to C. sonorensis among host larvae on nontoxic plants began to decline after 3 d; whereas, parasitism by C. nigriceps increased over the 1st few days and remained high through 8 d.

The Life-Histories of Parasitoid Wasps Developing in Small Gregarious Broods

Theory predicts that small gregarious brood sizes are evolutionarily unstable in parasitoid wasps due to the evolution of brood reduction behaviour in larvae competing for developmental resources. Despite this, many species of parasitoid wasp develop in small gregarious broods. Here, I catalogue the life-history properties of 87 such species, and attempt to explain their apparent stability in brood size. Small gregarious broods are taxonomically widespread, occurring in at least 15 of the 26 families containing gregarious species. The life-histories represented in this species list are extremely varied. Several species display properties consistent with an increase in the stability of non-siblicidal behaviour, but few of these properties are common to a large proportion of species. This suggests that either several factors contribute towards clutch size stability, or some important unknown variable is responsible. I discuss stochasticity in brood size as a novel but widespread factor contributing to the stability of non-siblicidal behaviour.