Some factors affecting the composition of tropical ichneumonid faunas

An increasing accumulation of data shows that tropical ichneumonid faunas are no more species-rich than extra-tropical ones, despite the fact that most of their host groups show increased tropical species-richness. This lack of increase in ichneumonid species-richness can be attributed to the absence of groups whose hosts are not present (e.g. Ctenopelmatinae) and poor tropical representation by many groups of diurnal koinobionts (e.g. Campopleginae). Low host density has been postulated as a barrier to tropical koinobiont species-richness, but it is here suggested that this is not the only limiting factor as groups of nocturnal koinobionts, such as the Ophioninae, show increased tropical species-richness. It is postulated that koinobionts have the capability of being able to locate sparse hosts, but as they host-search in flight, a prolonged daytime host-searching period in the tropics would expose them to a high level of predation pressure. By being active at night koinobionts can avoid diurnally active predators. It is also postulated that sparse hosts may be located more easily at night and more habitats may be climatically suitable for ichneumonid activity when they are not subject to direct sunlight. Idiobionts, such as the Mesostenini and Pimplini, are more species-rich and morphologically diverse in the tropics than they are in extra-tropical regions. It is suggested that this results from the fact that tropical idiobionts can be active during the whole of the diapausing period, when their hosts are available, whereas activity by temperate idiobionts is prevented by inclement weather. Although many idiobionts are probably less exposed to predators than koinobionts, many have evolved obvious protective devices.     

The interaction between host and host plant influences the oviposition and performance of a generalist ectoparasitoid

The successful development of parasitoids of herbivores depends on the quality of their host, which is often affected by the host plant. Therefore, a parasitoid’s oviposition decisions will directly depend on the host, but also on plant quality. Here, we investigated the direct effects of host species and the indirect effects of the host’s food plant on the oviposition decisions and performance of the gregarious ectoparasitoid Euplectrus platyhypenae Howard (Hymenoptera: Eulophidae). With a series of no‐choice experiments, we determined the oviposition and performance of the parasitoid on: (1) two caterpillar species, fall armyworm, Spodoptera frugiperda JE Smith (Lepidoptera: Noctuidae), and velvet armyworm, Spodoptera latifascia Walker, reared on maize (Zea mays L., Poaceae), (2) the same caterpillars reared on maize, bean (Phaseolus vulgaris L., Fabaceae), or squash (Cucurbita pepo L., Cucurbitaceae) leaves, and (3) S. latifascia caterpillars reared on leaves of wild and cultivated lima bean, Phaseolus lunatus L. All these insects and plants originate from Mesoamerica where they have coexisted for thousands of years in the traditional agricultural system known as Milpa in which maize, beans, and squash are planted together. We found that the preferred and best combination of host and host plant for parasitoid performance was S. frugiperda on maize. Parasitoids laid larger clutches, had higher survival, and more females and larger adults emerged from S. frugiperda reared on maize. However, when both caterpillar species were reared on squash, S. latifascia was the better host. Contrary to the literature, S. frugiperda was not able to develop on bean plants. Results from the lima bean experiment showed that parasitoid performance was best when S. latifascia was reared on leaves of cultivated compared to wild lima bean. These findings are discussed in the context of mixed cropping in which the ability of generalist parasitoids to switch among hosts and host plant species could be advantageous for pest management.     

Host age and fitness-related traits in a koinobiont aphid parasitoid

Abstract.  1. Trade-offs play a key role in species evolution and should be found in host–parasitoid interactions where the host quality may differ between host age categories.
2. The braconid wasp, Aphidius ervi , is a solitary endoparasitoid that allows its aphid hosts to continue to feed and grow after parasitisation. The hypotheses that host age influences their quality and that female parasitoids exploit their hosts based on that quality were tested under laboratory conditions using no-choice tests.
3.  Aphidius ervi females accepted the aphid Myzus persicae for oviposition and their progeny developed successfully in all host ages. The fitness-related traits of parasitoids did not increase linearly with the host age in which they developed. Host quality was found to be optimal at intermediate host ages and the females preferred to parasitise these hosts. The shortest progeny development time and a more female-biased sex ratio were observed in hosts of intermediate age.
4. This study suggests the existence of multiple interactive trade-offs occurring during host–parasitoid interactions according to host age related quality.     

The effect of host instar on the size and sex ratio of the endoparasitoidDacnusa sibirica

Older instars of the leafminer hostChromatomyia syngenesiae Hardy (Diptera: Agromyzidae) at parasitization resulted in larger female adults of the larval-pupal endoparasitoidDacnusa sibirica Telenga (Hymenoptera: Braconidae). The sex ratio in the parasitoid offspring became increasingly female biased and development period decreased with increasing age of the host at parasitization. Older host instars were therefore considered to produce a fitter parasitoid.     

Structural refuges in two stem-boring weevils on Rumex crispus

Abstract.

• 1 Differences in feeding positions, parasitoid complexes and parasitization rates of the two dominant insect species in stems of Rumex crispus were investigated. Apion miniatum (inhabiting the lower 10% of the stem) and Apion violaceum (occurring all along the stem) exploited distinctly different feeding positions, partly due to differences in time of oviposition.
• 2 A.miniatum suffered less parasitization thanA.violaceum, measured both by number of parasitoid species and parasitization rates. Its feeding position constitutes a refuge.
• 3 Parasitization of A.violaceum was least in individuals feeding in basal, thick parts of the stem. There is a trend that the developed refuge increases with increasing stem diameter.
• 4 The refuge of A.violaceum in basal positions only affected idiobiont ectoparasitoids, because koinobiont endoparasitoids attack the weevils before they enter the stem. Moreover, the koinobiont did benefit from the refuge itself by avoiding hyperparasitism.
• 5 Densities of A.violaceum increased with plant size, whereas percentage parasitism did not differ. Its three most abundant parasitoid species showed small-scale density-independent patterns of attack over all plants. Therefore a constant proportion of weevil larvae mine within the refuge independent of its potential size.
• 6 Synchronization between weevils and plants, as well as between parasitoids and hosts, appears to be the dominant factor for the spatial community structure of the stem borers of Rumex crispus and their enemies.

     

Competitive Superiority of Early Acting Species: A Case Study of Opiine Fruit Fly Parasitoids

Koinobiont parasitoids that attack an early host stage may have an advantage in suppressing competing parasitoids that attack later stages of the same host. We examined the competitive interaction between the two most important parasitoids of tephritid fruit flies in Hawaii, Fopius arisanus (Sonan), and Diachasmimorpha longicaudata (Ashmead) (Hymenoptera: Braconidae). The former species attacks host eggs while the latter attacks host larvae, and both species emerge as adults from the host puparia. F. arisanus physiologically suppressed egg development of D. longicaudata. Over 90% of D. longicaudata eggs died in the presence of F. arisanus larvae within host larvae of either the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) or the oriental fruit fly, Bactrocera dorsalis (Hendel). D. longicaudata appeared not to discriminate against hosts previously parasitized by F. arisanus. The mechanism that F. arisanus uses to eliminate D. longicaudata is similar to that which it employs to eliminate five other larval fruit fly parasitoids so far reported in Hawaii. This suggests that there is a broad competitive superiority of the early acting species in fruit fly parasitoids. We discuss the implication of this in relation to future biological control introductions against tephritid fruit flies.     

Growth rate adjustment of two Drosophila parasitoids in response to the developmental stage of hosts

1. Generalist koinobiont parasitoids often exhibit high flexibility in their development; their larvae shorten or prolong the developmental period depending on the host quality at parasitisation. However, flexibility of the growth rate of parasitoid larvae has rarely been investigated so far. 2. This study investigated how the koinobiont parasitoid wasps Asobara japonica and Leptopilina ryukyuensis regulate their larval growth when they parasitise host Drosophila larvae with varying larval periods. 3. In both parasitoid species, the preimaginal period was longer when they parasitised 1‐day‐old larvae of Drosophila rufa than when they parasitised older larvae of D. rufa or when they parasitised larvae of Drosophila simulans, a species with a shorter larval period than D. rufa. After host pupariation, A. japonica accelerated its growth, thereby showing a biphasic growth curve. On the other hand, L. ryukyuensis did not accelerate its growth after host pupariation. 4. Growth retardation of parasitoid larvae in 1‐day‐old D. rufa larvae would contribute to avoiding excess growth before host pupariation, because the excess growth of parasitoid larvae would have negative effects on host growth. The growth rate acceleration of A. japonica after host pupariation suggests that they enhance resource utilisation in a host that has reached maximum body mass. It remains uncertain as to why L. ryukuensis does not show clear accelerated growth after host pupariation. Nonetheless, these results suggest that parasitoid larvae have the ability to detect the developmental stage of hosts in a species‐specific manner.     

Comparing and contrasting life history variation in four aphid hyperparasitoids

1. In primary parasitoids, significant differences in life history and reproductive traits are observed among parasitoids attacking different stages of the same host species. Much less is known about hyperparasitoids, which attack different stages of primary parasitoids. 2. Parasitoids exploit hosts in two different ways. Koinobionts attack hosts that continue feeding and growing during parasitism, whereas idiobionts paralyse hosts before oviposition or attack non‐growing host stages, e.g. eggs or pupae. 3. Koino‐/idiobiosis in primary parasitoids are often associated with different expression of life history trade‐offs, e.g. endo‐ versus ectoparasitism, high versus low fecundity and short versus long life span. 4. In the present study, life history parameters of two koinobiont endoparasitic species (Alloxysta victrix; Syrphophagus aphidivorus), and two idiobiont ectoparasitic species (Asaphes suspensus; Dendrocerus carpenteri) of aphid hyperparasitoids were compared. These hyperparasitoids attack either the parasitoid larva in the aphid before it is killed and mummified by the primary parasitoid or the parasitoid prepupa or pupa in the dead aphid mummy. 5. There was considerable variation in reproductive success and longevity in the four species. The idiobiont A. suspensus produced the most progeny by far and had the longest lifespan. In contrast, the koinobiont A. victrix had the lowest fecundity. Other developments and life history parameters in the different species were variable. 6. The present results reveal that there was significant overlap in life history and reproductive traits among hyperparasitoid koinobionts and idiobionts, even when attacking the same host species, suggesting that selection for expression of these traits is largely association specific.     

“Veils” Against Predators: Modified Web Structure of a Host Spider Induced by an Ichneumonid Parasitoid, Brachyzapus nikkoensis (Uchida) (Hymenoptera)

When the larva of Brachyzapus nikkoensis, a koinobiont ectoparasitoid of Agelena limbata forming a funnel web, is a penultimate instar, “veils” of very fine and dense threads covering the spider and parasitoid larva were observed in the tunnel of funnel web. As the veils are not formed until the parasitoid became penultimate instar and the parasitoid larvae are on active hosts at this stage, the veils are considered to be formed by the spider under the manipulation of the parasitoid larva. Removal of the veil indicated that penultimate larvae and pupae were more likely to fail to emerge in manipulated webs compared to the controls. The modified web seems resistant against predators and scavengers such as ants. This is the first record of web modification demonstrating its resistance to predators.     

Development of the solitary endoparasitoid Microplitis demolitor: host quality does not increase with host age and size

Abstract.  1. Many studies examining the relationship between host size, an index of host quality, and parasitoid fitness use development time and/or adult parasitoid size as currencies of fitness, while ignoring pre-adult mortality. Because the physiological suitability of the host may vary in different stages, sizes, or ages of hosts, a misleading picture of host quality may therefore be obtained in cases where fitness is based on only one or two developmental traits.
2. The development of the solitary koinobiont endoparasitoid Microplitis demolitor is examined in different larval age-classes of its host the soybean looper Pseudoplusia includens . Hosts were parasitised on days 1–8 after hatching from the egg, and development time, adult body size, and mortality of the parasitoid were compared.
3. A comparison of larval growth trajectories (using dry body mass) of M. demolitor revealed that parasitoid larvae attained over twice as much body mass in old hosts than in younger hosts. Similarly, adult parasitoid size at eclosion generally increased with host size, although parasitoids developing in smaller hosts lost a much lower proportion of mass between pupation and eclosion.
4. Overall egg-to-adult development was most rapid in intermediate-aged hosts, and longer in hosts at opposite ends of the age continuum. Moreover, parasitoid mortality varied non-linearly with host stage, and was generally higher in very young and older hosts.
5. Based on these results and other empirical data for koinobionts, it is argued that fitness functions in this group of parasitoids are not simply a positive function of host size or age, but instead may be distinctly dome-shaped, both patterns reflecting the degree of physiological and nutritional compatibility between the two organisms.