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Wheat stem sawfly (WSS), Cephus cinctus Norton (Hymenoptera: Cephidae), is a major pest of cereal crops throughout the Northern Great Plains of North America. Native parasitoids, Bracon cephi (Gahan) and B. lissogaster Muesebeck (Hymenoptera: Braconidae), play a key role in suppressing WSS populations and limiting associated damage. Smooth brome grass (Bromus inermis Leyss.) serves as a potential trap reservoir for WSS when grown in areas surrounding wheat (Triticum aestivum L.) fields in Montana. Its unique biology allows it to support high WSS infestation while promoting significant larval mortality throughout the growing season. Late-season WSS survivors can then serve as hosts for WSS parasitoids. Our study investigated smooth brome as a host refuge for WSS parasitoids. We measured WSS larval infestation and survival rate inside smooth brome grown within WSS-inclusion cages, finding a maximum infestation of 66.5% and an end-of-year WSS survival of 5.7%. In addition, we collected stems from sites in central and north-central Montana to measure the WSS infestation and parasitoid prevalence in wheat and adjacent smooth brome. WSS infestation was high in both Big Sandy (64.5% smooth brome, 65.7% adjacent wheat) and Moccasin, MT (50.6%, 38.6%).Year-end WSS larval mortality was 43.6% greater in smooth brome compared to adjacent wheat at both field sites, but both hosted similar numbers of WSS parasitoids. This research underscores the importance of smooth brome in providing a sustainable host refuge for WSS parasitoids and highlights its significant role in supporting the economics of wheat cultivation.

Colonies of house flies (Musca domestica L. [Diptera: Muscidae]) and four species of parasitoids (Muscidifurax raptor Girault and Sanders, Muscidifurax zaraptor Kogan and Legner, Spalangia cameroni Perkins and Spalangia endius Walker) were established by making collections from dairy farms near Bell, FL, Beatrice, NE, Minneapolis, MN, and San Jacinto, CA. Colonies were assessed for heat tolerance by comparing life history parameters at 25–27°C and fluctuating hot (26.7–41.7°C) temperatures. Muscidifurax raptor, S. cameroni, and S. endius produced 24–28% as many progeny under hot conditions as at 25°C. Colonies of M. zaraptor were more heat-tolerant and produced an average 46.9% as many progeny under the hot regime compared with moderate conditions. There was little evidence for higher heat tolerance in parasitoid populations from historically hot locations (CA desert and FL). Colonies of M. raptor and S. endius that had been in culture for 24 yr were the least heat-tolerant with regard to progeny production. House flies collected from the same locations varied little in longevity, fecundity, or egg-to-adult survival under either hot or moderate regimes. Flies reared under hot conditions laid about half as many eggs (89/female) and had about half the egg–adult survival rate (47.3%) under hot compared with moderate conditions, indicating that heat stress had less effect on flies than on all of the parasitoids except M. zaraptor. An attempt to select for heat tolerance in flies by subjecting them to incremental increases in rearing temperatures for 20 generations resulted in little change in tolerance among the selected flies.

We conducted a review of published information on Tuta absoluta parasitoids for the Neotropical region to (1) corroborate species records, (2) analyze associations including the T . absoluta , other insect and plant hosts and (3) identify research directions for enhancing their use as biological control agents. The literature review shows more than 50 species or morphospecies of Hymenoptera associated with T . absoluta , but less than a half (23) could be confirmed as parasitizing T . absoluta . Erroneous reports or invalid names of species, two new species records were found. Over a 100 pests and non-economically important insect and cultivated and non-cultivated plants directly or indirectly interact with T . absoluta in the region. Four T . absoluta parasitoid species include in their host range predatory insects or act as hyperparasitoids, a negative feature considered for a biological control agent. Five larval parasitoids have a narrow host range and could be considered for classical biological control programs in the areas of new invasion. Six Trichogrammatidae species are commercially used in various countries; of those, T . minutum and T . pretiosum are considered to be moderately generalist, being able to exploit several insect hosts. Apart from Apanteles gelechiidivoris and Pseudapanteles dignus , other native species have been the subject of field studies as biological control agents. The review presented here provides useful insights for identifying species that deserve further evaluation as T . absoluta biological control agents through augmentative or conservation strategies in South America, as well as for potential classical biological control programs in other continents.

The wheat stem sawfly, Cephus cinctus Norton, is a major pest of cultivated wheat (Triticum aestivum L.) and other cereals in North America. The native congeneric parasitoids Bracon cephi (Gahan) and B. lissogaster Muesebeck are important biocontrol agents and play a crucial role in managing wheat stem sawfly outbreaks and damage. Smooth brome grass (Bromus inermis Leyss) has been found to be an effective wheat stem sawfly sink and parasitoid source when grown in areas neighboring wheat fields in Montana. To better understand the ecology of the system, we investigated both the wheat stem sawfly-induced volatile organic compounds produced by smooth brome and winter wheat, and the electrophysiological and behavioral response of B. cephi and B. lissogaster to the collected volatiles via coupled electroantennography and gas chromatography-flame ionization detection. Volatile concentration analysis indicated significantly increased production of (Z)-3-hexenyl acetate, 6-methyl-5-hepten-2-one, and (E)-2-hexenal in wheat stem sawfly-infested smooth brome, and elevated production of 6-methyl-5-hepten-2-one in infested smooth brome and winter wheat when compared to their uninfested counterparts. Both B. cephi and B. lissogaster exhibited significant electrophysiological and behavioral response to (Z)-3-hexenyl acetate, 6-methyl-5-hepten-2-one, and hexahydrofarnesyl acetone. Our results provide important evidence supporting habitat management recommendations that will enhance the effectiveness of biological control, contributing to more sustainable agricultural practices and the preservation of vital ecological functions.

Several agroecological and integrated pest management strategies focus on landscape management to increase complexity and foster biodiversity. However, landscape complexity does not always enhance biological control and in some cases may lead to increased pest populations. We examined the prevalence of two Bracon parasitoids, Bracon cephi Gahan and Bracon lissogaster Muesebeck (Hymenoptera: Braconidae), and their host the wheat stem sawfly Cephus cinctus Norton, a major pest of wheat. We assessed the degree of noncrop and crop host plant use and responses to landscape composition. We found no instances of parasitism by either Bracon species in our three-year, statewide winter wheat survey but found small populations of Bracon in noncrop landscapes throughout eastern and western Colorado. We used model selection to examine how local (500 m scale) and landscape (5 km scale) cover of suitable noncrop and crop habitats potentially affects abundances of Bracon and wheat stem sawfly. Our best fit model for wheat stem sawfly suggests that a decrease in noncrop cover at the landscape scale leads to an increase in wheat stem sawfly infestation. Our best fit model for Bracon parasitism suggests an increase in wheat cover at the local level results in the greatest increase in the odds of parasitism by either species of Bracon. Herbaceous cover at local and landscape scales were also significant predictors of Bracon parasitism. The results of this study suggest that pest and natural enemies respond differently to landscape composition and these responses should be evaluated before management decisions are made.

A major goal of insect community ecology is to understand how and why herbivorous insect species vary in the diversity of their parasitoid assemblages and the rates of parasitism that they experience. Most studies investigating these issues with Lepidoptera as hosts have relied on literature records of parasitoid-host associations that are often of limited quality and that do not necessarily reflect local interactions between hosts and parasitoids. We sampled externally feeding Lepidoptera in mesquite-oak savanna habitats of southeastern Arizona (USA) to assess the ecological and evolutionary determinants of parasitoid community structure. We focused on parasitoids in the family Tachinidae (Diptera) due to their dominance as larval parasitoids of macrolepidoptera at our site. Host abundance, morphology, coloration, gregariousness, and diet breadth of the host were all significantly correlated with tachinid species richness among hosts. Tachinid species richness also varied according to host taxonomy (family), but most of this variation appeared to be better explained by morphology and ecology than by phylogenetic position. Characteristics of host habitat and body size had no significant effect on tachinid species richness. Tachinid parasitism rates were higher for abundant, hairy, non-aposematic, and gregarious hosts. Hymenopteran parasitism rates were low and variable with only host family explaining a significant amount of variation. In general, we found that a substantial amount of the variation in tachinid species richness and parasitism rates among hosts can be explained by ecological attributes, and that interactions of host species with their host plants and predators may determine their suitability as hosts for parasitoids.

Two or more species cannot coexist on a single limiting resource in a constant environment unless each species can increase when rare. In this paper, we show, theoretically and empirically, how trade-offs in life history characters have the potential to mitigate the effects of interspecific competition and promote persistence in a host-multiparasitoid interaction. Theoretically, we show how niche partitioning between competing parasitoids can arise through differences in resource breadth and utilization. We demonstrate, empirically, how trade-offs between parasitoid larval competitive ability and wasp life history characters (adult wasp longevity and the ability to paralyze hosts) can be mediated. Differences, not only in the mean, but also in the variance of these life history traits can influence the outcome of competition, and we discuss how species life history trade-offs can promote species coexistence.

Dispersal has profound effects on parasite populations. Understanding the dispersal behavior of parasites is fundamental to our appreciation of their virulence, epidemiology, and host specificity. Very few host-parasite systems, however, allow for studying how parasites optimize their transmission rates. Here, we investigated the dispersal behavior of a common ectoparasite of European passerine birds, the flea Ceratophyllus gallinae. This flea primarily infests hole-nesting species and breeds during its host's breeding season. Once the host leaves the nest, flea larvae build cocoons, pupate, and remain dormant before initiating their host search. There is considerable variation in the time at which they hatch and disperse from the nest boxes. Some offspring disperse before the hosts choose their nest sites at the beginning of the next breeding period, while others wait until after that stage to disperse. By experimentally manipulating the density of fleas in the nests of their breeding hosts we were able to investigate density-dependent processes that would later affect the dispersal behavior of flea offspring. We found that the density of offspring in the nests was negatively correlated with the proportion of early-dispersing individuals and negatively affected the phenotypic quality of dispersers. Flea offspring of poor phenotypic quality in terms of body size dispersed earlier and had lower potential fecundity than bigger individuals. In a laboratory experiment, we found that the intensity of larval competition strongly affected offspring development, body size at maturity, and overwintering capacity. Thus, in order to maximize their chance of transmission, C. gallinae individuals adjust their dispersal behavior according to their phenotypic quality. In this species, dispersal in time may be explained by the carryover effects of variation in the amount of competition experienced at the larval stage.

Parasites are ubiquitous, but widely neglected, components of food webs. Because of their trophic position, parasites may have major direct and indirect effects on community structure. Because the magnitude of these effects is likely to depend on parasite density, this study was undertaken to explore temporal patterns in the density of parasites and the processes responsible for those patterns. Parasitism by Howardula nematodes was monitored in four species of mycophagous Drosophila (D. falleni, D. neotestacea, D. putrida, and D. recens) that were sampled monthly from 1984 through 1998 near Rochester, New York, USA. Adult flies were collected by sweep netting over naturally occurring mushrooms or commercial Agaricus bisporus mushrooms that had been set out as baits. These adults were then dissected to determine whether they were parasitized by Howardula nematodes. The prevalence of parasitism varied substantially among species, being greatest in D. neotestacea (23.0% infected) and least in D. recens (4.8%). Parasitism was greatest in the spring and fall, and least in midsummer. Prevalence of parasitism covaried significantly through time among the three principal host species, D. falleni, D. neotestacea, and D. putrida, in part because the different species often breed in the same individual mushrooms, the site where nematode transmission from adult flies to larvae occurs. The mean prevalence of parasitism across these three species at the end of the season (September) was significantly correlated with the total precipitation from May through August. The mean prevalence of parasitism in the spring collections was significantly correlated with the mean prevalence the previous fall, because flies overwinter as adults. Consequently, the mean prevalence of parasitism in spring was highly correlated with May through August precipitation the previous year. It is hypothesized that rainfall, which stimulates the production of mushrooms (the flies' breeding sites), interacts with density-dependent nematode transmission to influence the prevalence of parasitism in these populations.

Symbiotic microbes have become increasingly recognized to mediate interactions between natural enemies and their hosts. The ecologies of these symbioses, however, are poorly understood in many systems, and a predictive framework is needed to guide future studies. To achieve this, we focus on heritable, defensive microbes of insects. Our review of laboratory‐based studies identifies diverse bacterial species that have independently evolved to protect a range of insects against parasitoids, parasites, predators and pathogens. Although defensive mechanisms are typically unknown, some involve toxins or the upregulation of host immunity. Despite substantial benefits of infection in the presence of natural enemies, the protective symbionts of insects are often found at intermediate levels in natural populations. Using a host‐centred population genetics approach made possible by the host restriction and cytoplasmic inheritance of these microbes, we propose that balancing selection plays a major role in symbiont maintenance, with protective benefits in the presence of enemies and infection costs in their absence. Other mediating factors are likely to be important, including temperature, superinfections and transmission dynamics. While few studies have provided evidence for defence in the field, several studies have shown symbiont infection frequencies to be dynamic, varying across temporal and spatial gradients and food–plant associations. Newly presented data from our pea aphid research reveal that temporal shifts in defensive symbiont prevalence can be quite rapid, with Hamiltonella defensa showing 10–20% shifts around a seasonal average of c. 50%. Such findings contrast with more unidirectional changes seen in laboratory population cages, suggesting temporal changes in the costs and benefits of symbionts in the field. To frame future research on defensive symbiont ecology, we briefly consider a range of studies needed to test laboratory‐ and field‐derived predictions on defensive symbiosis. Included are investigations of defensive mechanisms, symbiont‐driven co‐evolution and community‐level effects. We also consider the need for more thorough and highly resolved molecular diagnostics of natural infections, laboratory studies on functional differences between symbiont strains and species and studies on the relative costs and benefits of defenders in nature. The emerging theme of symbiont‐mediated defence across eukaryotes suggests that knowledge of the functional mechanisms behind protection and natural symbiont dynamics could be key to understanding many of the world's antagonistic species interactions. Thus, the development of insects as a model for such studies holds promise for these organisms and beyond.