Learning and natal host influence host preference,handling time and sex allocation behaviour in a pupal parasitoid

The host choice and sex allocation decisions of a foraging female parasitoid will have an enormous influence on the life-history characteristics of her offspring. The pteromalid Pachycrepoideus vindemiae is a generalist idiobiont pupal parasitoid of many species of cyclorrhaphous Diptera. Wasps reared in Musca domestica were larger, had higher attack rates and greater male mating success than those reared in Drosophila melanogaster. In no-choice situations, na?ve female P. vindemiae took significantly less time to accept hosts conspecific with their natal host. Parasitoids that emerged from M. domestica pupae spent similar amounts of time ovipositing in both D. melanogaster and M. domestica. Those parasitoids that had emerged from D. melanogaster spent significantly longer attacking M. domestica pupae. The host choice behaviour of female P. vindemiae was influenced by an interaction between natal host and experience. Female P. vindemiae reared in M. domestica only showed a preference among hosts when allowed to gain experience attacking M. domestica, preferentially attacking that species. Similarly, female parasitoids reared on D. melanogaster only showed a preference among hosts when allowed to gain experience attacking D. melanogaster, again preferentially attacking that species. Wasp natal host also influenced sex allocation behaviour. While wasps from both hosts oviposited more females in the larger host, M. domestica, wasps that emerged from M. domestica had significantly more male-biased offspring sex ratios. These results indicate the importance of learning and natal host size in determining P. vindemiae attack rates, mating success, host preference and sex allocation behaviour, all critical components of parasitoid fitness. Electronic Publication     

Daphne gnidium, a possible native host plant of the European grapevine moth Lobesia botrana, stimulates its oviposition. Is a host shift relevant?

Summary. We tested the hypothesis that Daphne gnidium is an ancestral host plant of Lobesia botrana. To this end, we studied the oviposition response of this moth to various aerial organs of this plant. Our results show that females prefer to lay eggs on daphne berries rather than on grapes (cv. chasselas) and that polar surface extracts of daphne berries and leaves strongly stimulate oviposition in a dosedependant way, whereas flower extracts are weakly stimulant and branch extracts are inactive or deterrent for oviposition. Furthermore, a total extract of daphne berries stimulates oviposition in semi-natural conditions when applied onto fresh grapes. Oviposition stimulants from berries were isolated by HPLC coupled with a dual-choice oviposition bioassay. The ecological value of D. gnidium for L. botrana is discussed and the possible use of oviposition stimulants from daphne, contrasting with the oviposition signal of the cultivated host plant, is proposed in the perspective of developing behavioural manipulation methods such as stimulo-deterrent diversionary strategies compatible with IPM programs.     

Evidence for two genetically and chemically defined host races of Tyria jacobaeae (Arctiidae, Lepidoptera)

Summary. A widely distributed host race of Tyria jacobaeae lives on Senecio jacobaea and related species and accumulates pyrrolizidine alkaloids (“PA race”), another race, which is restricted to the Alps and found on Petasites paradoxus, sequesters sesquiterpenes, such as petasol and isopetasol. Nucleotide sequences of the mitochondrial 16S rDNA gene show 1% sequence divergence, indicating that genetical differences exist between the PA exploiting and the terpene-sequestering host races of T. jacobaeae. This finding suggests that both host races of T. jacobaeae must have been separated for some time already, possibly since the Pleistocene. Received 2 May 2001; accepted 1 June 2001.     

Coexistence of nine anemonefish species: differential host and habitat utilization, size and recruitment

The region of Madang, Papua New Guinea, has the highest reported species diversity of both anemonefishes (nine species) and their host anemones (ten species). To determine which factors may allow so many anemonefish species to coexist at this location, we studied their patterns of distribution, abundance, and recruitment. Population surveys at three replicate reef sites within four zones situated at varying distances from the mainland (nearshore, mid-lagoon, outer barrier, and offshore) indicated that each species of host anemone and anemonefish lived within a particular range of zones. Each species of anemonefish lived primarily with one species of host. Anemonefish species that lived with the same host species usually had different distribution patterns among zones (e.g., Amphiprion percula occupied Heteractis magnifica in nearshore zones, while A. perideraion occupied H. magnifica in offshore zones). Monitoring of natural populations showed that there were few changes (losses or recruitment) in the number or species of fishes associated with each individual anemone over periods ranging from 3 to 9 months. Recruitment was monitored on anemones with and without residents (resident fishes were removed) within each of three zones (nearshore, mid-lagoon, outer barrier). Significantly more anemonefishes recruited to anemones without resident fishes than to anemones with resident fishes. Each anemonefish species recruited to particular host species and zones. The distribution and abundance of the recruits of each fish species among zones were positively correlated with the distribution and abundance of resident fishes in the benthic habitat. This suggests that the spatial patterns of recruitment among zones strongly determined the distribution and abundance patterns of the benthic populations, and they were not the result of post-recruitment mortality or movement. Coexistence of the nine anemonefish species on the limited anemone resource was considered possible because of niche differentiation (i.e., differences in host and habitat utilization among zones), and the ability of two small species (i.e., Amphiprion sandaracinos and A. leucokranos) to cohabit individual anemones with other anemonefish species. Received: 29 July 1999 / Accepted: 1 September 2000     

Induction of host specificity in larvae of Manduca sexta: chemical dependence controlling host recognition and developmental rate

Summary. The tobacco hornworm, Manduca sexta L. (Lepidoptera: Sphingidae) is a specialist on Solanaceae. This host specificity is induced as the larva feeds on solanaceous foliage, so that solanaceous-reared larvae will refuse to feed on other plants. Experiments were designed to determine the role of dietary constituents on the induction of host specificity and the effects of these on development of M. sexta. Choice assays using leaf discs of cowpea, Vigna sinensis (Fabaceae), were used to monitor the isolation of relevant chemical cues from foliage of potato. An aqueous extract of potato foliage strongly stimulated feeding. This extract was partitioned with n-butanol under alkaline conditions to obtain a highly active butanol extract. Reversed phase flash chromatography with a water-methanol gradient gave an active fraction that was used as a supplement for wheat germ-based artificial diet. Larvae reared on this s-diet became dependent on potato allelochemicals for initiation and continuation of feeding activity. These larvae also developed faster than larvae reared on the control p-diet, but no effect on adult mass was detected. Further flash chromatography of the active fraction under alkaline conditions provided a highly active sub-fraction, and semi-preparative HPLC using gradients of water and acetonitrile resulted in the isolation of a single compound. Bioassays indicated that this compound alone can account for host recognition by solanaceous-reared larvae. The results suggest that the mechanism of induced host specificity in M. sexta involves development of dependence on this compound. Received 21 December 1999; accepted 14 March 2000     

Fitness consequences of body-size-dependent host species selection in a generalist ectoparasitoid

In insect parasitoids, offspring fitness is strongly influenced by the adult females choice of host, particularly in ectoparasitoids that attack non-growing host stages. We quantified the fitness consequences of size-dependent host species selection in Dirhinus giffardii, a solitary ectoparasitoid of tephritid fruit fly pupae. We first showed a positive correlation between the size of emerged D. giffardii wasps and the size of their host fruit fly species (in order of decreasing size): Bactrocera latifrons, B. cucurbitae, B. dorsalis or Ceratitis capitata. We then manipulated individual wasps to show that the parasitoid preferred to attack the largest (B. latifrons) to the smallest (C. capitata) host species when provided with a choice, and laid a greater proportion of female eggs in B. latifrons than in C. capitata. There were no differences in developmental time or offspring survival between individuals reared from these two host species. Finally, we compared the foraging efficiency of large versus small wasps (reared from B. latifrons vs C. capitata) under two different laboratory conditions: high versus low host habitat quality, given that realized fecundity in parasitoids may be influenced by either egg-limited or time-limited factors. Under both conditions, large wasps parasitized more hosts than did small ones as a consequence of high searching efficiency in the host-poor habitat, and high capacity for adjusting egg maturation in response to host availability in the host-rich habitat. Considering the flexibility of body growth, the apparent lack of cost of achieving large body size in either development or survival, and the strong dependence of realized reproductive success on a females size, we argue that body size may be a key to understanding evolution of host species selection in ectoparasitoids. We also discuss constraints upon the evolution of size-dependent host species selection in parasitoids.Communicated by D. Gwynne     

Quantifying the burden of managing wildlife diseases in multiple host species

Mitigation of infectious wildlife diseases is especially challenging where pathogens affect communities of multiple host species. Although most ecological studies recognize the challenge posed by multiple-species pathogens, the implications for management are typically assessed only qualitatively. Translating the intuitive understanding that multiple host species are important into practice requires a quantitative assessment of whether and how secondary host species should also be targeted by management and the effort this will require. Using a multiple-species compartmental model, we determined analytically whether and how intensively secondary host species should be managed to prevent outbreaks in focal hosts based on the reproduction number of individual host species and between-species transmission rates. We applied the model to the invasive pathogenic fungus Batrachochytrium salamandrivorans in a 2-host system in northern Europe. Avoiding a disease outbreak in the focal host (fire salamanders [Salamandra salamandra]) was impossible unless management also heavily targeted the secondary host (alpine newts [Ichthyosaura alpestris]). Preventing an outbreak in the community required targeted removal of at least 80% of each species. This proportion increased to 90% in the presence of an environmental reservoir of B. salamandrivorans and when the proportion of individuals removed could not be adjusted for different host species (e.g., when using traps that are not species specific). We recommend the focus of disease-mitigation plans should shift from focal species to the community level and calculate explicitly the management efforts required on secondary host species to move beyond the simple intuitive understanding that multiple host species may all influence the system. Failure to do so may lead to underestimating the magnitude of the effort required and ultimately to suboptimal or futile management attempts.     

Nemertean,copepod, and amphipod symbionts of the dimorphic ascidian Pyura stolonifera near Melbourne,Australia: specificities to host morphs,and factors affecting prevalences

On the central coast of Victoria. Australia, the dimorphic ascidian Pyura stolonifera (Heller, 1878) harbors three endosymbionts: the nemertean Gononemertes australiensis Gibson, 1974, the copepod Doropygus pulex (Thorell, 1859), and the amphipod Paraleucothoe novaehollandiae (Haswell, 1880). The specificities of these symbionts to two host colour morphs were studied during 1989 to 1991 as part of a multidisciplinary investigation aimed at determining whether the two morphs are genetically distinct. Distributional surveys revealed that nemerteans and copepods occur only in yellow and brown ascidians, respectively, and that amphipods live in both forms. These specificities held true not only when the two morphs were in allopatry, but also in sympatry. These observations, especially the sympatric data, suggest that the two host morphs might be genetically distinct. For example, the two morphs might have different genetically encoded internal milieus that favour the survival of nemerteans in yellow ascidians, and copepods in brown hosts. In transplant experiments, which involved moving ascidian morphs within and between habitats, the wrong symbionts never colonised the wrong hosts. These results, although consistent with the hypothesis of genetic maintenance of specificity, were deemed inconclusive because of the difficulty of establishing reliable controls (i.e. vacant hosts). The relationships between symbiont prevalences and several factors (season, year, site within host, host individual, host habitat, host size/age, host breeding condition, and co-occurrence of other symbiont species) were also analysed. Both simple (e.g. greater prevalences for large hosts) and complex (e.g. prevalence x season x gonad state of host) interactions were detected for all three symbiont species. These are among the very few quantitative analyses of factors affecting prevalences of ascidicolous nemerteans and amphipods. The present report identifies one of very few definite nemertean-ascidian symbioses. Since no differences in gross condition were ever noticed between occupied and vacant hosts, it is suggested that all three symbionts are commensals rather than parasites or mutuals.     

Tactics of parasitic American coots: host choice and the pattern of egg dispersion among host nests

Summary I examined the tactics adopted by a conspecific brood parasite, the American coot (Fulica americana), and the degree to which these tactics reflect sources of mortality for parasitic eggs. Only 8% of parasitic eggs produced independent offspring, compared to a 35% success rate for non-parasitic eggs, and most mortality was due to egg-rejection by hosts or the consequences of laying eggs too late in the host's nesting cycle. Parasites usually laid parasitically before initiating their own nests and usually parasitized immediate neighbours. Parasites did not remove host eggs before laying their own egg, and egg disappearance in general was not more common at parasitized nests. I found no evidence for non-random host choice, either on the basis of stage of the host's nesting cycle or the host's brood size. The absence of adaptive host choice is likely a consequence of the fact that, due to host limitation, only a small proportion of parasites had meaningful variation among potential hosts to choose from. The pattern of egg dispersion among host nests by individual parasites appears to be a compromise between constraints imposed by host limitation and the increased success obtained from spreading eggs among nests. Most females laying fewer than five parasitic eggs laid them in a single host nest while females laying five or more eggs normally parasitized two or more hosts. An examination of egg rejection and survival rates showed that parasites would maximize success by laying a single egg per host nest, and the pattern of laying several eggs per host nest is likely a consequence of host limitation. However, no egg that was the fifth laid, or later, parasitic egg in a host nest was ever successful and this probably explains why most females laying five or more eggs parasitized more than one host.     

Learning affects host discrimination behavior in a parasitoid wasp

Learning is generally predicted not to be important in host discrimination by parasitoids, because the stimuli involved are less variable than those used in habitat location. However, Anaphes victus (Hymenoptera: Mymaridae), an egg parasitoid of Listronotus oregonensis (Coleoptera: Curculionidae) apparently learns to associate external pheromones with the presence of a conspecific in a host. In this species, females can reject a parasitized host either after antennal drumming (antennal rejection) or after the insertion of their ovipositor (sting rejection). When they encountered a series of parasitized hosts, females A. victus learned to associate the presence of the external pheromone with the presence of the internal one. Learning lasted less than 4 h and occurred earlier in a series when the female marking the egg and the one detecting that mark were close relatives. This behavior could be adaptive because antennal rejection is faster than sting rejection. Received: 11 March 1997 / Accepted after revision: 30 August 1997     

Incidence,occurrence and distribution of the nematode Echinomermella matsi in its echinoid host,Strongylocentrotus droebachiensis,in northern Norway

The incidence, occurrence and distribution of the nematode Echinomermella matsi (Jones and Hagen) in its host, the echinoid Strongylocentrotus droebachiensis Müller, were examined in Norwegian waters from Nordmre (63° N) to the Russian border (70° N) in a range of habitats (e.g. kelp cover and community type) from 1982 to 1992. The results are used to examine the likely influence of the parasite on fitness (individual growth and gonad production) and its potential in regulating the population size of its echinoid host. The nematode infections are restricted to areas between Vikna (Vega) and middle Troms, with the highest prevalence (40 to 88% infected) in the Bodø area, the focus of the distribution. The prevalence rates are similar on kelp beds and barren grounds. No difference in growth rates between infected and noninfected sea urchins was found. Nematode-infected sea urchins in kelp bed populations had gonads weighing about one-sixth of those of equivalent uninfected individuals. This indicates that nematodes are really parasites causing reduced reproductive capacity. Areas with high prevalence (>40% infected) of nematodes had low sea urchin densities. High nematode intensities were found in the areas with high prevalence. The mean abundance in high prevalence areas was highest (eight nematodes) in the 4-to 7-yr-old age group and decreased to zero among the oldest age groups. The ratio of variance/mean abundance was also highest among 5-to 8-yr-old sea urchins. At highly and slightly infected locations, the relationships between the log of the variance and the log of the mean abundance of parasites were 1.35 and 1.69, respectively. Measurements of the frequency distribution of nematodes in sea urchins and the corresponding estimates of the negative binomial distribution showed significantly higher counts in estimated than observed subpopulations exceeding 20 nematodes per sea urchin. These results may explain nematode-induced sea urchin mortality. The low density of the host in the Bodø area, which had high prevalence, may be a result of increased host mortality caused by nematodes, by lower recruitment because of reduced gonad production, or by other factors. Low prevalence among older sea urchins may be caused by nematodeinduced host mortality or by cycles in infection. However, studies on the life cycle of the nematode and long term investigations of host density and nematode intensity/prevalence will be needed before any true conclusions can can be drawn.     

Distribution of larval host plants of the fruit piercing moth,Othreis fullonia

Summary The adult fruit piercing moth,Othreis fullonia, a native of the indo-Malaysian region, causes severe damage to fruits grown throughout the tropical and subtropical belt from Africa through Asia and Australia to the Pacific Islands. Plants of the family Menispermaceae and the genusErythrina (Fabaceae) serve as larval hosts but the adult moths prefer Menispermaceae plants for oviposition. In Africa, Asia and Australia, the moth does not lay eggs onErythrina since members of the Menispermaceae are abundant. However in the insular Pacific region, where most islands have few or no species of Menispermaceae, the introduced fruit piercing moth utilizesErythrina as an alternate larval host, and either depletes, endangers or causes the possible extinction of Menispermaceae.     

Chemical camouflage of the slave-making ant Polyergus samurai queen in the process of the host colony usurpation (Hymenoptera: Formicidae)

Founding queens of the obligatory social parasite ant Polyergus samurai usurp the host ant Formica japonica colony. The aggressive behaviors of F. japonica workers on the parasite queen disappear after the parasite queen kills the resident queen. To determine whether the parasite queen chemically mimics the host ants, we examined the aggressive behavior of F. japonica workers toward glass dummies applied with various extracts of the parasite queen and host workers. The crude extracts and hydrocarbon fraction reproduced the host workers’ behavior to the live ants. The extracts of the post-adoption parasite queen, as well as the nestmate extracts of F. japonica, did not elicit the aggressive behavior, but the extract of the pre-adoption parasite queen triggered attacks by the host workers. The nestmate recognition of host workers did not change, regardless of contact with the parasite. The gas chromatography and gas chromatography–mass spectrometry analyses indicated that the cuticular hydrocarbon (CHC) profile of the parasite queen drastically changed during the process of usurpation. Discriminant analysis showed the successfully usurped P. samurai queen had colony-specific CHC profiles. CHC profiles of the P. samurai queen who killed the host queen were more similar to those of the host queen than the workers, while the P. samurai queen who usurped the queenless colony had a profile similar to those of host workers. These results suggest that the P. samurai queen usually acquires the CHCs from the host queen during the fight, but from host wokers in queenless host colonies.     

Quantitative analysis of pyrrolizidine alkaloids sequestered from diverse host plants in Longitarsus flea beetles (Coleoptera, Chrysomelidae)

Summary. Several species of the flea beetles genus Longitarsus are able to sequester pyrrolizidine alkaloids (PAs) from their host plants. In five Longitarsus species we compare the concentration of PAs present in their host plants belonging to the Asteraceae or Boraginaceae with those found in the beetles. To get an estimate of the intrapopulation variability, three samples of five beetles each and five individual plants were analyzed for each comparison. A strong intrapopulation variability could be detected both among plant and beetle samples. The total concentration found in the beetles varied strongly between species. The local host plant and its phenology influence the concentrations present in the beetles as evidenced in comparisons of a single beetle species from two different hosts and of one beetle species collected at the same site at different times of the year. In addition, different beetle species apparently vary in their capacity to sequester the alkaloids, at the lowest extreme the mean PA concentration in the beetles (0.034 μg PA/mg dry weight) was 1/30 of the mean concentration found in the plant leaves (L. aeruginosus from Eupatorium cannabinum), at the highest extreme (2.098 μg PA/mg dw) the concentration in the beetles was a 1000 fold higher than in the plant leaves (L. nasturtii from Symphytum officinale). The highest mean concentration found in the beetles was 3.446 μg/mg dw (L. exoletus from Cynoglossum officinale). The absolute concentrations found in the beetles are comparable to other insects which have been shown to be effectively defended against their potential predators. Received 22 June 1999; accepted 25 August 1999     

Cuckoo growth performance in parasitized and unused hosts: not only host size matters

The quality and quantity of food delivered to young are among the major determinants of fitness. A parental provisioning capacity is known to increase with body size. Therefore, brood parasitism provides an opportunity to test the effects of varying provisioning abilities of different-sized hosts on parasitic chick growth and fledging success. Knowledge of growth patterns of common cuckoo, Cuculus canorus, chicks in nests of common hosts is very poor. Moreover, no study to date has focused on any currently unused hosts (i.e., suitable cuckoo host species in which parasitism is currently rare or absent). Here, I compare the growth performance of cuckoo chicks in nests of a common host (the reed warbler, Acrocephalus scirpaceus) and two unparasitized hosts (the song thrush, Turdus philomelos, and the blackbird, Turdus merula). Parasitic chicks were sole occupants of the observed nests, thus eliminating the confounding effect of competition with host chicks. Experiments revealed striking differences in parasitic chick growth in the two closely related Turdus hosts. Cuckoo chicks cross-fostered to song thrush nests grew much quicker and attained much higher mass at fledging than those in nests of their common reed warbler host. Alternatively, parasitic chicks in blackbird nests grew poorly and did not survive until fledging. I discuss these observations with respect to host selection by parasitic cuckoos.     

The role of trail pheromones in host nest recognition of the social parasitic bumblebees Bombus bohemicus and Bombus rupestris (Hymenoptera: Apidae)

Bumblebees of the subgenus Psithyrus are obligate social parasites of Bombus colonies. Parasitic females enter host colonies and replace the host queen. The offspring of the parasite is reared by the host workers. Females locate host colonies by nest searching flights and recognition of species-specific nest odours at the entrance. We investigated inter- and intraspecific odour variation of 45 hydrocarbons of nests of potential hosts by coupled gas chromatography/mass spectrometry and tested the preferences of the parasite females of B. bohemicus and B. rupestris for these mixtures in Y-olfactometer choice tests. Interspecific and intracolonial differences in the odour bouquets of the host species were found to be predominantly due to different patterns of alkenes. Furthermore, we found intercolonial differences within the single species. In behavioural assays, females of the two species showed different preferences for the offered nest odours, implicating different host spectra. Bombus rupestris showed a clear preference for the scent of its host, B. lapidarius. Bombus bohemicus females were attracted by B. terrestris, B. lucorum, and B. cryptarum in a similar manner. The results show that volatile signals enable parasite females to discriminate between potential host species.     

Glucose and glycerol uptake by isolated zooxanthellae fromCassiopea xamachana: Transport mechanisms and regulation by host homogenate fractions

We investigated the mechanisms by which glycerol and glucose enter freshly isolated zooxanthellae (FIZ),Symbiodinium microadriaticum Freudenthal, of the mangrove jellyfish,Cassiopea xamachana Bigelow, and the specific sites of host factor interaction. Glucose entry into FIZ is accomplished by a Na⁺-dependent symport system driven by an electrochemical gradient generated via a Na⁺-K⁺ ATPase. Inhibition of glucose uptake by a low molecular weight fraction of host homogenates [mol. wt<2 kilodaltons (kD)] occurs through the interaction of a putative host factor with the carrier protein and not the ATPase. Glycerol entry is apparently accomplished by simple or facilitated diffusion and is not affected by host homogenate fractions.     

The role of chemosensory behavior of Symbiodinium microadriaticum,intermediate hosts,and host behavior in the infection of coelenterates and molluscs with zooxanthellae

Symbiotic dinoflagellates, Symbiodinium microadriaticum (=zooxanthellae), may gain access to aposymbiotic hosts (i.e., those lacking zooxanthellae) by chemosensory attraction of the motile algae by the potential host or via an intermediate host. Laboratory experiments showed that motile zooxanthellae were attracted to intact aposymbiotic host animals, but not to starved symbiotic hosts. Fed symbiotic hosts and brine shrimp (Artemia sp.) nauplii also attracted motile zooxanthellae. The attraction of these zooxanthellae was directly correlated with nitrogen levels in the seawater surrounding the hosts; thus ammonia and possibly nitrate could be atractants. Brine shrimp nauplii, acting as intermediate hosts actively ingested both motile and non-motile zooxanthellae. the ingested zooxanthellae tended to remain morphologically unaltered during and after passage through the gut of the brine shrimp. Capture and ingestion of brine shrimp containing zooxanthellae by aposymbiotic scyphistomae of the jellyfish Cassiopeia xamachana led to infection of the scyphistomae with zooxanthellae. Zooxanthellae isolated from 17 different species of coelenterates and molluscs could be transferred via brine shrimp to the endodermal cells of the scyphistomae. However only 10 of these isolates persisted to establish a permanent association with C. xamachana. Scyphistomae in suspensions of motile zooxanthellae responded by a classical coelenterate feeding response, which may facilitate ingestion of the potential symbionts and establishment of a symbiosis.     

Spatial and temporal occurrence of the parasitic dinoflagellate Duboscquella cachoni and its tintinnine host Eutintinnus pectinis in Chesapeake Bay

Eutintinnus pectinis (Kofoid, 1905) Kofoid and Campbell is a seasonally important component of the Chesapeake Bay, USA, microzooplankton. During the summers of 1986 and 1987, E. pectinis populations commonly reached densities well above 10³ cells l^-1 and were often heavily infected by Duboscquella cachoni Coats, 1988, a lethal parasitic dinoflagellate. The temporal and spatial occurrence of D. cachoni suggests that this parasite has a significant impact on E. pectinis populations and may, under appropriate conditions, regulate host abundance. Infection levels above 10% were frequently encountered and epizootic events with 20 to 50% of host individuals parasitized by dinoflagellates were observed over broad areas. Epizootic infections were usually recorded in regions of the Bay that had high concentrations of E. pectinis, and data for vertical profiles, when integrated with depth, showed a significant positive correlation (p0.01) between host abundance and parasite prevalence. However, peak E. pectinis density and maximum levels of parasitism were often not vertically coincident at stations, and a clear relationship between host abundance and parasite prevalence was not evident for data from discrete samples. These vertical distributions and the absence of a correlation between host density and parasite prevalence for discrete samples may reflect death of E. pectinis as epizootics spread through the host population. Bay-wide infection levels averaged 10.4% (±1.73 SE) for eight cruises in 1986 and indicate that parasite-induced mortality removes 7 to 24% of the E. pectinis standing stock per day. Comparison of these values to ingestion rates for copepods on tintinnine ciliates reveals that parasite-induced mortality of E. pectinis is comparable to predation pressure by the dominant mesozooplankton grazers in Chesapeake Bay.     

Differences in the electroantennal responses of apple- and hawthorn-infesting races of Rhagoletis pomonella to host fruit volatile compounds

Summary. Domestic apple (Malus pumila)- and hawthorn (Crataegus sp.)-infesting races of Rhagoletis pomonella, Walsh (Diptera: Tephritidae) provide an excellent model to examine the role that host plant specificity plays during sympatric speciation (i.e., divergence in the absence of geographic isolation). Previous work has shown that these races differ in their propensities to accept apple and hawthorn fruits in behavioral choice assays, and that this discrimination translates into "host fidelity" in the field (i.e., apple flies tend to mate on and oviposit into apples and hawthorn flies on hawthorns). ?We present the results of a study examining possible physiological factors contributing to host choice differences in R. pomonella. We tested whether apple and hawthorn flies differ in their electroantennogram (EAG) responses to biologically relevant volatile compounds emitted from apples and hawthorns. Significant differences were found in the relative EAG responses of apple and hawthorn flies to host fruit compounds at five of six paired study sites across the eastern United States. The geographic pattern of EAG variation was complex, however, with local populations of apple and hawthorn flies tending to be more similar to one another than to flies of the same race at distant sites. This pattern was largely due to EAG responses for several compounds showing longitudinal or latitudinal clines, the latitudinal clines being similar to those observed for allozyme loci in the host races. We also found evidence for sex-related differences, as males tended to have higher mean EAG responses to compounds than females. Host-associated differences were therefore nested within geographic and sex-related differentiation in R. pomonella.?Further behavioral studies are needed to distinguish whether the EAG differences are responsible for, as opposed to being a consequence of, host-plant fidelity and adaptation. Crosses are also required to establish a genetic basis for the EAG responses, although we did find significant correlations between EAG scores for several compounds and the allozymes NADH-Diaphorase-2 and Hydroxyacid dehydrogenase at one of the study sites. Questions therefore remain concerning the evolutionary significance of the EAG response differences between apple and hawthorn fly races. Nevertheless, these differences raise the possibility that antennal responses to fruit-related volatile compounds contribute to host plant discrimination in R. pomonella. Regardless, the EAG responses represent another set of traits, in addition to diapause/eclosion time phenotypes and allozyme frequencies, differing between apple and hawthorn host races of R. pomonella. Received 17 March 1998; accepted 21 September 1989.