Specificity of attachment sites of larval water mites (Hydrachnidia, Acari) on their insect hosts (Chironomidae, Diptera) – evidence from some stream-living species

This study concerns the parasite-host associations of water mite larvae and their chironomid hosts in a small stream on the North German Plain. The different feeding sites on the host were tested as to whether they represented a strategy of the parasites regarding host partitioning. The attachment sites of nine ectoparasitic water mite species were observed in detail: Aturus fontinalis, Atractides nodipalpis, Feltria rouxi, Hygrobates nigromaculatus, Protzia eximia, Sperchonopsis verrucosa, Sperchon clupeifer, S. setiger and Lebertia inaequalis. Aturus fontinalis, A. nodipalpis, F. rouxi and H. nigromaculatus distinctly preferred sites on the abdomen of the host, whereas the other species preferred feeding sites on the thorax. The four species that preferentially attached to the abdomen of the host showed a distinct specificity for selected segmental and/or intersegmental regions. All species differed in their sites along the length of the abdomen. The order of attachment on the abdominal segments was, from anterior to posterior: H. nigromaculatus, F. rouxi, A. fontinalis and A. nodipalpis. The sites were analysed with regard to segmental and intersegmental attachment, the proportion of dorsal and ventral sites and the differences between attachment to different host species. Larvae attached to their hosts as single individuals showed only slight differences from the sites of mites on hosts that carried many mite larvae. The finding that less than 10% of the chironomids were parasitized by more than one water mite species suggested that, at least in the zoocoenosis of the studied collecting site, the interspecific competition for attachment sites was not strong. However, the specificity of attachment sites clearly had the potential of diminishing competition between water mite species by host partitioning. Intra- and interspecific competition for preferred sites and preparasitic constraints are discussed.     

Parasite-host associations and life cycles of spring-living water mites (Hydrachnidia, Acari) from Luxembourg

Larval water mites are parasites of various insect species. The main aim of the present study was to analyse the host range of spring dwelling water mites. The investigation focuses on seven spring sites in Luxembourg. Some 24 water mite species were recorded either from the benthos or as parasites attached to flying insects captured in emergence traps. For 20 mite species 35 host species from four Nematocera (Diptera) families were recorded. About 80% of the host species and over 90% of the host individuals were Chironomidae, the others were Limoniidae, Dixidae and Simuliidae. For all water mite species recorded we present the observed host spectrum and/or potential hosts as well as the intensity of parasitism and the phenology of the mites. For 10 mite species the hosts were previously unknown. For another ten species the known host spectrum can be confirmed and extended. The host spectrum ranged from one host species (e.g. for Sperchon insignis) to at least 10 host species (for Sperchon thienemanni, Ljania bipapillata), but the effective host range could not be definitively estimated due to the lack of corresponding data. The hypothesised host preference of the water mites, of which most are strictly confined to spring habitats, for similarly spring-preferring hosts could not be proven. The mean intensity of parasitism was highest for Thyas palustris (10.8 larvae/host) and lowest for Sperchon insignis and Hygrobates norvegicus (1.2 larvae per host for each). The hydryphantid mite Thyas palustris occurred at maximal intensity (41 larvae per host) and the two abdominal parasites Ljania bipapillata and Arrenurus fontinalis showed higher mean intensities than the thoracic parasites did. Larval water mites parasitising chironomids did not exhibit a preference for host sex. The phenology of the larval mite species was varied, some species were only present in samples early in the year and others exclusively in the summer. Another species showed two peaks of occurrence, springtime/early summer and late summer/autumn. In conclusion, the water mite larvae in the studied springs showed differences in host spectra and phenology but there are no clear evidences in both for host partitioning. Maybe, the relative low species diversity of water mites in individual springs and the low inter-specific competition for suitable hosts in combination with the high host abundances and species richness makes springs such favourable habitats for the mites.     

Larval parasitism of spring-dwelling alpine water mites (Hydrachnidia,Acari): a study with particular reference to chironomid hosts

During faunistic investigations on spring habitats in the alpine National Park Berchtesgaden (Bavaria, Germany), water mites were found to be the group with the highest share of species strongly adapted to springs. At four sample sites at two spring complexes, insect emergence was screened for parasitism by larval water mites. A total of at least 36 host species were recorded as being parasitized by 19 water mite species. As in many other habitats, the most important host taxon was shown to be the nematoceran family Chironomidae, both in the number of species and individuals parasitized. Likewise, the number of water mite species attached to chironomids was high. Further host species were found among the Plecoptera, Trichoptera, Coleoptera, Limoniidae and Empididae (Diptera). These taxa were only parasitized by a single water mite species in each case. For 13 mite species, new hosts were recorded for the first time. For another six species, the known host spectrum could be confirmed and/or supplemented. The parasitological data presented (e.g., prevalences, selected attachment sites on the host, larval phenology, intensity of parasitism) provide, in most cases, basic information concerning previously unknown parasite–host associations. At this time, the reason for the strong crenobiosis in water mites cannot be explained by their parasitism.     

Parasitism ofAcyrthosiphon pisum byAllothrombium pulvinum (Acariformes: Trombidiidae): Host attachment site,host size selection,superparasitism and effect on host

Several aspects of parasitism of the pea aphid,Acyrthosiphon pisum (Harris), by the mite parasiteAllothrombium pulvinum Ewing, were examined in the laboratory. Larvae ofA. pulvinum were fastmoving mites that find their host by contact of their foretarsi with the host. They can attach to all parts of the host body, but insert their chelicerae only into weakly sclerotized parts such as intersegmental membranes. Of the attached larval mites, most (63.5–74.1%) were on the thorax of their hosts, regardless of host size. In hosts of small and medium size, the ventral side receives most parasitism, whereas in large hosts the lateral sites are most often attacked. Larval mites prefer large hosts when allowed to select between pairs of large and small hosts, but show no significant preference when allowed to choose between pairs of large and medium hosts or pairs of medium and small hosts. In two-choice tests, larval mites prefer previously parasitized hosts to umparasitized hosts, which results in superparasitism of the hosts. When the mite load is fiveA. pulvinum kills all small hosts within three days, and all medium hosts and 50% of large hosts within four days, the reproduction of surviving adult aphids were significantly reduced. Host-finding behavior, attachment site preference, host size selection, superparasitism, and effect on hosts are briefly reviewed for larval parasites of Trombidiidae. The potential role of larvalAllothrombium in integrated and biological aphid control is also discussed.     

The host specificity of Lepeophtheirus pectoralis (Müller, 1776) (Copepoda: Caligidae)

The host specificity of Lepeophtheirus pectoralis (Müller) was examined experimentally by exposing different fish species to infection by artificially reared copepodid larvae. Copepodids which were hatched from eggs of adults parasitic on plaice ( platessae copepodids) preferred plaice to all other fishes tested, whereas copepodids which were hatched from eggs of adults parasitic on flounder ( flesi copepodids) preferred flounder to all other fish species. These behavioural differences suggest that two strains of L. pectoralis exist, one ( platessae ) adapted to plaice as its host and the other ( flesi ) to flounder. Comparison of an experimentally derived order of host preference with a table of occurrence obtained from the literature, suggests that a third strain of L. pectoralis , adapted to dab as its host, might also occur.
The process of infection by L. pectoralis copepodids is also described. It comprises a host location phase, during which the copepodid enters the habitat of its flatfish hosts and locates a host individual, and an attachment phase. The host location phase appears to be governed by changes in the activity of the copepodid and by its positively rheotactic response to water currents produced by the host. The attachment phase is probably based on the response of the copepodid to chemical factors produced by the host.     

Development of Habrolepis rouxi Compere (Hymenoptera: Encyrtidae) in two armoured scale hosts (Homoptera: Diaspididae) and parasite egg encapsulation by California red scale

Abstract. 1. California red scale Aonidiella aurantii (Maskell) and oleander scale Aspidiotus nerii Bouché (Homoptera: Diaspididae) are not completely suitable for the development of the encyrtid endoparasite Habrolepis rouxi Compere.
2. Mortality of parasite immatures in California red scale is caused mainly by egg encapsulation. Although oleander scale is unable to encapsulate H. rouxi , it is nutritionally unfavourable for the parasite's development.
3. Some immunity to the parasite is conferred to the oleander scale by the particular host plant used.
4. The parasite's rate of development is not influenced by the host species studied.
5. The mortality of parasite immatures in California red scale is lower than in oleander scale. The former host also is more acceptable and preferred by the parasite for ovipositon. These characteristics of the California red scale render it more suitable than oleander scale as host for H. rouxi.
6. Studies on the encapsulation of H. rouxi eggs by the California red scale indicate that (i) encapsulation does not prevent third instar host larvae from maturing and producing progeny; (ii) neither rearing temperature nor host age affect encapsulation frequency.     

Competitive strategies between Anastrepha fraterculus and Anastrepha obliqua (Diptera: Tephritidae) regulating the use of host fruits

Interspecific competition between individuals of different species can result in reductions in their fecundity, growth or survival, reflecting differential exploitation of resources that become intensified due to spatial co-occurrence, ecological similarity and increased population densities. As two species cannot occupy the same niche, coexistence is only possible if the available resources are used in non-overlapping manners such as niche partitioning or the use of refuges. Among agricultural insect pests, such as fruit flies of the family Tephritidae, competitive interactions can result in competitive displacement, host changes, or the expansion or restriction of the numbers of hosts utilized that can have negative consequences for human agricultural activities. We evaluated the competitive interactions between two fruit fly species of the genus Anastrepha, Anastrepha obliqua (Macquart, 1835) and Anastrepha fraterculus (Wiedmann, 1830), on their respective preferred hosts (mangoes and guava). Experiments of larval competition and competition for ovipositioning sites by adult females were performed to compare the parameters of larval development time, numbers of pupae and emerged adults and numbers of ovipositions in the presence or absence of interspecific competition. We observed that the interactions between those species were asymmetrical and hierarchical, and our results suggest a competitive displacement of A. fraterculus by A. obliqua when those two species are present on the same fruit, whether mangoes or guavas.     

Distribution of chigger mites (Acari: Trombiculidae) over hosts,parasitopes, collection localities,and seasons in northern Iran

We studied the distribution of chigger mite species over mammal hosts, attachment sites on the host body, habitats, and seasons in Iran. The study was based on 2155 specimens of 36 chigger species collected from 10 species of Muridae, Cricetidae, and Soricidae across six provinces of northern Iran. A high level of mixed infestation by chiggers was recorded—76% of hosts parasitized by chiggers were infested by more than one (2–8) species. Statistically significant differences in the preference for anterior and posterior parts of the host body were found. Three species—Neotrombicula lubrica, N. delijani, and Cheladonta firdousii—preferred the posterior part of the host body; 12 species were characterized by the occurrence in the anterior part and differed from one another by the frequency of presence in the posterior part. One species, Hirsutiella alpina, was found only in the anterior part of the host body (inside the ears of rodents). The most diverse chigger fauna was on the fringe of Golestan National Park (species richness?=?21, Shannon–Wiener index?=?2.823). The chigger fauna of the high-mountain localities on the Alborz Range was the least diverse (species richness?=?16, Shannon–Wiener index?=?2.439). The seasonal aspect of activity was evident for Neotrombicula elegans, which exposed the autumn–winter period of the occurrence on hosts, and N. vernalis, with the winter-spring peak of abundance.

     

Insect emergence from a shallow southern West German lake,with special reference to the parasitic host-associated water mite larvae

Insect emergence was studied with floating box traps in the littoral of Lake Mindelsee from July 1st until September 30th, 1984. The material was collected every 48 h. Out of the total catch of 6897 specimens per m², 75.1 % belonged to the Chironomidae, 4,9% to the Chaoboridae, 15,4% to the Ephemeroptera and 3,2% to the Trichoptera. Endochironomus sp., Lauterborniella agrayloides, Polypedilum pullum (Chironomidae) and Oocetis testacea (Trichoptera) are reported for the first time from Lake Mindelsee. The seasonal course of the total emergence showed two distinct peaks: one at the end of July, the other at the beginning of September. Diurnal emergence was studied on two occasions, in July and in August. Most of the insects emerged during dusk and dawn. Species composition and degree of water mite infection of the collected nematocerans were studied. Host range and specific attachment sites of the different water mite taxa could only be established on the genus or family-level, with the exception of the most common water mite species Hydrodroma despiciens. In addition to its main host Chaoborus flavicans, this mite species infested 16 chironomid species with Corynoneura sp., Cryptocladopelma viridula, Pseudochironomus prasinatus and Tanytarsus lactescens as new records. The larvae of Hydrodroma despiciens were attached almost exclusively to the thorax of the imagines. Chaoborus flavicans, and within the Chironomidae Procladius sp. of the subfamily Tanypodinae, showed the highest average mite load. Species of the subfamily Chironominae were also heavily infested (e.g. Microtendipes sp., infestation rate 76 % with a maximum mite load of 18). Within the representatives of the subfamily Orthocladiinae however, only low infestation rates occurred with maximum one larva per host. Finally, problems like the synchronization of the life-cycles of hosts and parasites and strategies of coexistence between the different water mite taxa parasitizing one host are discussed.     

不同寄主对小线角木蠹蛾幼虫消化解毒酶的影响

小线角木蠹蛾Streltzoviella insularis是一种重要的林木蛀干害虫,在我国北方地区广泛分布,其寄主范围广,尤其对城市园林树种威胁严重。本研究测定了小线角木蠹蛾幼虫分别取食3种喜食寄主(洋白蜡、银杏和国槐)和4种普通寄主(旱柳、栾树、香椿、毛桃)后的解毒酶(细胞色素P450酶、谷胱甘肽S转移酶和羧酸酯酶)和消化酶(内切-β-1,4-葡聚糖酶、外切-β-1,4-纤维二糖水解酶、β-葡萄糖苷酶和木聚糖酶)的活性。结果表明,不同寄主对小线角木蠹蛾幼虫的消化酶和解毒酶都存在显著影响,其中取食普通寄主的幼虫3种解毒酶的平均活性(P450、GST和CarE酶活性分别为21.12、4876.86、0.035μmoL/min/mg protein)均高于取食喜食寄主的幼虫(P450、GST和CarE酶活性分别为15.87、3760.55、0.027μmoL/min/mg protein),且普通寄主的平均次生代谢物质含量更高(普通寄主总酚、类黄酮和单宁含量分别为173.10、110.07、88.77 mg/g,喜食寄主为105.22、14.05、55.74 mg/g);而取食普通寄主与喜食寄主的幼虫EG和CBH酶活性之间没有显著差异,仅取食普通寄主的幼虫β-葡糖苷酶活性和木聚糖酶活性略高于取食喜食寄主的幼虫,幼虫的消化酶活性(纤维素酶和半纤维素酶)和寄主营养物质(纤维素和半纤维素)并未在普通寄主和喜食寄主中表现出明显趋势。因此本研究认为小线角木蠹蛾的寄主适应性与寄主次生代谢物质(总酚、类黄酮和单宁)含量的关联性更强,而与营养物质(纤维素和半纤维素)含量的相关性较弱。     

Abundance patterns of two Oropsylla (Ceratophyllidae: Siphonaptera) species on black-tailed prairie dog (Cynomys ludovicianus) hosts.

Behavioral, genetic, and immune variation within a host population may lead to aggregation of parasites whereby a small proportion of hosts harbor a majority of parasites. In situations where two or more parasite species infect the same host population there is the potential for interaction among parasites that could potentially influence patterns of aggregation through either competition or facilitation. We studied the occurrence and abundance patterns of two congeneric flea species on black-tailed prairie dog (Cynomys ludovicianus) hosts to test for interactions among parasite species. We live-trapped prairie dogs on ten sites in Boulder County, CO and collected their fleas. We found a non-random, positive association between the two flea species, Oropsylla hirsuta and O. tuberculata cynomuris; hosts with high loads of one flea species had high loads of the second species. This result suggests that there is no interspecific competition among fleas on prairie dog hosts. Host weight had a weak negative relationship to flea load and host sex did not influence flea load, though there were slight differences in flea prevalence and abundance between male and female C. ludovicianus. While genetic and behavioral variation among hosts may predispose certain individuals to infection, our results indicate apparent facilitation among flea species that may result from immune suppression or other flea-mediated factors.     

Niche Partitioning of Feather Mites within a Seabird Host, Calonectris borealis

According to classic niche theory, species can coexist in heterogeneous environments by reducing interspecific competition via niche partitioning, e.g. trophic or spatial partitioning. However, support for the role of competition on niche partitioning remains controversial. Here, we tested for spatial and trophic partitioning in feather mites, a diverse and abundant group of arthropods. We focused on the two dominant mite species, Microspalax brevipes and Zachvatkinia ovata, inhabiting flight feathers of the Cory’s shearwater, Calonectris borealis. We performed mite counts across and within primary and tail feathers on free-living shearwaters breeding on an oceanic island (Gran Canaria, Canary Islands). We then investigated trophic relationships between the two mite species and the host using stable isotope analyses of carbon and nitrogen on mite tissues and potential host food sources. The distribution of the two mite species showed clear spatial segregation among feathers; M. brevipes showed high preference for the central wing primary feathers, whereas Z. ovata was restricted to the two outermost primaries. Morphological differences between M. brevipes and Z. ovata support an adaptive basis for the spatial segregation of the two mite species. However, the two mites overlap in some central primaries and statistical modeling showed that Z. ovata tends to outcompete M. brevipes. Isotopic analyses indicated similar isotopic values for the two mite species and a strong correlation in carbon signatures between mites inhabiting the same individual host suggesting that diet is mainly based on shared host-associated resources. Among the four candidate tissues examined (blood, feather remains, skin remains and preen gland oil), we conclude that the diet is most likely dominated by preen gland oil, while the contribution of exogenous material to mite diets is less marked. Our results indicate that ongoing competition for space and resources plays a central role in structuring feather mite communities. They also illustrate that symbiotic infracommunities are excellent model systems to study trophic ecology, and can improve our understanding of mechanisms of niche differentiation and species coexistence.     

Parasitism of Hylodes phyllodes (Anura: Cycloramphidae) by Hannemania sp (Acari: Trombicuudae) in an area of Atlantic Forest, Ilha Grande, southeastern Brazil

We studied some parameters of the parasitism by the mite Hannemonia sp. on the endemic frog Hylodes phyllodes in the Atlantic Forest of Ilha Grande (Rio de Janeiro State, Brazil). Prevalence, mean abundance, mean intensity and total intensity of infestation, body regions infected, and host sexual differences in parasitism rate of the larvae of Hannemania sp. on individuals of H. phyllodes were assessed. Prevalence was 86.5% (N = 37; total of 1,745 larvae of Hannemania sp) for male hosts and 91.7% (N = 12; total of 330 larvae) for female hosts, with no significant difference between the sexes. Overall prevalence of Hannemania sp. on H. phyllodes was 87.7%. Mean intensity of infestation in males (54.5 +/- 42.5; range 1-173 larvae) was higher than in females (29.9 +/- 47.6; range 1-166). We conclude that the rates of intensity, abundance, and prevalence of Hannemania larvae parasitizing Hylodes phyllodes at Ilha Grande were considerably high, suggesting that this species of anuran constitutes a relevant host for this mite species to complete its life-cycle in the area. Differences between males and females in infestation rates probably reflect their differential use of space in the forest.     

The attachment and stylostome of Trombidium newelli (Acari: Trombidiidae), an ectoparasitic mite on adults of alfalfa weevil, Hypera postica (Coleoptera: Curculionidae)

Larvae of the mite Trombidium newelli Welbourn and Flessel are ectoparasitic on adult alfalfa weevil, Hypera postica (Gyllenhall), an insect pest of alfalfa. The mite larvae are found under the elytra, attached to the dorsal surface of weevils abdomen. T. newelli larvae use their chelicerae to penetrate the pliable and weakly sclerotized areas of the hosts integument, and to hold on firmly to the host. The attachment sites associated with larval mites of different degrees of engorgement were examined using both light and scanning electron microscopy. The ventral inside surface of parasitized host tergites revealed a characteristic injury as spots varying in color from white-yellow to golden-brown depending on the engorged size of the mite. In addition, spots associated with fully engorged mite larvae showed an aggressive dendritically radiating mass expanding outward from the site of cheliceral penetration. This mass, known as feeding tube or stylostome was branched, with each branch ending in a cluster of closed bulbs. Stylostomes appear to exist independently in the hosts tissue. Stylostome mass corresponded with the engorgement state of the mite, increasing as the mite larva increased in size. The possible nature and mechanism of stylostome formation is discussed.     

小菜蛾主要寄生性天敌——菜蛾绒茧蜂与菜蛾啮小蜂间的相互作用

在28℃下,以小菜蛾3龄幼虫作寄主,研究了菜蛾绒茧蜂与菜蛾啮小蜂间的相互关系．当寄主供2种蜂同时产卵寄生时,与只供1种蜂时相比。绒茧蜂的寄生率无显著变化,而啮小蜂的寄生率则显著下降;2种蜂的合计寄生率与任一种蜂单独存在时相比无显著差异．当寄主先供绒茧蜂寄生,再供啮小蜂寄生时,绒茧蜂的成功寄生率不受影响,而啮小蜂的寄生率仅为8％～13％;啮小蜂能寄生在寄主体内的绒茧蜂高龄幼虫．绒茧蜂能寄生已被啮小蜂寄生的寄主幼虫,其子代部分个体能正常发育至成虫羽化．当已被绒茧蜂寄生和未被寄生的寄主同时存在时,啮小蜂主要寄生未被寄生的寄主．表明绒茧蜂具有竞争优势。但这种优势可因啮小蜂的寄生而被削弱．     

Behavioural responses of Varroa destructor (Acari: Varroidae) to extracts of larvae, cocoons and brood food of worker and drone honey bees, Apis mellifera (Hymenoptera: Apidae)

Abstract. Varroa destructor is a parasitic mite of the honey bee species Apis cerana Fabr . and A. mellifera L. Mature females reproduce on the immature stages of their hosts, producing more viable female offspring on drone hosts than on worker hosts. Thus, immature drones are more likely to be infested with mites than immature workers. To investigate the hypothesis that differences in host chemistries underlie the biased distribution of mites between worker and drone brood, the arrestment responses of mites to solvent extracts of a number of stimuli normally encountered by a mite during its life cycle were measured. Mites were arrested by cuticular extracts of worker and drone larvae obtained at 0, 24 and 48 h prior to the time when cell capping is completed. Mites were also arrested by extracts of worker and drone, brood food and cocoons, and by a blend of synthetic fatty acid esters previously shown to be active in the host acquisition process. In a wind tunnel bioassay, mites were attracted to odours from living fifth-instar worker and drone larvae, but not to volatiles from cocoons, brood food or a blend of fatty acid esters. The sex of the host was not an important factor affecting the behavioural responses of the mites in any assay. We conclude that host kairomones play a role in the host acquisition process, but we found no evidence to support the hypothesis that mites use these substances to differentiate between worker and drone brood.     

Seasonal abundance of Allothrombium monochaetum and Allothrombium pulvinum in Navarra-Nafarroa (northern Spain), with notes on larval host preference and rate of parasitism

In this paper we report on the seasonal abundance of velvet mite larvae of Allothrombium pulvinum Ewing and Allothrombium monochaetum Goldarazena and Zhang in a meadow located in the mediterranean Navarra-Nafarroa region (northern Spain). Larvae of A. monochaetum peaked in numbers on Aphis fabae in mid-June while larvae of A. pulvinum peaked in the third week of July. Both species preferred the aphid thorax as attachment site. We also provide a list of aphid hosts of both mites and observed density of parasites on the hosts.     

Competitive interactions among four pest species of earth mites (Acari: Penthaleidae)

Earth mites are major winter pests of a variety of crops and pastures in southern Australia. Competition between four earth mite species was investigated using field and shadehouse experiments. The influence of different plant hosts on the frequency and intensity of competitive interactions also were examined. This information is important, because control attempts that eradicate one species of mite could be directly followed by an increase in abundance of another earth mite species. There were strong effects of intraspecific competition on the reproductive rate of species, while interspecific interactions between Halotydeus destructor (Tucker) and Penthaleus species and between the three Penthaleus species also were detected. Competitive abilities were altered on the different plant types. On pasture, the competitive advantage swayed between Penthaleus major (Dugés), H. destructor, and Penthaleus falcatus (Qin & Halliday). Penthaleus sp. x was the strongest competitor in a mixture of wheat, Triticum aestivum (L.), and oats, Avena sativa (L.), whereas on canola, Brassica napus (L.), and bristly ox-tongue, Picris echioides (L.), P. falcatus, and H. destructor were superior competitors. These results suggest that competition is a strong force influencing the abundance of earth mites in the field and that host plant factors are important in shaping the type of interactions. This highlights the importance of identifying mite species when considering control options and suggests that effective control recommendations need to be developed for each individual species.     

Parasitism of the backswimmer Buenoa scimitra (Hemiptera: Notonectidae) by the water mite Hydrachna virella (Acari: Hydrachnellae)

SUMMARY. The life cycle of Hydrachna virella , a water mite parasitic on the notonectid backswimmer Buenoa scimitra , is described. The mite parasitizes immature instars 2–5 as well as adult males and females of the notonectid. Parasitized immature backswimmers spent longer periods of time between moults than unparasitized immature backswimmers. Initially attaching mites appeared to be generally distributed over the host body surface but previously attached mites that reattached after host ecdysis were found primarily on the dorsum of the thorax. The observed frequency distribution of mite larvae on the host was similar to that predicted by the negative binomial distribution.     

Host specificity among Unionicola spp. (Acari: Unionicolidae) parasitizing freshwater mussels

Water mites of Unionicola spp. are common parasites of freshwater mussels as adults, living on the gills, or mantle and foot of their hosts and using these tissues as sites of oviposition. The present study addresses specialization among North American Unionicola mussel-mites using 2 measures of host specificity: (1) the number of host species used by a species of mite; and (2) a measure that considers the taxonomic distinctness of the hosts utilized by mites, weighted for their prevalence in the different hosts. Results of this study indicate the Unionicola spp. mussel-mites are highly host specific, with most species occurring in association with 1 or 2 species of hosts. If 2 or more host species are utilized, they are typically members of the same genus. These data are consistent with studies examining the dispersal abilities and host recognition behavior for members of the group. When the average values of host specificity for Unionicola subgenera were mapped on a phylogenetic tree for these taxa, a clade comprised of gill mites appeared to be more host specific than a clade consisting of mantle mites. There were, however, no apparent patterns of host specificity within each of the clades. Differences in specificity between the 2 lineages may reflect either a long evolutionary history that gill mites have had with host mussels or the intense competition among gill mites for oviposition sites within unionid mussels, leading to increased host specialization.