In a laboratory experiment the survival of two amphipod species Corophium volutator and C. arenarium was differentially affected by the digenetic trematode Microphallus claviformis, the former suffering from parasite induced mortality. No clear species specific parasite preference was observed, but there was a difference between the two frequency distributions of parasite number per host, indicating a preference for the competitively superior C. volutator. In both species the parasite metapopulation was overdispersed, but to the highest degree in C. volutator. Formation of a penetration cyst on the Corophium cuticle preceeds the penetration of the M. claviformis cercariae, a common feature of microphallid digeneans, but not earlier reported from this species. Dead individuals of both species carried significantly more penetration cysts than the live ones which suggests that massive invasion of cercariae during a short period of time is the cause of death of Corophium, rather than crude parasite intensity. The potential for M. claviformis to act as a structuring agent in communities of Corophium species is discussed.

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In the Danish Wadden Sea the intertidal distribution of the introduced bivalve Ensis americanus (syn. E. directus) is restricted to a narrow zone around the mean low water level. To test the possible impact of birds and submersion time on dynamics and distribution of the clams, adult specimens of E. americanus collected near the low water line were transplanted to two intertidal sites and established in open and net-covered experimental plots for 9 weeks (autumn 2001). The lowest survival of clams was registered at the low-shore-site (LSS) in plots open to bird predators, suggesting that birds such as Common Eider (Somateria mollissima) or Oystercatcher (Haematopus ostralegus) may control the abundance of E. americanus at the lower tidal levels. For clams showing increment in shell length during the study period, the shell growth rates were highest at the LSS and lowest in the open plots at the highshore-site (HSS). DiVerences in immersion time and thus food supply may explain this pattern. Body mass index (BMI) of the clams showed basically the same pattern as the survivorship: lowest BMI in open plots at the LSS and highest in the covered plots at this site. Clams from the HSS were intermediate in their BMI. Disturbance by birds in the open plots at the LSS may explain the low BMI. In conclusion birds may be an important factor controlling abundance of E. americanus in the lower intertidal zone.

The phenology of microphallid trematodes within their intermediate host populations has been studied on an intertidal mud flat. The parasites use the mud snail Hydrobia ulvae and the infaunal amphipod Corophium volutator as first and secondary intermediate host, respectively. Migratory shorebirds act as final hosts. Our results show a general trend of decline in the density of infected intermediate hosts during both spring and autumn, which could mainly be ascribed to shorebird predation. During summer the density of both infected snails and infected amphipods increased considerably, with a culmination in June within the snail population (1000 infected m 2) and in August within the amphipod population (40 000 infected m 2). This time lag in parasite occurrence could be related to (1) the development time of larval trematodes within the snails, (2) higher ambient temperatures in late summer increasing parasite transmission between snails and amphipods during this period, and (3) a general increase in the Corophium population during late summer. From samples collected between 1990 and 1995 it is shown that microphallid trematodes occasionally may give rise to mass mortality in the amphipod population. The prerequisites for such an event are a high parasite prevalence within the first intermediate host population and unusually high ambient temperatures, facilitating parasite transmission to the secondary intermediate host, C. volutator.

Investigations on flying Cephalcia spp. imagines were carried out in a Danish spruce plantation 1980–81 by different means of trapping. Five species were caught, viz C. arvensis, C. abietis, C. alashanica, C. pallidula and C. erythrogaster, the two former species dominating the catches. The three latter species were scarce but occurred in at least three out of six spruce stands investigated. The flight period of the species are described, C. erythrogaster appearing as the first species in May followed by C. abietis and C. pallidula and later by C. arvensis and C. alashanica. The overall male:female ratio was 85:1. Males of all species were mainly caught in yellow traps situated on the soil surface, whereas the few females were caught in traps within the tree canopy as well. The results of the trappings are discussed in relation to flight periods of Cephalcia elsewhere in Europe, development time, trap efficiency and insecticide treatment. Zusammenfassung Zur Phänologie und räumlichen Verteilung von Cephalcia (Hym., Pamphiliidae)‐Imagines in dänischen Fichtenforsten In einem dänischen Fichtengebiet wurden 1980–81 mittels verschiedener Fallen die fliegenden Cephalcia spp.‐Imagines untersucht. Fünf Arten wurden gefangen: C. arvensis, C. abietis, C. alashanica, C. pallidula und C. erythrogaster, wobei die beiden ersten dominierten. Die 3 letzteren waren selten, traten aber jeweils in mindestens 3 der 6 Fichtenbestände auf. Die Flugperiode der Arten wird beschrieben. C. erythrogaster erschien als erste im Mai, gefolgt von C. abietis und C. pallidula. Zuletzt erschienen C. arvensis und C. alashanica. Das Gesamtgeschlechterverhältnis betrug 85:1 zugunsten der Männchen. Die Männchen aller 5 Arten wurden hauptsächlich in gelben Fallen an der Bodenoberfläche gefangen, während die wenigen Weibchen sich in Fallen in den Baumkronen befanden. Die Fallenfangergebnisse werden erörtert in bezug auf die Flugperiode von Cephalcia in europäischen Ländern, auf die Entwicklungszeit, Fallenwirksamkeit und Insektizidausbringung.