Jan Landin

Diving beetles (Dytiscidae) as predators of mosquito larvae (Culicidae) in field experiments and in laboratory tests of prey preference

Field experiments were performed in artificial ponds to evaluate how the density of predatory diving beetles (Dytiscidae) would affect the population levels of mosquito larvae (Culicidae). Mosquitoes colonizing the ponds were predominantly species of the genus Culex. In 2000, most of the dytiscids colonizing the ponds were small (Hydroporus spp.), and these predators had no impact on the size of larval mosquito populations, not even in ponds with added dytiscids. In 2001, larger beetles (Ilybius, Rhantus, and Agabus spp.) were more common, and there were significantly fewer mosquito larvae in ponds with the highest numbers of dytiscids. There was a negative correlation between numbers of diving beetles in the ponds and the mean body length of mosquito larvae. In neither year could dytiscid densities be maintained above a certain level owing to emigration. In laboratory tests, there were marked differences between three common dytiscid species in regard to preferences for Daphnia and Culex species as prey: Colymbetes paykulli Erichson chose mosquito larvae more often, whereas both Ilybius ater (De Geer) and I. fuliginosus (Fabricius) preferred Daphnia spp. All of the tested dytiscids consumed large numbers of prey. Since some dytiscid species can efficiently decrease populations of mosquito larvae, they are probably important in the natural control of these dipterans.

Biological diversity versus risk for mosquito nuisance and disease transmission in constructed wetlands in southern Sweden.

Abstract.  In southern Sweden, many wetlands have been constructed, and maintaining or increasing biological diversity is often included in the aims. Some wetlands are constructed near human settlements, thus raising the problem of wetlands being associated with mosquitoes (Diptera: Culicidae). Increased biodiversity (including mosquito diversity) is considered desirable, whereas mosquito nuisance from a human point of view is not. Adult mosquito abundance, diversity and species assemblages of constructed wetlands were compared to natural wetlands. The potential of constructed wetlands for mosquito nuisance and transmission of mosquito‐borne viruses was evaluated. The study areas included five constructed and four natural wetlands. Mosquito abundance and species richness were higher in the natural than in the constructed wetlands, and showed a positive correlation with wetland size. Mosquito species assemblages formed three clusters, which were not explained by origin, size and water permanence of wetlands. In a redundancy analysis, however, mosquito faunas showed significant relationships with these variables, and size and origin of wetlands were most important. Major nuisance species (multivoltine species feeding on mammals and laying eggs on soil) were found in all wetlands, although in relatively low numbers. Risk assessment for Sindbis virus transmission showed moderate risk for two constructed wetlands near human settlements. It is concluded that small size of constructed wetlands has the advantage of low mosquito numbers from a human point of view. The use of functional groups is recommended as a tool for presenting mosquito data to the public, and for helping communication between scientists and administrative decision makers.

DIVING BEETLE (DYTISCIDAE) ASSEMBLAGES ALONG ENVIRONMENTAL GRADIENTS IN AN AGRICULTURAL LANDSCAPE IN SOUTHEASTERN SWEDEN

We investigated diving beetle (Dytiscidae) assemblages in twelve wetlands in an agricultural landscape in southeastern Sweden. Beetles were trapped in wetlands that varied in permanence (temporary or permanent), area (25 to 4,800 m2), age (11 to > 50 yr), and shading (open to wooded surroundings). Our objective was to determine if those environmental factors are important in structuring the local assemblages of diving beetles and how the combination of different types of wetlands influence the diversity of diving beetles in a landscape. Generally, species-area relationships were weak, and shaded wetlands, both permanent and temporary, of intermediate size (240-1,100 m2) had the highest richness after a rarefaction analysis. It was not possible to discern a certain type of wetland where diversity was highest (measured by index α and Shannon-Wiener’s index), although, again, intermediate sized wetlands did tend to be more diverse than others. Similarities in species compositions were highest among environmentally similar wetlands, and assemblage structure differed substantially between different types of wetlands. Results of ordination (CCA) and variance partitioning revealed that permanence and degree of shading were the most important factors in structuring assemblages. Our findings imply that high diversity of the diving beetles depends on the number of wetland types represented in a landscape. It is possible to achieve high diversity in a small area by combining permanent and temporary wetlands, as well as many age and successional stages, located in wooded and open environments.

Population structure and movements of a threatened butterfly (Lopinga achine) in a fragmented landscape in Sweden

Abstract The red-listed butterfly Lopinga achine was studied by mark–recapture methods in southern Sweden for three seasons. We examined movement within and between populations and egg production in relation to age. The majority of the movements were small with mean movements between recaptures of 45–54 m for males and 94–116 m for females. There were few movements between sites, 20 of 996 recaptured males moved and 36 of 391 recaptured females, even though the distance to other sites was in many cases

The relationship between population density and body size: the role of extinction and mobility

The relationship between population density and body size: the role of extinction and mobility

Influence of landscape structure on mosquitoes (Diptera: Culicidae) and dytiscids (Coleoptera: Dytiscidae) at five spatial scales in Swedish wetlands

Patterns of species diversity and community structure depend on scales larger than just a single habitat and might be influenced by the surrounding landscape. We studied the response of two insect families, mosquitoes (Diptera: Culicidae) and dytiscids (Coleoptera: Dytiscidae), to landscape variables at five spatial scales. We studied adult mosquito and dytiscid abundance, diversity, and species assemblages in relation to water permanence (area of permanent water bodies versus temporary wetlands) and forest cover (area covered by forest versus open land) within nested circles of 100 to 3000 m around trap sites in four wetlands in southern Sweden and in five wetlands in central Sweden. We found that mosquito abundance was greatest in areas with plentiful forest cover and a high proportion of temporary water, while most dytiscids favored open areas with a high proportion of permanent wetlands. However, diversity of both mosquitoes and dytiscids was positively correlated with high permanence and little forest cover. Mosquito species assemblages were mainly influenced by forest cover at a large spatial scale, whereas permanence was more important at local scales. Dytiscid species assemblages were mainly influenced by water permanence, especially at intermediate spatial scales. These results can be explained by the flight capability and dispersal behavior of mosquito and dytiscid species. The observed landscape associations of mosquitoes and dytiscids could be useful when creating new wetlands. Mosquito colonization could be reduced by creating permanent wetlands in an open landscape, which would favor colonization by dytiscids, a potential predator of mosquito larvae, while also supporting the diversity of both taxa.

Distribution of occupied and vacant sites and migration of Lopinga achine (Nymphalidae: Satyrinae) in a fragmented landscape

The distribution of occupied and vacant sites and migration of the threatened butterfly Lopinga achine were studied in the province of Ostergotland, Sweden. The probability of occupation increased with increasing patch area and decreasing distance to the nearest occupied patch, presumably due to different probabilities of colonisation and survival of the populations inhabiting the patches. Probability of female emigration from and immigration to a patch increased with decreasing area. Middle-sized patches produced the largest number of female migrants, although the highest fraction was noted for the smallest patches, and the greatest number of females was marked in the largest patch. The fraction of resident females, but not males, increased with increasing area. The observed occupancy and migration pattern have important conservation implications: all but two populations comprising three or more individuals were within 740 m of the nearest neighbour, indicating the need for networks of suitable, closely situated patches.

Production of wetland Chironomidae (Diptera) and the effects of using Bacillus thuringiensis israelensis for mosquito control

Massive mosquito nuisance problems, caused by the flood-water mosquito Aedes sticticus, occur after floods in the flood plains of the River Dalälven, central Sweden. Since 2002, the biological mosquito larvicide Bacillus thuringiensis var. israelensis (Bti) has been used to control these mosquitoes. Here, we report results from the first six years of monitoring Chironomidae, the most susceptible non-target organisms, in three wetlands with Bti-treatment against mosquitoes and in three wetlands without treatment. Emergence traps were used for continuous insect sampling from May to September each year, 2002-2007, and were emptied once a week. A total of 21,394 chironomids of 135 species were collected, and the subfamily Orthocladiinae dominated the fauna followed by Chironominae and Tanypodinae. The annual chironomid production in the wetlands was low, with an average of 1917 individuals per m(2), and 42 g ash-free dry weight per m(2). We found no reduced production of chironomids at neither family, nor subfamily level, in Bti-treated as compared to untreated wetlands. This is the first long-term follow-up study of the possible effects of Bti-based mosquito larval control on chironomid species production. In the short-term view, one species had higher production in treated areas. In the long-term view, four species had higher and one species had lower production in treated areas. We conclude that the Bti-based control of flood-water mosquitoes does not cause any major direct negative effects on chironomid production, and therefore does not seem to induce any risk for indirect negative effects on birds, bats or any other predators feeding on chironomids.

Habitats, life histories, migration and dispersal by flight of two water‐beetles Helophorus brevipalpis and H. strigifrons (Hydrophilidae)

Populations of two Helophorus species from a Swedish lake were sampled during the ice‐free periods of two years. Dispersing beetles were (rapped in light‐reflecting glass traps. Both species are univoltine and breed in the spring. The populations of adult H. brevipalpis are largest during the summer when great numbers emerge and migrate. Very few H. strigifrons adults emerge before spring. Its adult populations are largest in the spring, and small numbers of mature individuals fly with mature H. brevipalpis specimens. This is discussed in relation to their habitat utilization: H. sirigifrons inhabits permanent waters only and is dimorphic for flight musculature. H. brevipalpis inhabits both permanent and ephemeral waters and is always equipped with a functioning flight apparatus. H. brevipalpis is an effective colonizer of ephemeral waters due to its large dispersal capacity and to the fact that the habitat of juveniles differs from that of adults. Age, sexual maturation and feeding in flying and non‐flying groups are compared. Food is seldom found in the gut of fliers of either species. In the spring flying H. brevipalpis females have larger oocytes than non‐flying ones; flying H. strigifrons females have smaller oocytes than non‐fliers. It is possible that the summer migrations of H. brevipalpis favour outbreeding since fliers are sexually immature; spring fliers, being sexually mature, are more efficient as colonizers.

A six-year study of insect emergence from temporary flooded wetlands in central Sweden, with and without Bti-based mosquito control

In temporary wetlands in the River Dalälven floodplains, recurrent but irregular floods induce massive hatching of the flood-water mosquito Aedes sticticus, which causes enormous nuisance. Flood-water mosquito control using the biological larvicide Bacillus thuringiensis var. israelensis (Bti) was commenced in parts of the floodplains during 2002, and here we report the first six years of full-season monitoring of general insect emergence from temporary wetlands with and without treatment. Emergence traps, which were emptied weekly, were used from May to September each year. A total of 137,153 insects of 13 taxonomic orders were collected. Diptera was highly dominating and especially the sub-order Nematocera with 18 families was a very prominent taxon. Bti-treatment effects were analysed by taxonomic order, by sub-order in Diptera and Hemiptera, and by family for Nematocera and Coleoptera for the whole study period. We found no significant negative effects of Bti treatments on the production of insects by taxonomic order, with the exception of Coleoptera in the long term. However, no significant negative effects were found for the Coleoptera families, neither in the short term nor in the long term. There was no significant negative treatment effect on Nematocera production, neither when analyzed for the whole sub-order nor when analyzed by family. However, abundance of Ceratopogonidae was significantly higher in experimental than in reference wetlands. We conclude that Bti-treatment effects on insect production may be minute in comparison to other environmental factors structuring the insect fauna of the temporary wetlands studied.