Beurteilung des Wiedererholungs-Potenzials emergierender Insekten am Beispiel der Büschelmücke Chaoborus crystallinus in Mesokosmosstudien unter Berücksichtigung der Populationsstruktur (Metapopulation) und der Verbreitungsmechanismen (English shop)

Preview

Table of Contents, Datei (63 KB)
Extract, Datei (230 KB)

Experimental mesocosm ponds in the Aachen area were treated twice, at an interval of two weeks, with a synthetic pyrethroid, the insecticide FASTAC SC 100 (alpha-cypermethrin a. i.). The concentration was 0.02 μg/L on each occasion. The study investigated the internal and external recovery of the phantom midge Chaoborus crystallinus. A special feature of the experimental design were tent-like net systems over some of the ponds in order to prevent migration. There were treated and untreated single ponds, a network of interconnected treated and untreated ponds, and treated ponds without nets. The insecticide concentration to be applied was determined in a preceding toxicity test in the laboratory. In the laboratory test, the sensitivity of both C. crystallinus and the potential prey organism Daphnia magna was examined.

Heterozygosity was calculated on the basis of AFLPs in order to detect the influence of the net installations and of the insecticide on population genetic parameters. In addition, genetic distance was calculated in order to analyse the relatedness of C. crystallinus in the mesocosm ponds and in comparison with other ponds in the surrounding area. Possible migration distances were also derived from this.

Shortly after the applications, it was mainly the younger larvae that responded to the contamination. Therefore, four weeks after the second application, a decline in abundance was observed due to the lack of offspring of the first generation. Although C. crystallinus showed population declines only in the younger larval stages after the first and second insecticide applications, a clear decrease in abundance became apparent five to six weeks after the second application in emergence, egg clutches, egg numbers and larvae. Owing to the small number of replicates, the result was not always significant, but a trend was always evident.

Recovery took place relatively quickly and, depending on the endpoint considered, occurred as early as six to seven weeks after the second application. Net isolation supported recolonisation because of the absence of emigration. Recovery of treated ponds occurred more quickly when control ponds were accessible and midges from the controls were able to emigrate into the treated ponds.

In the open treated ponds, recovery was somewhat slower. Emigration in addition to the insecticide effect was assumed here. Overall, C. crystallinus therefore possesses a recovery potential in isolated habitats, which is, however, all the greater when untreated ponds are present. After the decimation of the population, a reduced individual density had led to lower mortality and probably contributed to a faster recovery. A positive correlation between egg clutch density and mortality indicated this.

The significance of the migration events in the mesocosm study became clear from the number of egg clutches per female. In principle, the natural oviposition success in the isolated controls was low, at 0.33 clutches per female. A possible temperature influence and the surplus of males in this study were discussed. Owing to the toxic exposure, isolated treated ponds, with a value of 0.15, recorded around 50 % fewer egg clutches per female than the isolated controls. Oviposition success in the six-pond network, with an average of 0.28 egg clutches per female, was only slightly lower than in the single-net control. This was favoured by control individuals that had hatched and, after the contamination, immigrated into the treated ponds to lay eggs. In the entire six-pond network, the number of egg clutches per female was thus reduced by only 15 % by the insecticide. Compared with the single-net treatment, two events led to a reduction of the population in the open ponds: the insecticide contamination and emigration. The decrease due to migration was estimated at one third.

It was striking that in the open ponds and in the treated single ponds the proportion of large egg clutches was somewhat increased. Possibly more highly fecund females survived in both pond types and laid larger egg clutches. Thus previous population losses could be compensated for through larger egg clutches and a faster recovery. For the open ponds this could be a further indication of emigration. Furthermore, a connection between emigration and smaller egg clutches is assumed, because larger egg clutches were laid more frequently and oviposition was lower than in net-isolated treated ponds. The females remaining at the pond had possibly laid larger egg clutches more frequently.

Overall, the absence of emigration due to the nets, the development of several generations, the rapid degradation kinetics of the insecticide and presumably the adsorption of the insecticide to organic material favoured a rapid recovery.

An insecticide effect on the genetic population structure of C. crystallinus could not be demonstrated; however, the alleles of the net-isolated individuals at the institute were more similar to one another than in continuously open populations. The net installations had possibly prevented genetic exchange and caused a founder effect. A phylogenetic tree – the so-called Neighbor-Net – calculated that individuals in the net-isolated mesocosm ponds were more closely related to ponds 1.4 kilometres away than to ponds located 2.7 and 3.0 kilometres away. As a rule, flight distances of C. crystallinus of less than two kilometres are known from the literature and could thus be confirmed. However, a relationship could also be established with a palsa pond 25 km away in the Eifel. With dispersal over several generations, a relationship might also be possible with ponds located 25 kilometres away, such as the palsa pond.