The overwintering biology of the hoverfly Episyrphus balteatus (DeGeer) (Diptera: Syrphidae) was studied and analysed. In Europe, females of this species can overwinter as adults in a facultative, reproductive diapause. The diapause phase is characterized by the ovaries ceasing to develop and by hypertrophy of the fat body. Diapause was induced during the second and third larval instars. The critical photoperiod for inducing diapause was between 11.8 and 11.9 h, corresponding to the day lengths that occur during mid‐ to late September in Hannover, Germany. When temperatures were lower, insects could be induced into diapause at longer day lengths, similar to those that occur in early September in Hannover. A semifield study was done during the winter of 1999/2000 to confirm the results obtained under laboratory conditions and to obtain additional information on the over‐wintering development and mortality of E. balteatus. The results suggested that mortality was correlated with the duration of the experiment and with humidity, rainfall, and temperature. The importance of these results for the regulation of cereal aphid populations by this hoverfly is discussed.
A polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis using mitochondrial (A+T-rich region; mtDNA) and genomic (zen-region; nDNA) DNA was performed on 182 female individuals of Episyrphus balteatus (DeGeer), a widespread aphidophagous hoverfly with supposed migratory behaviour. Specimens originated from 13 sampling sites in six European countries. The analyses revealed 12 and 18 haplotypes, respectively, for the two DNA types, several of them with a wide distribution, although seven and eight haplotypes, respectively, occurred only in one location. In contrast to other studies on mobile insects, the genetic diversity was relatively high. However, lack of population subdivision, low genetic distances between populations, the very high gene flow rates, and the complete lack of isolation by distance suggest that E. balteatus populations are largely connected and that there is an absence of large-scale geographic structuring. These results support the hypothesis that E. balteatus is a migratory hoverfly species, capable of moving over large distances. These findings related to the seasonal migrations of this species are discussed.
Only few studies are available dealing with the relation between winter host density and spatial distribution and spring colonization of winter cereals by the host-alternating cereal aphid species Rhopalosiphum padi and Metopolophium dirhodum. Large-scale studies in climatically different agroecosystems in Germany from 2004 to 2006 revealed for R. padi and M. dirhodum larger spring/summer populations in landscapes with higher densities of winter hosts. A small-scale study was performed in winter wheat fields adjacent to a large hedge with several typical winter hosts plants, bird cherry (Prunus padus) and wild rose species (Rosa spp.) to indentify distance effects (0-8, 8-24 and 24-60 m). Weekly measurements of aphid density between May to July showed significantly higher densities of R. padi compared with those of other aphids. Statistical analysis (Tukey-Kramer test and regression analyses) revealed significant gradients from the hedge to the field centre for R. padi and M. dirhodum. In comparative studies, winged R. padi from winter and adjacent summer hosts were genotyped using four microsatellite markers. The results showed that individuals from a certain winter host were not genetically similar with individuals from neighbouring summer hosts; it, therefore, seems that winter host clones did not significantly contribute to population built-up in cereal fields over short distances. It could be concluded that on a regional scale, the density of sources for early migrants of R. padi is important for colonization intensity of surrounding summer hosts, but that the high local movement intensity and the relative small proportion of aphids that could be analysed in such tracking studies are blurring close spatial relations within short time periods.
Western flower thrips (WFT) (Frankliniella occidentalis) is an introduced pest that harms French bean production in Kenya and other countries. Since new WFT management approaches are being developed, a closer look at the genetic makeup of WFT populations can give new insights into source habitats, crop colonization patterns or host plant preferences, which are prerequisites for integrated pest management (IPM) strategies. For this purpose, we used six microsatellite loci to analyse the genetic structure, diversity and gene flow of WFT sampled on French beans, intercrops and weeds in Kenyan French bean production areas. The results of this preliminary study indicate that the available microsatellites are sufficiently polymorphic for more detailed analyses on local dispersal patterns of WFT in Kenya. Even with the limited data set, the results reveal that F. occidentalis populations show considerable genetic differentiation between host plant species but not between regions, which suggests reduced gene flow and a possible development of biotypes. Possible consequences of the results on IPM are discussed.
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