What is the required minimum landscape size for dispersal studies?

Among small animals dispersal parameters are mainly obtained by traditional methods using population studies of marked individuals. Dispersal studies may underestimate the rate and distance of dispersal, and be biased because of aggregated habitat patches and a small study area. The probability of observing long distance dispersal events decreases with distance travelled by the organisms. In this study a new approach is presented to solve this methodological problem. An extensive mark-release-recapture programme was performed in an area of 81 km(2) in southern Sweden. To estimate the required size of the study area for adequate dispersal measures we examined the effect of study area size on dispersal distance using empirical data and a repeated subsampling procedure. In 2003 and 2004, two species of diurnal burnet moths (Zygaenidae) were studied to explore dispersal patterns. The longest confirmed dispersal distance was 5600 m and in total 100 dispersal events were found between habitat patches for the two species. The estimated dispersal distance was strongly affected by the size of the study area and the number of marked individuals. For areas less than 10 km(2) most of the dispersal events were undetected. Realistic estimates of dispersal distance require a study area of at least 50 km(2). To obtain adequate measures of dispersal, the marked population should be large, preferably over 500 recaptured individuals. This result was evident for the mean moved distance, mean dispersal distance and maximum dispersal distance. In general, traditional dispersal studies are performed in small study areas and based on few individuals and should therefore be interpreted with care. Adequate dispersal measures for insects obtained by radio-tracking and genetic estimates (gene flow) is still a challenge for the future.     

Suitability of different fluorescent powders for mass-marking the Chrysomelid, Diabrotica virgifera virgifera LeConte

Abstract:  The Western Corn Rootworm, Diabrotica virgifera virgifera LeConte (Col., Chrysomelidae), is an invasive alien pest of maize, Zea mays , in Europe. The suitability of 14 fluorescent powders for mass-marking the adults was studied in laboratory and in field cages. The visual discrimination between remaining spots of each colour on the beetles was investigated under ultraviolet (UV) light, as well as their retention time and the influences of those colours on the beetle survival and flight take-off response. The two best recognizable orange colours (i.e. of Radiant Colour and of Fiesta Colours Swada) were proposed for field experiments in first priority, followed by an orange and a yellow (both Magruder Colour), another yellow (Fiesta) and a pink (Radiant), as all did not affect beetle survival and flight take-off response and were recognizable under UV light for at least 10 days in the field. In contrast, the colours yellow and green (Radiant), red and blue (Magruder), yellow (Ciba Geigy) and pink (Fiesta) were unsuitable, because they either quickly disappeared from the beetles or adversely affected beetle survival or flight take-off response. For mass releases with differently marked beetles, only the use of a single orange colour together with a single yellow colour or the use of a pink colour together with a yellow colour can be used since few spots can clearly be discriminated from each other under UV light.     

Movement of two grassland butterflies in the same habitat network: the role of adult resources and size of the study area

Abstract. 1.  Movement patterns of two butterfly species (meadow brown Maniola jurtina L. and scarce copper Lycaenae virgaureae L.) were studied in a 172 ha area within a landscape with a high percentage of suitable habitats for mark–release–recapture experiments.
2.  Adult resource density, but not patch size or larval food plant abundance, influenced the numbers and the fractions of residents, emigrants, and immigrants.
3.  Differences between species were observed in movement frequency and maximum distances moved but not in mean distances moved.
4.  The scarce copper showed much greater movement ability than expected from the results of published studies. This is believed to be a result of the comparatively large size of the study area and the high cover of suitable habitat (>50%).
5.  The mean and maximum distances travelled by butterflies reflected differences in the size of the study area.     

Population sizes and dispersal pattern of tsetse flies: rolling on the river?

The West African trypanosomoses are mostly transmitted by riverine species of tsetse fly. In this study, we estimate the dispersal and population size of tsetse populations located along the Mouhoun river in Burkina Faso where tsetse habitats are experiencing increasing fragmentation caused by human encroachment. Dispersal estimated through direct (mark and recapture) and indirect (genetic isolation by distance) methods appeared consistent with one another. In these fragmented landscapes, tsetse flies displayed localized, small subpopulations with relatively short effective dispersal. We discuss how such information is crucial for designing optimal strategies for eliminating this threat. To estimate ecological parameters of wild animal populations, the genetic measures are both a cost- and time-effective alternative to mark–release–recapture. They can be applied to other vector-borne diseases of medical and/or economic importance.     

Evaluation of traps used to monitor southern pine beetle aerial populations and sex ratios

1 Various kinds of traps have been employed to monitor and forecast population trends of the southern pine beetle (Dendroctonus frontalis Zimmermann; Coleoptera: Scolytidae), but their accuracy in assessing pine‐beetle abundance and sex ratio in the field has not been evaluated directly. 2 In this study, we used fluorescent powder to mark pine beetles emerging from six isolated infestations. We then compared estimates of total abundances and proportions of males emerging from within each infestation to the estimates from three types of traps: passive sticky traps (2, 5, 10 and 20 m away from the source of beetles), multi‐funnel traps baited with pine beetle attractants (100 m away) and pine trees baited with attractants (also 100 m away). 3 We found that the proportion of males captured in traps was significantly affected by the type of trap used. 4 Within an infestation, equal proportions of males and females were marked (0.53 ± 0.02 males; mean ± SE), but the proportions captured in trap trees and passive traps were more female biased (0.42 ± 0.03 and 0.46 ± 0.01 males, respectively). On the other hand, funnel traps provided an estimate of the proportion of males that was nearly identical to the proportion from within infestations (0.51 ± 0.03). 5 Numbers of marked beetles captured in traps were uncorrelated with the numbers of marked beetles emerging from the focal infestations. This suggests that traps positioned around an infestation may not be effective at estimating relative abundances of beetles within the infestation.     

Response of Cydia pomonella to selection on mobility: laboratory evaluation and field verification

1. The codling moth Cydia pomonella (L.), largely regarded as a sedentary species, shows great variation in flight capacity among individuals in the laboratory, as has been reported for tethered flight. The occurrence of individuals with the ability to fly over long distances is considered ecologically significant for the colonisation of new habitats in response to deteriorating environmental conditions. The work reported here was designed to investigate the importance of the genetic component in regulating dispersal in C. pomonella. 2. Bi‐directional selection on mobility measured by actograph was carried out in the laboratory. Both male and female codling moths responded significantly to the upward and downward selections. Divergence between the two selected strains occurred after one generation of selection and increased as further selections continued. The realised heritabilities for mobility were estimated as 0.43 in males and 0.29 in females when averaged from the two selection directions. 3. The dispersal capacity of the selected mobile and sedentary strains was verified by mark–release–recapture experiments in the field, though only the released male moths were captured effectively. Flight distances by males differed significantly between the two strains, with the mobile strain showing a greater dispersal capacity than the sedentary strain. 4. This study demonstrated a positive correlation between mobility measured by actograph and field dispersal capacity in the codling moth. Furthermore, it provides the first experimental data to show field performances of different genotypes in insect dispersal.     

Dispersal of the Old World screw-worm fly Chrysomya bezziana

Abstract. Dispersal of the Old World screw-worm fly, Chrysomya bezziana Villeneuve, was studied in Papua New Guinea by releasing radio-isotope labelled, laboratory-reared flies and collecting their labelled egg masses from sentinel cattle. A log-linear model was developed to describe recapture rate. Distance was found to dominate the model and was represented by a bilinear ('broken-stick') term as log-distance. Further terms in the model such as attractiveness of the site (estimated from the number of non-labelled egg masses), the season of the year and a time trend were statistically significant but of minor importance. From the model, the median distance females dispersed before depositing an egg mass was 10.8 km. The maximum distance from the release site that egg masses were recovered was 100 km.
The dispersal ability of C. bezziana is discussed in terms of its impact on the prospects of eradicating this species using SIRM if an outbreak occurred in Australia.     

Within field movement of overwintered Colorado potato beetle: a patch-based approach

Abstract:  After comparing the persistence of four marking techniques, a mark–release–resight study was performed to characterize mid-season movement of the Colorado potato beetle [ Leptinotarsa decemlineata (Say); Col., Chrysomelidae] simultaneously in a fallow and in a wheat field. Isolated patches of potatoes were installed in a random spatial arrangement on both fields similarly. Overwintered beetles were individually marked and released. Beetles showed limited inter-patch movement activity (15.9% of recovery events) with an overall mean daily dispersal of 0.309 m (0.0–7 m). There was a significant difference in the insects' movement distance between the fallow and wheat field but there was no difference between the movement distances of males and females. The distance between the patches varied between 1 and 7.81 m, and inter-patch movement was infrequent (15.9%). Results suggest that surrounding fields by wheat rather than fallow grounds should be studied as a possible strategy to reduce the movement of overwintered beetles between potato fields.     

Movement of the Apollo butterfly Parnassius apollo related to host plant and nectar plant patches

1. The movement of adults of the endangered Apollo butterfly, Parnassius apollo, was studied using mark–recapture data, within a population consisting of discrete patches of the species’ host plant (n = 43), which were segregated spatially from patches of the species’ main nectar plants (n = 14). 2. The Apollo routinely moved large distances (median 260 m, maximum 1840 m), and moved frequently between the two types of patches. Only 27% (28/105) of the recaptures were made on the same host plant patch as the release. 3. The population acts as a patchy population where the adults mix over the whole area, but successful reproduction can only take place in the discrete host plant patches. 4. Occurrence on a host plant patch was restricted by the area size of the host plant patch and the spatial configuration of nectar plant patches. Thus, although the Apollo is a good flyer, its movement over the patches is still constrained by the segregation of adult and larval resources.