Recent theoretical and empirical studies suggest that an animals's long—distance dispersal behavior can have a major influence on its population dynamics. In this study I examined the role of dispersal in the population dynamics of a goldenrod leaf beetle, Trirhabda virgata. I measured the long—distance emigration rates of this beetle in the field under different population densities. I manipulated densities of Trirhabda larvae early in the season to create crowded, heavily defoliated host patches and uncrowded, lightly defoliated patches. Newly emerged, teneral beetles were added to these two density treatments and their long—distance emigration rates were measured. Beetles flew from host patches that had high densities of adults and heavy damage caused by larvae. No flight was detected from low—density patches with little defoliation. Beetles that had developed in these crowded and uncrowded patches were placed on lush, undefoliated plants as teneral adults. Their emigration rates did not differ. In a final experiment, female beetles were placed on plants with or without mates. The plants were one of two types, either heavily defoliated or lush. On lush plants, females without mates showed a stronger tendency to leave than females that had access to mates. On defoliated plants, females left at high rates whether or not mates were present. These experiments indicate that long—distance emigration is triggered by conditions experienced during the teneral phase. Heavy host defoliation inflicted by high population densities appears to induce flight. This density—dependent emigration should reduce local population pressure and result in the spread of local outbreaks across a larger area. The influence of dispersal on population dynamics will vary depending on the pattern of beetle densities across the region.
The ability of an insect to disperse to new habitat patches is difficult to quantify, but key to the establishment and persistence of populations. In this study, we examined dispersal of the phytophagous chrysomelid beetle, Galerucella calmariensis, which is currently being introduced into North America for the biological control of purple loosestrife (Lythrum salicaria), an aggressive wetland weed. We used a mark, release, and recapture approach to determine how rates of colonization of host patches by this beetle are influenced by the distance of the patch from the source of dispersers, and by the presence of conspecifics at the patch. We released color-coded beetles at six distances from a long, linear patch of purple loosestrife that was divided into segments with and without conspecifics. We observed initial flight directions as beetles left the release points and collected all beetles that settled at the target patch. We found a bias in initial flight toward the target for distances up to 50 m. Over the 7 days of the experiment, beetles arrived at the target from all release points, including the farthest release point, 847 m away. G. calmariensis was strongly attracted to conspecifics when settling after dispersal; 86% of the 582 recovered beetles came from the segments inhabited by conspecifics. The probability of an individual arriving at the patch declined steeply with release distance. This relationship fits a model in which beetles move in a random direction and stop if they intercept the target patch, and where beetles are lost at a constant rate with distance travelled. The dispersal and patch-colonizing behavior of G. calmariensis is likely to have important consequences for the biological control program against purple loosestrife.
Dalechampia brownsbergensis and D. fragrans co-occur in Suriname, and both are pollinated by fragrance-collecting male euglossine bees. Dalechampia brownsbergensis appears to bloom year-round and is pollinated by relatively few species of bees, including Euglossa tridentata and E. gaianii. In contrast, D. fragrans appears to bloom from late October through early December and is visited and pollinated by at least 13 species ofeuglossines. Field observations of pollination indicated that the two species did not share pollinators. However, when the flowers ofDi fragrans were "transplanted" into a population of D. brownsbergensis, the main pollinator of D. brownsbergensis also visited the flowers of D. fragrans. The pollinators of D. fragrans, however, did not visit the flowers of D. brownsbergensis. Partial sharing of pollinators may have only a small negative impact on the two sympatric plant species at this site because they flower simultaneously only part of the year, and they are often spatially separated from one another.
Dalechampia brownsbergensis and D. fragrans co‐occur in Suriname, and both are pollinated by fragrance‐collecting male euglossine bees. Dalechampia brownsbergensis appears to bloom year‐round and is pollinated by relatively few species of bees, including Euglossa tridentata and E. gaianii. In contrast, D. fragrans appears to bloom from late October through early December and is visited and pollinated by at least 13 species of euglossines. Field observations of pollination indicated that the two species did not share pollinators. However, when the flowers of D. fragrans were “transplanted” into a population of D. brownsbergensis, the main pollinator of D. brownsbergensis also visited the flowers of D. fragrans. The pollinators of D. fragrans, however, did not visit the flowers of D. brownsbergensis. Partial sharing of pollinators may have only a small negative impact on the two sympatric plant species at this site because they flower simultaneously only part of the year, and they are often spatially separated from one another.
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