Post-glacial colonization of the Fennoscandian coast by a plant parasitic insect with an unusual life history.

Species that exhibit very peculiar ecological traits combined with limited dispersal ability pose a challenge to our understanding of ecological and evolutionary mechanisms. This is especially true when they have managed to spread over long distances, overcome physical barriers, and colonize large areas. Climate and landscape changes, trophic web relations, as well as life history all interact to shape migration routes and present-day species distributions and their population genetic structures. Here we analyzed the post-glacial colonization of northern Europe by the gall midge Contarinia vincetoxici, which is a monophagous parasite on the perennial herb White swallowwort (Vincetoxicum hirundinaria). This insect not only has a narrow feeding niche but also limited dispersal ability and an exceptionally long dormancy. Gall midge larvae (n = 329) were collected from 16 sites along its distribution range in Denmark, Sweden, and Finland. Using microsatellite loci and knowledge of the species and the regions' history, we investigated the role of landscape change, host plant distribution, insect population dynamics, and life history in shaping the population genetic structure of the insect. We devoted particular interest to the role of the insect's presumed poor dispersal capacity in combination with its exceptionally extended diapause. We found significant levels of local inbreeding (95% highest posterior density interval = 0.42-0.47), low-level within-population heterozygosity (mean H E = 0.45, range 0.20-0.61) with private alleles in all populations except two. We also found significant (p < .001) regional isolation-by-distance patterns, suggesting regularly recurring mainly short-distance dispersal. According to approximate Bayesian computations, C. vincetoxici appears to have colonized the study area via wind-aided flights from remote areas approximately 4600-700 years before present when the land has gradually risen above the sea level. Extremely long dormancy periods have allowed the species to "disperse in time", thereby aiding population persistence despite generally low census population sizes.

Role of weather and other factors in the dynamics of a low-density insect population.

Insect population dynamics are the result of an interplay between intrinsic factors such as density dependence, trophic web interactions, and external forces such as weather conditions. We investigate potential mechanisms of population dynamics in a natural, low-density insect population. Eggs and larvae of the noctuid moth, Abrostola asclepiadis, develop on its host plant during summer. The population density, and mortality, was closely monitored throughout this period during 15 years. Densities fluctuated between one and two orders of magnitude. Egg-larval developmental time varied substantially among years, with lower survival in cool summers with slower development. This was presumably due to the prolonged exposure to a large guild of polyphagous arthropod enemies. We also found a density-dependent component during this period that could be a result of intraspecific competition for food among the last larval instars. Dynamics during the long period from pupation in late summer through winter survival in the ground to adult emergence and oviposition the next year displayed few clear patterns and more unexplained variability, thus giving a more random appearance. The population hence shows more unexplained or unpredictable variation during the long wintering period, but seems more predictable over the summer egg-larval period. Our study illustrates how weather-via a window of exposure to enemies and in combination with density-dependent processes-can determine the course of population change through the insect life cycle.

Seed production and predation in a changing climate: new roles for resource and seed predator feedback?

Climate change may cause changes in the dynamics of populations beyond comparatively simple directional effects. To better understand complex effects on dynamics requires long-term studies of populations that experience changes in climatic conditions. We study the dynamics of a seed-production-seed-predation system, consisting of a perennial herb and its two seed predatory insects, over a 40-yr period during which climate change has caused the annual growing season to increase by 20 d. During this period, plant patches have increased almost threefold in size and seed production has slipped into a pattern of alternate high and low years with a higher variance than in the beginning of the period. We find that seed production is associated with precipitation of the present summer and a non-linear feedback from seed production of the previous year. When previous year's seed production is low, weather forcing and unexplained noise determine the extent of seed production. When previous seed production is high, depleted resources limit seed production. Resource depletion happened frequently in the latter parts of the study but rarely in the beginning. The changing patterns of seed production in turn affect the dynamics of seed predation, which is dominated by one of the seed predators. Its dynamics are strongly linked to seed density fluctuations, but its population growth rate is satiated when resource fluctuations become too large. In the latter part of the study period, when seed fluctuations were alternating between years of high and low density, satiation was common and there was a large increase in surviving seeds in good years. Our study illustrates that a changing climate can fundamentally influence patterns of long-term dynamics at multiple trophic levels.

Small-scale resource tracking in a population of a long-lived insect.

How plant-feeding insects distribute themselves and utilize their host plant resources is still poorly understood. Several processes may be involved, and their relative roles may vary with the spatial scale considered. Herein, we investigate small-scale patterns, namely how population density of a gall midge is affected by individual growth form, phenology, and microsite characteristics of its herb host. The long-lived plant individuals vary much with regard to number of shoots, flower abundance, and flowering phenology. This variation is connected to site characteristics, primarily the degree of sun exposure. The monophagous insect galls the flowers of the host plant - an easily defined food resource. It is a poor disperser, but very long-lived; diapausing larvae can stay in the soil for many years. Galls were censused on individual plants during 5 years; from a peak to a low in gall population density. Only a very small fraction of the flowers produced (<0.5%) were galled even in the peak year. Nevertheless, most plant individuals had galls at least 1 year. In a stepwise multiple regression, plant size (number of shoots) was found to be the most important predictor of gall density (galls/flower). However, gall density decreased more than one order of magnitude over the plant size range observed. There was also a weak effect of plant phenology. Early flowering plants had lower gall densities than those starting later. Sun exposure had no direct effect on gall density, but a path analysis revealed indirect effects via the timing of flowering. Gall population change was highly synchronous in different parts of the study area with no significant decrease in synchrony with distance.

Density dependence vs. independence, and irregular population dynamics of a swallow-wort fruit fly.

Although most long-term studies of consumer-resource (e.g., predator-prey) interactions select species showing cyclic population dynamics, strong consumer-resource interactions can also produce irregular, noncyclic dynamics. Here, we present a case in which a seed predator, the tephritid fruit fly Euphranta connexa, shows fluctuations in density of more than two orders of magnitude over a 22-year period. To explain these fluctuations, we analyzed a stage-specific data set to quantify the density-dependent and density-independent components of larval survivorship and realized fecundity. Both larval survivorship and realized fecundity were strongly density dependent. Larval survivorship dropped from 0.62 at low larval density to 0.081 at high larval density, whereas fecundity dropped from 84.3 to 0.32 eggs per individual, more than a 100-fold decrease. We divided density-independent variation in E. connexa population dynamics into components for variability in (1) larval survivorship, (2) realized fecundity, and (3) annual fruit abundance. Of these components, 96% of the density-independent variance in per capita population growth rates was caused by fluctuations in fruit abundance. This highlights the importance of the strong consumer-resource interactions in driving fluctuations in E. connexa abundance. It also demonstrates that E. connexa dynamics are remarkably simple, and aside from the 4% of unexplained variance in per capita population growth rates, our understanding of E. connexa dynamics is remarkably complete.

The role of variable weather for the dynamics of a seed-seed predator system.

A simple model is presented describing the interaction between weather conditions, seed production of a longlived herb, Vincetoxicum hirundinaria (Asclepiadaceae), and abundance of a predispersal seed predator, Euphranta connexa (Diptera, Tephritidae). The model is used to investigate the role of weather-induced fluctuations in seed set on the interaction between plant and seed predator and the resulting longterm production of seeds escaping predispersal seed predation.As weather variability increases Euphranta populations become less effective in tracking their food resources, leading to increased longterm production of dispersing seeds. Occasional years of crop failure due to drought stress may thus in the long run be of benefit to the plant.

Induction of diapause in a migratory seed bug, Neacoryphus bicrucis (Say) (Heteroptera: Lygaeidae).

Neacoryphus bicrucis hibernates in the adult stage and exhibits a facultative reproductive diapause. Constant photoperiods of the durations encountered in the field do not evoke the diapause response but Shortening of the photoperiods does, providing the decrease in daylength is of a certain length and occurs within a certain photoperiodic interval. The sensitive stages are larval instars II to V. The percentage of diapausing bugs increases as temperature is lowered.It is suggested that the response to daylength shortening in N. bicrucis is an adaptation to ensure seasonal synchronization in a highly migratory species where successive generations may breed at different latitudes.

Relations between environment, migration and reproduction in a seed bug, Neacoryphus bicrucis (Say) (Heteroptera: Lygaeidae).

The seed feeding bug Neacoryphus bicrucis migrates by flight, often at great altitudes, and evidently is able to disperse over long distances. Non-diapausing females provided with ample food and mates histolyze their flight muscles soon after adult eclosion, and thus undertake very little flight. If food density is below a certain threshold flight ability is retained at a constant high level through most of adult life, while egg production becomes a linear function of the logarithm of food density. Bugs totally starved from adult eclosion onwards survive for a month or more with only a minor decrease in flight ability. Females also retain their flight potential when subjected to lack of mates or diapause-inducing photoperiodic conditions. Most males are able to fly throughout their adult lives under all conditions studied, but the tendency to fly is enhanced by lack of food or mates.

Maturation of post-hibernation flight behaviour in the Coccinellid Coleomegilla maculata (DeGeer).

Coleomegilla maculata flies from its hibernation sites to breeding areas in the spring. Flight behaviour among the hibernating beetles was studied using the tethered flight technique. It was found that there is a successive maturation of flight behaviour in the spring. This process is mainly controlled by temperature acting over an extended period. From a standstill at 15°C the rate of the maturation process increased as the temperature was raised. Short photoperiods exerted a depressing effect on glight development early in the winter, but later this effect disappears.