Influence of distance from adjacent forest fragments on aphids (Hemiptera: Aphididae) parasitoidism in wheat fields in Parana State, Brazil

In agricultural landscapes, non-crop habitats, such as forest fragments, may not only play an important role for natural enemies, but may also favor some pests. However, there is a lack of studies in Neotropical regions about these interactions. Thus, the aim of this study was to investigate aphid parasitoidism at two distances (25 m and 525 m) from edges of forest fragments adjacent to wheat crops. This research was carried out in four wheat fields in the north of the Parana state, Brazil, during the planting season of 2009. In total, 8,392 aphids were counted, representing two species. There were 589 total aphid-parasitized (“mummies”), represented by six species. In all four areas, aphid abundance and parasitoidism showed no clear relationship to distance from forest fragments (25 m and 525 m). In addition, inverse density dependence between aphids and parasitoidism was observed. In summary, this study does not support the hypothesis of higher parasitoidism of aphids near edges of forest fragments. However, this is the first study in the Neotropical region and more research must be done to better understand this tritrophic interaction.

Influence of habitat and population density on recruitment and spatial dynamics of the sea urchin Paracentrotus lividus: implications for harvest refugia

We studied the spatial variation in recruitment and the population dynamics of the sea urchin Paracentrotus lividus, analysing the effect of depth and presence of sea urchin aggregations on population structure. Over 90% of the observed recruits (individuals under 1 year of age) were concentrated in aggregations found in shallow waters. In these areas, a positive linear relationship was found between adult and recruit density, possibly due to higher survival rates of juveniles taking refuge among the spines of adults. The scarcity of recruits and the presence of adult sea urchins at depths of 8 and 12 m suggest the migration of part of the population towards deep areas when individuals reach a size of ∼40 mm. In light of these results, the implementation of harvest refugia for sea urchins in shallow areas could constitute an effective fishery management tool for this species. High population densities could enhance recruitment, given the inverse density dependence of this process, while their migratory pattern would guarantee biomass exportation towards deeper fishing grounds.

Conditions for caribou persistence in the wolf-elk-caribou systems of the Canadian Rockies

Woodland caribou populations are considered threatened in Alberta and have declined in the Canadian Rocky Mountain National Parks of Banff and Jasper despite protection from factors causing caribou populations to decline outside of parks. Recent research emphasizes the importance of the numeric response of wolves to moose in moose-caribou-wolf systems to caribou persistence. Moose are rare in the Canadian Rockies, where the dominant ungulate prey for wolves is elk. Few studies have explored wolf-elk dynamics and none have examined implications for caribou. We used data collected in Banff to estimate the numeric response of wolves to elk from 1985 to 2005. Because no caribou kill-rate data exist for the Rockies, we explore the consequences of a range of hypothetical kill-rates based on kill-rates of alternate prey collected from 1985 to 2000 in Banff. We then multiplied the numeric response of wolves by the estimated caribou kill-rates to estimate the wolf predation response on caribou as a function of elk density. Caribou predation rates were inversely density dependent because wolf numbers depend on prey species besides caribou in multiple prey species systems. We then combined this simple wolf-elk-caribou model with observed demographic and population estimates for Banff and Jasper caribou from 2003-2004 and solved for the critical kill-rate thresholds above which caribou populations would decline. Using these critical kill-rate thresholds, Jasper caribou are likely to persist when wolf densities are below 2.1 - 4.3 wolves/1000km2 and/or when elk densities are below 0.015- 0.033 elk/km2. Thresholds for Banff caribou persistence are much lower because of inverse density dependence. Future research is needed on some of the necessary assumptions underlying our modeling including multi-prey wolf numeric responses, wolf kill-rates of caribou, caribou mortality by other predators, and spatial aspects of wolf-elk-caribou dynamics.

Regulation of nematode fecundity in the ring-necked pheasant (Phasianus colchicus): not just density dependence

Patterns of nematode fecundity were investigated for infections of the caecal worm Heterakis gallinarum in the ring-necked pheasant (Phasianus colchicus). Worm length was a good predictor of parasite fecundity. After controlling for worm length no other factors, including parasite intensity, were related to worm fecundity. Density dependence in worm size was detected in natural infections at parasite intensities above a threshold of 96 worms (worm size decreased with increasing parasite intensity). However, below this threshold, worm size actually decreased with decreasing parasite intensity (inverse density dependence). The interaction between density dependence and inverse density dependence in regulating the development and subsequent fecundity of H. gallinarum worms in ring-necked pheasants was demonstrated in an infection experiment. Density dependence was observed in the stunted growth of worms in heavily infected hosts, relative to worms in lightly infected hosts. Inverse density dependence in worm size was the common pattern across hosts by the end of the experiment, when parasite intensities were below the density dependence threshold. This is the first study to document both density dependence and inverse density dependence in parasite fecundity in the same host–helminth system.

Impact and oviposition behaviour ofAgeniaspis fuscicollis(Hymenoptera: Encyrtidae), a polyembryonic parasitoid of the apple ermine moth,Yponomeuta malinellus(Lepidoptera: Yponomeutidae)

The distribution and extent of parasitism of the apple ermine mothYponomeuta malinellusZeller by the polyembyronic encyrtid parasitoidAgeniaspis fuscicollis(Dalman) were examined in a three year field study and related to oviposition behaviour in the laboratory.Ageniaspis fuscicollisattacks egg batches of its host and kills the final instar larvae, which feed gregariously from within tents. Host population densities in the field were low, from 1.5 to 2.2 tents per 100 leaf clusters, and parasitism increased from 7.8% to 18% over the three year period. Parasitism was independent of host density at the whole tree scale, but at the individual tent scale, the probability of a tent containing parasitized host larvae increased and percent parasitism decreased with the number of host larvae per tent. Observations on the oviposition behaviour ofA. fuscicollisin the laboratory revealed that parasitoids distributed their eggs randomly within host egg batches. On average, they spent almost two hours on an egg batch and laid 44% of their egg load of 132 eggs into the first egg batch visited, leading to a mean of 1.4 eggs laid per host egg through frequent self-superparasitism of hosts. The percentage of eggs receiving one or more ovipositions was independent of the size of an egg batch, contradicting our field observations of inverse density dependence. Factors that might account for the differences in rates of parasitism and attack distributions between laboratory and field data are discussed.