Resources and dispersal as factors limiting a population of the tussock moth (Orgyia vetusta), a flightless defoliator

The western tussock moth (Orgyia vetusta) is very abundant on one stand of bush lupine (Lupinus arboreus) at the Bodega Marine Reserve (Sonoma Co, Calif., USA), but is sparse or absent on nearby stands. To determine what controls its abundance, both within the outbreak area and more globally, I performed experimental manipulations of resource availability and dispersal. To measure resource limitation, I inoculated 30 caged and 30 uncaged bushes with a realistic range of numbers of tussock moth eggs. On caged bushes, starvation led to density-dependent reductions in survival, pupal weight and fecundity. Larvae on uncaged bushes experienced density-independent ant predation on early instars and density-dependent emigration by late instars. From the results of this experiment, I predicted the density of a resource-limited tussock moth population. The predictions agreed fairly well with data from the outbreak area in 1992. To measure dispersal by the moth, which has flightless adult females, I released 30,000 tussock moth eggs at a central point in each of two uninfested lupine stands, and censused larvae weekly in a circle of radius 16 m until pupation. Median displacement over one entire generation was only 2 m, or about 2 bush radii. Rearing experiments indicated that bushes outside the out-break area are fully nutritionally suitable for the moth. I conclude that two major factors limiting the population are resources (within the outbreak area) and inefficient dispersal (more globally).     

Origin of an insect outbreak: escape in space or time from natural enemies?

Initiation of insect outbreaks is poorly understood, and may involve sporadic events that temporarily release insect populations from predation or parasitism. While studying a declining outbreak of the western tussock moth (Orgyia vetusta) on bush lupine (Lupinus arboreus), we witnessed the onset of a new tussock moth outbreak, separated by 1,000 m in space and 2 months in phenological timing from the original population. This new population underwent explosive growth for 2 years and then collapsed because of a massive die-off of lupines. We tested whether during its growth phase, this new outbreak benefited by escaping in either space or time from the natural enemies attacking the original population. In experimental populations on single bushes, we compared predation and parasitism at the sites of the new and the old outbreak. At the site of the old outbreak, we compared predation and parasitism early and late in the season. Parasitism was significantly lower and population growth significantly higher at the new outbreak site than the old one. Neither seasonal timing, predator exclusion, nor their interaction significantly affected survival at either site. Thus the new outbreak appeared to escape in space from parasitism. These results corroborate our previous experimental findings, which suggest that as predicted by theory, the interaction between the tussock moth and its parasitoids can produce large-scale spatial patterning in population densities.     

Tests of the role of a nuclear polyhedrosis virus in the population dynamics of its host,douglas-fir tussock moth,Orgyia pseudotsugata (Lepidoptera: Lymantriidae)

Summary Outbreaks of the Douglas-fir tussock moth, Orgyia pseudotsugata (McDunnough), have recurred periodically, at 7- to 10-year intervals, since the first recorded observation in 1916 in Chase, British Columbia, Canada. Anderson and May (1981) hypothesized that microparasites are responsible for the periodic population fluctuations of some defoliating insects. We chose the association between the Douglas-fir tussock moth and a viral disease, caused by a nuclear polyhedrosis virus (NPV), to test whether their model, and variants thereof, can predict the observed population cycles. Density-dependent mortality, vertical transmission of the virus and an incubation period were added to the free-living stages model of Anderson and May (1981). Parameter values for the models were derived from published data and from an experiment.Sensitivity analyses conducted for each model showed that none of the models generated the behavior of the Douglas-fir tussock moth as observed in the field. Thus, the periodicity of the outbreaks in field populations of tussock moths cannot be explained solely by the dynamics of the viral disease as described by Anderson and May's class of models; the virus is too short-lived and the growth rate of the insect population too high. Dynamics of other system components such as predators, parasites or food of the tussock moth probably play a significant role in the insect's population dynamics.     

Exploring stable pattern formation in models of tussock moth populations

1. The western tussock moth ( Orgyia vetusta ) at the University of California Bodega Marine Reserve (Sonoma County, California, USA) exhibits dense, localized populations in the midst of extensive habitats where variation in host plant quality or predator abundance is unable to explain the restricted extent of the outbreaks.
2. Two primary features suggest that the host patterning is intrinsically generated: (i) female tussock moths are wingless, producing a low effective dispersal distance for the hosts; and (ii) the tussock moth population is attacked by several species of widely dispersing wasp and fly parasitoids.
3. We consider a set of spatially explicit host–parasitoid models to examine whether intrinsically generated patterns are possible within this system. These models include a spatially extended Nicholson–Bailey model to examine general features of pattern formation in host–parasitoid systems, and two system-specific models, an individual-based simulation and a population-level analytic model, to examine the details of this empirical system.
4. Both stable patterning and rapid extinction of the host population are initial-condition dependent outcomes of the general and specific models, implying that an intrinsically generated stable host pattern is feasible within the tussock moth system.
5. Stable patterning is enhanced by a large parasitoid-to-host dispersal ratio, local host resource limitation, and increased parasitism at the host patch's edge.     

Persistent, localized outbreaks in the western tussock moth Orgyia vetusta: the roles of resource quality, predation and poor dispersal

1. On stands of the abundant perennial lupins Lupinus arboreus and L. chamissonis on the California coast, the western tussock moth Orgyia vetusta Bdv. occurs as only a few localized populations, which are locally dense enough to defoliate the host plants and yet do not appear to spread.
2. To determine what limits the spatial distribution of this insect, and therefore its global abundance, experiments were performed at three sites, examining the roles of host-plant quality, predator abundance, rates of predation and dispersal.
3. At no site did host quality, predator abundance or rates of predation vary spatially in a way that would explain the location of the outbreak. However, adult female tussock moths are flightless, and rates of long-distance aerial dispersal ('ballooning') by first-instar larvae were very low at all sites. Poor dispersal is the only consistent explanation for the limited distribution of the tussock moth.     

Numerical and Functional Responses of Forest Bats to a Major Insect Pest in Pine Plantations

Global change is expected to modify the frequency and magnitude of defoliating insect outbreaks in forest ecosystems. Bats are increasingly acknowledged as effective biocontrol agents for pest insect populations. However, a better understanding is required of whether and how bat communities contribute to the resilience of forests to man- and climate-driven biotic disturbances. We studied the responses of forest insectivorous bats to a major pine defoliator, the pine processionary moth pityocampa, which is currently expanding its range in response to global warming. We used pheromone traps and ultrasound bat recorders to estimate the abundance and activity of moths and predatory bats along the edge of infested pine stands. We used synthetic pheromone to evaluate the effects of experimentally increased moth availability on bat foraging activity. We also evaluated the top-down regulation of moth population by estimating T. pityocampa larval colonies abundance on the same edges the following winter. We observed a close spatio-temporal matching between emergent moths and foraging bats, with bat activity significantly increasing with moth abundance. The foraging activity of some bat species was significantly higher near pheromone lures, i.e. in areas of expected increased prey availability. Furthermore moth reproductive success significantly decreased with increasing bat activity during the flight period of adult moths. These findings suggest that bats, at least in condition of low prey density, exhibit numerical and functional responses to a specific and abundant prey, which may ultimately result in an effective top-down regulation of the population of the prey. These observations are consistent with bats being useful agents for the biocontrol of insect pest populations in plantation forests.     

Bottom-up effects of a no-take zone on endangered penguin demographics

Marine no-take zones can have positive impacts for target species and are increasingly important management tools. However, whether they indirectly benefit higher order predators remains unclear. The endangered African penguin (Spheniscus demersus) depends on commercially exploited forage fish. We examined how chick survival responded to an experimental 3-year fishery closure around Robben Island, South Africa, controlling for variation in prey biomass and fishery catches. Chick survival increased by 18% when the closure was initiated, which alone led to a predicted 27% higher population compared with continued fishing. However, the modelled population continued to decline, probably because of high adult mortality linked to poor prey availability over larger spatial scales. Our results illustrate that small no-take zones can have bottom-up benefits for highly mobile marine predators, but are only one component of holistic, ecosystem-based management regimes.     

Imperfect Batesian mimicry—the effects of the frequency and the distastefulness of the model

Batesian mimicry is the resemblance between unpalatable models and palatable mimics. The widely accepted idea is that the frequency and the unprofitability of the model are crucial for the introduction of a Batesian mimic into the prey population. However, experimental evidence is limited and furthermore, previous studies have considered mainly perfect mimicry (automimicry). We investigated imperfect Batesian mimicry by varying the frequency of an aposematic model at two levels of distastefulness. The predator encountered prey in a random order, one prey item at a time. The prey were thus presented realistically in a sequential way. Great tits (Parus major) were used as predators. This experiment, with a novel signal, supports the idea that Batesian mimics gain most when the models outnumber them. The mortalities of the mimics as well as the models were significantly dependent on the frequency of the model. Both prey types survived better the fewer mimics there were confusing the predator. There were also indications that the degree of distastefulness of the model had an effect on the survival of the Batesian mimic: the models survived significantly better the more distasteful they were. The experiment supports the most classical predictions in the theories of the origin and maintenance of Batesian mimicry.     

In a warmer Arctic,mosquitoes avoid increased mortality from predators by growing faster

Climate change is altering environmental temperature, a factor that influences ectothermic organisms by controlling rates of physiological processes. Demographic effects of warming, however, are determined by the expression of these physiological effects through predator–prey and other species interactions. Using field observations and controlled experiments, we measured how increasing temperatures in the Arctic affected development rates and mortality rates (from predation) of immature Arctic mosquitoes in western Greenland. We then developed and parametrized a demographic model to evaluate how temperature affects survival of mosquitoes from the immature to the adult stage. Our studies showed that warming increased development rate of immature mosquitoes (Q₁₀ = 2.8) but also increased daily mortality from increased predation rates by a dytiscid beetle (Q₁₀ = 1.2–1.5). Despite increased daily mortality, the model indicated that faster development and fewer days exposed to predators resulted in an increased probability of mosquito survival to the adult stage. Warming also advanced mosquito phenology, bringing mosquitoes into phenological synchrony with caribou. Increases in biting pests will have negative consequences for caribou and their role as a subsistence resource for local communities. Generalizable frameworks that account for multiple effects of temperature are needed to understand how climate change impacts coupled human–natural systems.     

A note on the relationship between outbreaks of the oriental tussock moth,Euproctis flava and weather conditions

A relationship between outbreaks of the oriental tussock moth, Euproctis flava, in Akita Prefecture and weather conditions have been analyzed in the present paper. The results obtained are summarized as follows: Outbreaks were, in most cases, observed at intervals of 7–9 years, and the population fluctuation was considered to be regular, when small rises of population density were taken into consideration. The condition of low temperature and decreased percentage of sunny hours in spring and summer has a great influence upon the occurrence of outbreaks, and a big outbreak seems to come when a peak of the intrinsic population fluctuation occurs just simultaneously with such a weather condition.     

Extensive gypsy moth defoliation in Southern New England characterized using Landsat satellite observations

Southern New England is currently experiencing the first major gypsy moth (Lymantria dispar) defoliation event in nearly 30 years. Using a novel approach based on time series of Landsat satellite observations, we generated consistent maps of gypsy moth defoliation for 2015 (first year of the outbreak), 2016 (second year of outbreak), and 2017 (third year of outbreak). Our mapped results demonstrate that the defoliation event continued through the 2017 growing season. Moreover, the affected area more than doubled in extent each year and expanded radially to encompass 4386 km² of forested area in Rhode Island, eastern Connecticut, and central Massachusetts. The current gypsy moth outbreak is believed to be the result of a series of unusually dry springs in 2014, 2015, and 2016, which suppressed Entomophaga maimaiga, a fungal mortality agent that has historically reduced gypsy moth impacts in this region. The continuation and marked expansion of the outbreak in 2017 despite average spring rainfall suggests that caterpillars were active early in the growing season, and mortality from the fungus likely peaked after significant defoliation had already occurred. Our Landsat time series approach represents an important new source of data on spatial and temporal patterns in gypsy moth defoliation, and continued satellite-based monitoring will be essential for tracking the progress of this and other gypsy moth outbreaks.     

The coniferous tussock moth (<Emphasis Type="Italic">Calliteara abietis</Emphasis>, Lepidoptera,Lymantriidae) in Buryatia

The coniferous tussock moth Calliteara abietis Denis et Schiffermüller, 1775 (Lepidoptera, Lymantriidae) outbreaks occur in Eastern Siberia. The biological characteristics and outbreak dynamics of this species in the late twentieth century and in the twenty-first century are described. Outbreaks of the moth are shown to occur both separately and along with Siberian and white striped (in Sakhalin) lappet moths.     

Body size and tree species composition determine variation in prey consumption in a forest‐inhabiting generalist predator

Trophic interactions may strongly depend on body size and environmental variation, but this prediction has been seldom tested in nature. Many spiders are generalist predators that use webs to intercept flying prey. The size and mesh of orb webs increases with spider size, allowing a more efficient predation on larger prey. We studied to this extent the orb‐weaving spider Araneus diadematus inhabiting forest fragments differing in edge distance, tree diversity, and tree species. These environmental variables are known to correlate with insect composition, richness, and abundance. We anticipated these forest characteristics to be a principle driver of prey consumption. We additionally hypothesized them to impact spider size at maturity and expect shifts toward larger prey size distributions in larger individuals independently from the environmental context. We quantified spider diet by means of metabarcoding of nearly 1,000 A. diadematus from a total of 53 forest plots. This approach allowed a massive screening of consumption dynamics in nature, though at the cost of identifying the exact prey identity, as well as their abundance and putative intraspecific variation. Our study confirmed A. diadematus as a generalist predator, with more than 300 prey ZOTUs detected in total. At the individual level, we found large spiders to consume fewer different species, but adding larger species to their diet. Tree species composition affected both prey species richness and size in the spider''s diet, although tree diversity per se had no influence on the consumed prey. Edges had an indirect effect on the spider diet as spiders closer to the forest edge were larger and therefore consumed larger prey. We conclude that both intraspecific size variation and tree species composition shape the consumed prey of this generalist predator.     

Optimal Control of Gypsy Moth Populations

This study investigates an optimal strategy for the cost effective control of gypsy moth populations. Gypsy moth populations cycle between low sparse numbers to high outbreak levels and it is during the outbreak levels that the moths cause extensive damage to plant foliage which can lead to deforestation. Deforestation can result in significant economic damage to infested areas, and consequently, there have been many efforts to control moth populations. One effective method of control is the use of the biocontrol agent, Gypchek, but its production is costly. We develop a mathematical model which combines population dynamics and optimal control of the moth population to explore strategies by which the total cost of the gypsy moth problem (economic damage and cost of Gypchek) can be minimized.     

Bacterial predator–prey dynamics in microscale patchy landscapes

Soil is a microenvironment with a fragmented (patchy) spatial structure in which many bacterial species interact. Here, we explore the interaction between the predatory bacterium Bdellovibrio bacteriovorus and its prey Escherichia coli in microfabricated landscapes. We ask how fragmentation influences the prey dynamics at the microscale and compare two landscape geometries: a patchy landscape and a continuous landscape. By following the dynamics of prey populations with high spatial and temporal resolution for many generations, we found that the variation in predation rates was twice as large in the patchy landscape and the dynamics was correlated over shorter length scales. We also found that while the prey population in the continuous landscape was almost entirely driven to extinction, a significant part of the prey population in the fragmented landscape persisted over time. We observed significant surface-associated growth, especially in the fragmented landscape and we surmise that this sub-population is more resistant to predation. Our results thus show that microscale fragmentation can significantly influence bacterial interactions.     

An island-wide predator manipulation reveals immediate and long-lasting matching of risk by prey

Anti-predator behaviour affects prey population dynamics, mediates cascading effects in food webs and influences the likelihood of rapid extinctions. Predator manipulations in natural settings provide a rare opportunity to understand how prey anti-predator behaviour is affected by large-scale changes in predators. Here, we couple a long-term, island-wide manipulation of an important rodent predator, the island fox (Urocyon littoralis), with nearly 6 years of measurements on foraging by deer mice (Peromyscus maniculatus) to provide unequivocal evidence that prey closely match their foraging behaviour to the number of fox predators present on the island. Peromyscus maniculatus foraging among exposed and sheltered microhabitats (a measure of aversion to predation risk) closely tracked fox density, but the nature of this effect depended upon nightly environmental conditions known to affect rodent susceptibility to predators. These effects could not be explained by changes in density of deer mice over time. Our work reveals that prey in natural settings are cognizant of the dynamic nature of their predators over timescales that span many years, and that predator removals spanning many generations of prey do not result in a loss of anti-predator behaviour.     

Lionfish predators use flared fin displays to initiate cooperative hunting

Despite considerable study, mystery surrounds the use of signals that initiate cooperative hunting in animals. Using a labyrinth test chamber, we examined whether a lionfish, Dendrochirus zebra, would initiate cooperative hunts with piscine partners. We found that D. zebra uses a stereotyped flared fin display to alert conspecific and heterospecific lionfish species Pterois antennata to the presence of prey. Per capita success rate was significantly higher for cooperative hunters when compared with solitary ones, with hunt responders assisting hunt initiators in cornering the prey using their large extended pectoral fins. The initiators would most often take the first strike at the group of prey, but both hunters would then alternate striking at the remaining prey. Results suggest that the cooperative communication signal may be characteristic to the lionfish family, as interspecific hunters were equally coordinated and successful as intraspecific hunters. Our findings emphasize the complexity of collaborative foraging behaviours in lionfish; the turn-taking in strikes suggests that individuals do not solely try to maximize their own hunting success: instead they equally share the resources between themselves. Communicative group hunting has enabled Pteroine fish to function as highly efficient predators.     

Reversed predator–prey cycles are driven by the amplitude of prey oscillations

Ecoevolutionary feedbacks in predator–prey systems have been shown to qualitatively alter predator–prey dynamics. As a striking example, defense–offense coevolution can reverse predator–prey cycles, so predator peaks precede prey peaks rather than vice versa. However, this has only rarely been shown in either model studies or empirical systems. Here, we investigate whether this rarity is a fundamental feature of reversed cycles by exploring under which conditions they should be found. For this, we first identify potential conditions and parameter ranges most likely to result in reversed cycles by developing a new measure, the effective prey biomass, which combines prey biomass with prey and predator traits, and represents the prey biomass as perceived by the predator. We show that predator dynamics always follow the dynamics of the effective prey biomass with a classic ¼‐phase lag. From this key insight, it follows that in reversed cycles (i.e., ¾‐lag), the dynamics of the actual and the effective prey biomass must be in antiphase with each other, that is, the effective prey biomass must be highest when actual prey biomass is lowest, and vice versa. Based on this, we predict that reversed cycles should be found mainly when oscillations in actual prey biomass are small and thus have limited impact on the dynamics of the effective prey biomass, which are mainly driven by trait changes. We then confirm this prediction using numerical simulations of a coevolutionary predator–prey system, varying the amplitude of the oscillations in prey biomass: Reversed cycles are consistently associated with regions of parameter space leading to small‐amplitude prey oscillations, offering a specific and highly testable prediction for conditions under which reversed cycles should occur in natural systems.     

Toxin constraint explains diet choice,survival and population dynamics in a molluscivore shorebird

Recent insights suggest that predators should include (mildly) toxic prey when non-toxic food is scarce. However, the assumption that toxic prey is energetically as profitable as non-toxic prey misses the possibility that non-toxic prey have other ways to avoid being eaten, such as the formation of an indigestible armature. In that case, predators face a trade-off between avoiding toxins and minimizing indigestible ballast intake. Here, we report on the trophic interactions between a shorebird (red knot, Calidris canutus canutus) and its two main bivalve prey, one being mildly toxic but easily digestible, and the other being non-toxic but harder to digest. A novel toxin-based optimal diet model is developed and tested against an existing one that ignores toxin constraints on the basis of data on prey abundance, diet choice, local survival and numbers of red knots at Banc d''Arguin (Mauritania) over 8 years. Observed diet and annual survival rates closely fit the predictions of the toxin-based model, with survival and population size being highest in years when the non-toxic prey is abundant. In the 6 of 8 years when the non-toxic prey is not abundant enough to satisfy the energy requirements, red knots must rely on the toxic alternative.