Malacosoma Disstria Research Articles

Forest landscape structure influences the cyclic‐eruptive spatial dynamics of forest tent caterpillar outbreaks

AbstractA fundamental question in forest insect ecology is the role of forest landscape structure, particularly the amount and spatial configuration of host tree species, in shaping the dynamics of recurring forest insect outbreaks. For forest tent caterpillar (FTC), independent studies do not converge on a singular conclusion, although all indicate that forest structure influences outbreak dynamics. These studies also vary in how they treat climate as a covariate. We evaluated the relative importance of host forest landscape composition and configuration, as well as climate, for their influence on FTC outbreak cycling in the twentieth century. We predicted that FTC outbreaks would exhibit greater synchrony and intensity within areas associated with higher abundance of host trees. We reconstructed FTC outbreaks from 1928 to 2006 using tree‐ring analysis within a well‐structured experimental landscape located in northwestern Ontario and northern Minnesota. Time‐series clustering and spatial nonparametric covariance were used to determine whether similarities in time series and patterns in spatial synchrony corresponded with land management history. Using constrained ordination, we compared statistical properties of outbreak time series to landscape variables representing host abundance, forest configuration, and climate. We found no evidence of climatic effects at the scale of this study, but a significant albeit small influence of landscape structure on outbreak dynamics. Outbreaks were more synchronous and more cyclic within managed zones containing a greater relative abundance of aspen and other hardwood host tree species, compared with the more conifer‐dominated Wilderness area. Yet, we also observed asynchronous outbreak dynamics across the study area, such that correlations with slower‐changing forest landscape variables varied starkly among outbreak pulses. Consequently, the strength of relationship between landscape variables and FTC outbreak patterns varied strongly through time—a result that may explain why short‐term studies yield conclusions that are at odds with one another. Our results speak to the importance of long time series, contrasting landscape structure, use of multivariate methods, and controlling for climatic variation when investigating the effects of forest landscape structure on the cyclic‐eruptive spatial dynamics for forest defoliators.

A Continuous Model for Oscillating Outbreaks of the Population of a Phytophagous Moth, the Tent Caterpillar, Malacosoma disstria (Lepidoptera, Lasiocampidae)

Outbreaks of individual species populations are important phenomena in many aspects and are not alike in terms of the theory of multispecies community dynamics. Outbreaks of insect populations develop more quickly with long-lasting effects experienced by the forest industry. These events are considered as extreme unbalanced and transient processes. The mechanisms of the development and subsidence of insect outbreaks differ in different taxonomic groups of pests. The duration and occurrence of repeated outbreaks of psyllids and forest moths, which affect deciduous or coniferous forests in the same region, are different. Computational simulation is needed for understanding the dynamics of insect outbreaks. For the mathematical description of the outbreaks of forest tent caterpillar, in addition to the threshold version of the development of the insect outbreak, it is interesting to modify continuous computational models for the analysis of fluctuation dynamics. In this paper, we simulate the dynamics of spontaneously damping oscillations under a specific scenario during a population outbreak using a continuous model with delayed regulation and nonlinear counteraction by the biotic environment. The scenario described by the new phenomenological equation, which consists of a series of maxima of different sizes and final attenuation of peaks near balance, occurs for the pest tent caterpillar, Malacosoma disstria, which affects deciduous forests in North America leading to large-scale defoliation. The new scenario is qualitatively different from our model of the threshold development and subsidence of outbreaks of the psyllid Cardiaspina albitextura in Australia.

Density-dependent processes fluctuate over 50years in an ecotone forest.

Spatial patterns can inform us of forest recruitment, mortality, and tree interactions through time and disturbance. Specifically, successional trajectories of self-thinning and heterospecific negative density dependence can be interpreted from the spatial arrangement of forest stems. We conducted a 50-year spatial analysis of a forest undergoing succession at the ecotone of the southwestern Canadian boreal forest. The forest progressed from early to late sere and experienced repeated severe droughts, forest tent caterpillar outbreaks (Malacosoma disstria), as well as the outbreak of bark beetles. Cumulatively, the forest lost 70% of stems due to natural succession and a combination of disturbance events. Here, we describe spatial patterns displaying signals of successional self-thinning, responses to disturbance, and changes in patterns of density dependence across 50years. Forest succession and disturbance events resulted in fluctuating patterns of density-dependent mortality and recruitment that persisted into late seral stages. The combined effects of conspecific and heterospecific density-dependent effects on mortality and recruitment resulted in near-spatial equilibrium over the study period. However, the strength and direction of these demographic and spatial processes varied in response with time and disturbance severity. The outbreak of forest tent caterpillar, pronounced drought, and bark beetles combined to reduce stand aggregation and promote a spatial equilibrium. Density-dependent processes of competition and facilitation changed in strength and direction with succession of the plot and in combination with disturbance. Together these results reinforce the importance of successional stage and disturbance to spatial patterns.

Pests, climate and competition effects on survival and growth of trembling aspen in western Canada

Trembling aspen (Populus tremuloides Michx.) is experiencing increased drought stress resulting from climate change together with increases in damage by forest tent caterpillar and other defoliators. Coupled with effects of intraspecific and interspecific competition this could result in an overall decrease in survival and growth. In order to improve our understanding of the key limiting factors affecting trembling aspen we investigated survival probability and tree growth using data from an extensive network of permanent sample plots in Alberta (Canada). We developed mixed-effect non-linear models which included: tree size, competition, climate, pest incidence and time elapsed between consecutive measurements as predictor variables. Tree data from 1144 Permanent Sample Plots representing 39,394 trees for a total of 52,522 growth interval observations were used to develop these models. Annual data on aspen defoliation were available for the period 1990–2010, and the average Climate Moisture Index (CMI) was calculated for each measured interval by location. Intraspecific competition and competition from conifer species (i.e. spruce/fir and pine), had a strong negative effect on survival and growth. Aspen defoliators such as the forest tent caterpillar (Malacosoma disstria Hbn.), the large aspen tortrix (Choristoneura conflictana) and the Bruce spanworm (Operophtera bruceata) had a negative impact on survival, but a strong positive effect on growth of surviving trembling aspen (i.e. compensatory growth). Increasing levels of CMI, which are associated with relatively cooler and wetter conditions, had a positive effect on survival and growth. Model validation results indicated good performance for survival and tree growth predictions. This study indicates that trembling aspen is very sensitive to competition, insect damage, and climate (i.e. drought); respectively, and this behavior will likely intensify as climate warms. The models developed in this study enhance our understanding of trembling aspen survival and growth in boreal North America and could be used to improve the predictive ability of existing growth and yield models.

Distinguishing eastern North American forest moth pests by wing-scale ultrastructure: potential applications in paleoecology

The use of fossil moth wing scales has recently been introduced as a new method to reconstruct population histories of lepidopterans and provide a proxy for insect disturbance. We investigated the potential for using wing-scale ultrastructure to distinguish between the five most common outbreak species of moth pests in eastern North America: spruce budworm ( Choristoneura fumiferana Clemens), hemlock looper ( Lambdina fiscellaria Guenée), forest tent caterpillar ( Malacosoma disstria Hübner), blackheaded budworm ( Acleris variana Fernie), and jack pine budworm ( Choristoneura pinus Freeman). Using scanning electron images of scales, we made qualitative and quantitative comparisons of morphological traits at the ultrastructural level. We found that hemlock looper and eastern blackheaded budworm scales could be categorically separated from each other and from the three other species. We developed a quadratic discriminant function using measurements of ultrastructure traits that distinguishes scales of the three remaining species with an overall accuracy of 66%. We found that forest tent caterpillar could be well separated based on these traits, but we were less confident in distinguishing the closely related jack pine and spruce budworm. Our method offers potential advantages in scale identification for future studies in paleoecology, while providing the additional advantage of not requiring intact, unfolded, and undamaged scales.

Decline of an ecotone forest: 50 years of demography in the southern boreal forest

AbstractVariation in tree recruitment, mortality, and growth can alter forest community composition and structure. Because tree recruitment and mortality events are generally infrequent, long‐time scales are needed to confirm trends in forests. We performed a 50‐yr demographic census of a forest plot located on the southern edge of the Canadian boreal forest, a region currently experiencing forest die‐back in response to direct and indirect effects of recent severe droughts. Here, we show that over the last 30 yr biomass, basal area, growth, and recruitment have decreased along with a precipitous rise in mortality across the dominant tree species. The stand experienced periods of drought in combination with multiple outbreaks of forest tent caterpillar (Malacosoma disstria) and bark beetles. These insect disturbances interacted to increase mortality rates within the stand and decrease stand density. The interaction of endogenous and exogenous factors may shift forests in this region onto novel successional trajectories with the possibility of changes in regional vegetation type.

Changes in Nesting Density of Baltimore Orioles (1976-1995) and Other Species in the Dune- Ridge Forest, Delta Marsh, MB: Response to an Outbreak of Forest Tent Caterpillar?

Changes in Nesting Density of Baltimore Orioles (1976-1995) and Other Species in the Dune- Ridge Forest, Delta Marsh, MB: Response to an Outbreak of Forest Tent Caterpillar?

Drought and surface‐level solar radiation predict the severity of outbreaks of a widespread defoliating insect

AbstractDefoliating insects are a major factor impacting tree growth in temperate and boreal forests, but the effects of climate change on the severity and frequency of outbreaks of these insects are not well understood. Dendrochronological reconstruction of forest tent caterpillar (Malacosoma disstria) outbreaks on trembling aspen (Populus tremuloides) in southwestern Manitoba, Canada, from 1851 to 2010 was used to examine the effects of changes in temperature, incident surface‐level solar radiation, and drought severity on the cyclicity, frequency, and severity of outbreaks. Outbreak severity was determined by comparing the tree‐ring widths of trembling aspen with those of white spruce (Picea glauca), which is not a forest tent caterpillar host. Wavelet analysis suggested the cyclicity of outbreaks increased after 1930. From 1930 to 2010, the dominant period length of outbreak cycles steadily increased over time from three to seven years, coinciding with long‐term declines in drought severity. The strength of forest tent caterpillar suppression of trembling aspen growth in a given year was inversely related to incident solar radiation and drought severity during late spring (the larval feeding period) of the same year. The effect of late spring drought on the impact of forest tent caterpillar on ring growth differed from that of summer drought. There was a nonlinear relationship between growth suppression in a given year and summer drought severity in the previous year; slightly above‐average drought severity was associated with reduced impacts of the defoliator, whereas severe drought was associated with intense growth suppression. Strong suppression of aspen growth observed during outbreaks from 1983 to 2010 may be partly explained by observed long‐term trends of decreasing atmospheric transmittance of solar radiation (a phenomenon termed “global dimming”) and increasing soil moisture availability during late spring. However, as the severity and frequency of summer droughts are expected to increase over the 21st century, the nonlinear relationship between summer drought severity and growth suppression the following year suggests a future in which summer droughts precede severe outbreaks that threaten the health of boreal forest hardwood trees.

Host-plant mediated effects on group cohesion and mobility in a nomadic gregarious caterpillar

Gregarious animals often live in larger groups in food-rich environments. We compare cohesion and mobility of forest tent caterpillar (Malacosoma disstria) groups on high-quality (trembling aspen, Populus tremuloides) and low-quality (sugar maple, Acer saccharum) host trees. Following pheromone trails allows forest tent caterpillars to maintain group cohesion, but can cause groups to become trapped on poor-quality food sources. Trail-following decreases in food-deprived individuals. We combine laboratory and field experiments to show that this nutritionally mediated balance between trail fidelity and exploration generates fission-fusion dynamics, leading to increased mobility and smaller group sizes on poorer quality hosts. Indeed, caterpillar groups feeding on maple spent more time in locomotion, split into smaller groups, switched feeding site more frequently and traveled farther when compared to those feeding on aspen. These fission-fusion dynamics can be explained as an emergent property of individual responses to conspecifics: Individuals drift apart in contexts where responses to cues from conspecifics are weakened. Increased mobility appears as an adaptive response that improves the chances of relocating to a superior food source, but lower group size likely decreases thermoregulation and increases predation. Finally, we show that forest tent caterpillar fission-fusion dynamics not only control the spatial distribution of individuals in a tree, but they are also linked to considerable differences in growth and mortality and therefore can play an important role in population dynamics. Many gregarious animals split up into smaller groups when food is scarce. In many species, the behaviors by which group members stay together and follow each other are well-understood; how then do these behaviors respond to changing conditions? In particular, how does hunger drive groups to split up in response to decreased food availability? We compare mobility and group size of forest tent caterpillar colonies on good- and poor-quality host plants. These caterpillars use pheromone trails to move around together, but when hungry, they tend to leave these trails. We show how caterpillars on a poor-quality host plant are not only more mobile (presumably an adaptive response that improves chances of discovering a better food source), but they also split into smaller groups—a side effect of decreased trail fidelity that possibly exposes them to greater risks.

Evening Grosbeaks Feeding on Pupae of the Forest Tent Caterpillar in the Dune- Ridge Forest, Delta Marsh, Manitoba, Spring 1976

Evening Grosbeaks Feeding on Pupae of the Forest Tent Caterpillar in the Dune- Ridge Forest, Delta Marsh, Manitoba, Spring 1976

Effects of forest tent caterpillar defoliation on carbon and water fluxes in a boreal aspen stand

Insect outbreaks can significantly influence carbon (C) and water balances of forests. Forest tent caterpillars (FTC) (Malacosoma disstria Hübner) are one of the most prominent insects found in aspen forests in Canada and have the potential to considerably influence regional C and water fluxes. In the summer of 2016, an FTC infestation occurred in a ca. 100 -year-old trembling aspen (Populus tremuloides) stand in the southern boreal forest where the longterm research site known as Old Aspen (OA) is located. The infestation led to nearly complete defoliation of the canopy during the leafing out period when photosynthesis, and thus C uptake, is progressing towards maximum levels. We used 21 years of eddy-covariance (EC) and climate measurements covering pre-infestation and infestation periods to estimate the impact of the FTC infestation on net ecosystem production (NEP), gross ecosystem production (GEP) and evapotranspiration (E). Defoliation in 2016 reduced annual NEP to −130 g C m−2 y−1 and GEP to 798 g C m−2 y−1, respectively, which were much less than their 20-year means (NEP = 118 ± 53 g C m−2 y−1, GEP = 1057 ± 74 g C m−2 y−1), and resulted in the most negative annual NEP value of the 21 years of measurements at the OA site. NEP for 2016 was even lower than values observed during three drought years (2001–2003). However, FTC infestation caused little effect on annual E. FTC infestation reduced the near-surface remotely-measured greenness index, green chromatic coordinate (GCC), to ∼0.32 on June 10 in comparison to ∼0.40 in other years. The defoliation, observable from space as reductions in normalized difference vegetation index (NDVI) values, also showed a negligible effect on E but a large effect on the C fluxes.

Early 20<sup>th</sup> Century Climate-Driven Shift in the Dynamics of Forest Tent Caterpillar Outbreaks

Using tree-ring analysis, we show that the dynamics of forest tent caterpillar [Malacosoma disstria (Hbn.)] outbreaks in Alberta, Canada shifted at the turn of the 20th century from cyclic, synchronous behaviour 1850-1910 to complex, asynchronous behavior 1910-1993. This shift in dynamics coincided with the emergence in 1910 of a latitudinal gradient in outbreak stability and periodicity reflecting a similar gradient in the periodicity of winter temperatures. We postulate that the synchronizing strength of winter temperatures has diminished as a result of climate warming, and that any synchronizing strength due to inter-population migration has been superseded by regionalized patterns of periodic forcing caused by weak low-frequency variability in winter temperatures. We speculate that a decrease in polar vorticity at the start of the 20th century led to increased meridional jet stream flow and more frequent arctic weather anomalies, resulting in a loss of synchronous decadal periodicity in outbreak occurrence. These changes in insect disturbance probabilities, including rising uncertainty, have profound consequences for forest disturbance risk management.

DRINO IMBERBIS (WIEDEMANN) (DIPTERA: TACHINIDAE), A PARASITOID OF MALACOSOMA DISSTRIA HUBNER (LEPIDOPTERA: LASIOCAMPIDAE) CATERPILLARS IN IRAN

Malacosoma disstria Hubner (Lepidoptera: Lasiocampidae) is a widespread generalist defoliator of a wide variety of plants in Iran. This study was conducted to identify the indigenous parasitoids of the forest tent caterpillar in the Borazjan region of the Bushehr province, Iran. Samplings were conducted in different sites within a single cabbage field heavily infested with M. disstria . The forest tent caterpillars were reared in the laboratory until the emergence of parasitoids. A parasitoid species from the family Tachinidae was found, reared, and identified as Drino imberbis (Wiedemann). Malacosoma disstria is a new host record for D. imberbis in Iran.

Double allomimesis of advancing and retreating individuals maintains cohesion in exploring groups of nomadic caterpillars

Group-living entails that animals maintain cohesion during collective locomotion: This cohesion often requires that each individual respond to cues from several neighbours. Social insects generally use pheromone trails to integrate information from multiple group members. We demonstrate mechanisms used by nomadic social caterpillars to maintain cohesion when exploring off-trail. Our results show that forest tent caterpillars (Malacosoma disstria) use double allomimesis, responding to cues from both advancing and retreating neighbours. A group-level experiment measured cohesion and persistence of locomotion according to caterpillar age and group size. An individual-level experiment quantified responses to cues from neighbours, and a model was used to predict the group-level patterns that emerge from these responses. We show that double allomimesis generates feedback that maintains group cohesion, but at the price of locomotion efficiency, particularly in small groups of young caterpillars. We also show that the importance of allomimesis decreases as caterpillars age and show lesser responses to social cues. Finally, we demonstrate that, especially in the early instars, efficient collective locomotion is only possible in the large group sizes typically of field colonies. Simple do-what-your-neighbour-is-doing mechanisms underlie mass collective locomotion of gregarious animals. Social insects generally use pheromone trails to follow each other. How then to stay together when exploring away from these trails? Forest tent caterpillars provide an intriguing example: They follow trails faithfully, but on unexplored territory turn back frequently to rejoin the trail behind them. We show how direct physical contact with advancing and retreating neighbours provides an explanation. An off-trail caterpillar turns back less often if it is propelled by a forward-moving neighbour, but more often if that neighbour turns back. This dual imitation keeps the group together, but at the price of efficient movement, especially in small groups. Groups as large as natural colonies seldom turn back due to the steady supply of advancing neighbours to propel forward and eventually replace vanguard individuals.

A Forest Tent Caterpillar Outbreak Increased Resource Levels and Seedling Growth in a Northern Hardwood Forest.

In closed-canopy forests, gap formation and closure are thought to be major drivers of forest dynamics. Crown defoliation by insects, however, may also influence understory resource levels and thus forest dynamics. We evaluate the effect of a forest tent caterpillar outbreak on understory light availability, soil nutrient levels and tree seedling height growth in six sites with contrasting levels of canopy defoliation in a hardwood forest in northern lower Michigan. We compared resource levels and seedling growth of six hardwood species before, during and in the three years after the outbreak (2008–2012). Canopy openness increased strongly during the forest tent caterpillar outbreak in the four moderately and severely defoliated sites, but not in lightly defoliated sites. Total inorganic soil nitrogen concentrations increased in response to the outbreak in moderately and severely defoliated sites. The increase in total inorganic soil nitrogen was driven by a strong increase in soil nitrate, and tended to become stronger with increasing site defoliation. Seedling height growth increased for all species in the moderately and severely defoliated sites, but not in lightly defoliated sites, either during the outbreak year or in the year after the outbreak. Growth increases did not become stronger with increasing site defoliation, but were strongest in a moderately defoliated site with high soil nutrient levels. Growth increases tended to be strongest for the shade intolerant species Fraxinus americana and Prunus serotina, and the shade tolerant species Ostrya virginiana. The strong growth response of F. americana and P. serotina suggests that recurring forest tent caterpillar outbreaks may facilitate the persistence of shade intolerant species in the understory in the absence of canopy gaps. Overall, our results suggest that recurrent canopy defoliation resulting from cyclical forest insect outbreaks may be an additional driver of dynamics in temperate closed-canopy forests.

Root connections affect radial growth of balsam poplar trees

Connected root systems reduced the negative impact of August temperatures and insect outbreak on growth. Root connections between trees can be an ecological advantage of clonal plant species in environments with unevenly distributed resources. We investigated the effects of root connectivity in stands of balsam poplar in Quebec (Canada). We evaluated differences in growth response between groups of trees with and without root connections through climate-growth analyses, comparison of the growth dynamics, and analysis of growth response to a severe forest tent caterpillar (FTC) outbreak. Current May temperature had a positive influence on radial growth of both connected and non-connected trees. Growth of non-connected trees was negatively affected by August temperatures (r = −0.3) while connected trees did not reveal a significant relationship for that month. A mixed effect ANOVA showed a significant difference (F 1, 25 = 5.59, p = 0.02) in growth responses to FTC outbreak between connected and non-connected trees. Connected trees grew on average 16 % better than unconnected trees during the outbreak, with bootstrapped 95 % confidence range from 2.28 to 31.36 %. The study suggests a sharing of resources through root connections, affecting radial growth of connected balsam poplar trees under both average and extreme environmental conditions.

Effects of winter temperatures, spring degree-day accumulation, and insect population source on phenological synchrony between forest tent caterpillar and host trees

Global climate change has the potential to dramatically alter multiple ecosystem processes, including herbivory. The development rates of both plants and insects are highly sensitive to temperature. Although considerable work has examined the effects of temperature on spring phenologies of plants and insects individually, few studies have examined how anticipated warming will influence their phenological synchrony. We applied elevated temperatures of 1.7 and 3.4°C in a controlled chamberless outdoor experiment in northeastern Minnesota, USA to examine the relative responses in onset of egg eclosion by forest tent caterpillar (Malacosoma disstria Hübner) and budbreak of two of its major host trees (trembling aspen, Populus tremuloides Michaux, and paper birch, Betula papyrifera Marshall). We superimposed four insect population sources and two overwintering regimes onto these treatments, and computed degree-day models. Timing of egg hatch varied among population source, overwintering location, and spring temperature regime. As expected, the development rates of plants and insects advanced under warmer conditions relative to ambient controls. However, budbreak advanced more than egg hatch. The degree of phenological synchrony between M. disstria and each host plant was differentially altered in response to warming. The interval by which birch budbreak preceded egg hatch nearly doubled from ambient to +1.7°C. In the case of aspen, the sequence changed from egg hatch preceding, to following, budbreak at +3.4°C. Additionally, under temperature regimes simulating future conditions, some insect populations currently south of our study sites became more synchronous with the manipulated hosts than did currently coexisting insect populations. These findings reveal how climate warming can alter insect-host plant interactions, through changes in phenological synchrony, possibly driving host shifts among tree species and genotypes. They also suggest how herbivore variability, both among populations and within individual egg masses, may provide opportunities for adaptation, especially in species that are highly mobile and polyphagous.

The effects of genetic diversity, climate and defoliation events on trembling aspen growth performance across Canada

Tree genetic makeup may provide an important control of growth dynamics; however, no studies have previously attempted to evaluate its effects in natural trembling aspen stands. In this study, we examined the relative contribution of genetics (i.e. clonal diversity, observed heterozygosity) and environmental conditions (i.e. insects, climate) on aspen growth as represented by mean inter-tree correlation (RBAR), tree basal area increment (TBAI) and inter-annual growth variability (MS). We sampled 440 trees in 22 even-aged natural stands dominated by aspen along an east-west continental gradient of decreasing annual precipitation in the Canadian boreal forest. Linear and mixed-effect models tested the relationships between tree growth, genetics and environmental factors. We showed that clonal diversity and number of years with forest tent caterpillar (FTC) defoliation (NFTC) reduced and increased the level of growth synchronicity (RBAR), respectively. Clonal diversity explained 30 % of variation in RBAR among sites. TBAI was positively influenced by high moisture conditions while NFTC and climate explained the variation in MS among trees for each site. No genetic effect could explain either TBAI or the MS variation. Climate and NFTC drive annual growth variability in trembling aspen at stand and subcontinental scales. Tree genetic makeup contributed to these dynamics, the annual growth dynamics of multi-clonal stands being less homogeneous than those of monoclonal stands. Maintaining diverse aspen stands may ensure a wider range of growth responses to environmental variability, which in turn may help maintain resilience of aspen stands under future climate.

Forest tent caterpillar, Malacosoma disstria (Lepidoptera: Lasiocampidae), mate-finding behavior is greatest at intermediate population densities: implications for interpretation of moth capture in pheromone-baited traps

The forest tent caterpillar, Malacosoma disstria Hubner (Lepidoptera: Lasiocampidae) is a native forest defoliator with a broad geographic range in North America. Forest tent caterpillars experience cyclical population changes and at high densities, repeated defoliation can cause reduced tree growth and tree mortality. Pheromone-based monitoring of forest tent caterpillar moths can provide information on spatial and temporal patterns of incipient outbreaks. Pheromone-baited trap capture of male moths correlates to the number of eggs and pupae in a population but this relationship breaks down at high population densities, when moth trap capture declines. The objective of the current study is to understand the mechanisms that reduce trap capture at high population densities. We tested two different hypotheses: 1) at high population densities, male moth orientation to pheromone sources is reduced due to competition for pheromone plumes; and 2) moths from high density populations will be in poor condition and less likely to conduct mate-finding behaviors than moths from low density populations. A field study showed non-linear effects of density on male moth capture in female-baited traps. The number of males captured increased up to an intermediate density level and declined at the highest densities. Field cage studies showed that female moth density affected male moth orientation to female-baited traps, as more males were recaptured at low than high female densities. There was no effect of male density on the proportion of males that oriented to female-baited traps. Moth condition was manipulated by varying larval food quantity. Although feeding regimes affected the moth condition (size), there was no evidence of an effect of condition on mate finding or close range mating behavior. In the field, it is likely that competition for pheromone plumes at high female densities during population outbreaks reduces the efficacy of pheromone-baited monitoring traps.

Exploration costs promote conservative collective foraging in the social caterpillar Malacosoma disstria

Collective choices of a food source are generally thought to arise from the amplification of individual preferences. However, costs associated with foraging can also influence food choice: animals faced with a poor-quality food source may either continue feeding on it (a conservative strategy), or search for a potentially more profitable one (a more risky strategy). Groups of nomadic, trail-following Malacosoma disstria caterpillars were offered a leaf of two different hosts, either side by side to determine preference, or at a distance to investigate how exploration costs affect food choice. Arenas with and without pheromone trails were used to distinguish between direct travel costs and unknown territory costs. When two leaves were side by side, groups of second-instar caterpillars showed clear preferences between host trees, whereas groups of fourth-instar larvae did not. When the two leaves were distant from each other, groups of second-instar caterpillars usually remained on the known source, irrespective of food preference or group size. These groups were more likely to relocate to a preferred food source when pheromone trails were present, suggesting that trails effectively decrease unknown territory costs. Groups of fourth-instar caterpillars were more likely to leave the known food source even when no trail was present, suggesting a developmental shift in costs associated with exploration. Our results show that collective choices reflect food preference when exploration costs are low (second-instar larvae with pheromone trails), but not when they are high (second-instar larvae without trails). Such a conservative foraging strategy can be adaptive if the exploration costs of leaving a known food source are greater than the potential benefits of discovering a better alternative, for example, in a homogenous environment like that of a tree.