Impact of individual movement and changing resource availability on male–female encounter rates in an herbivorous insect

Species often confront changing resource distributions that result from natural and anthropogenic processes. For species that reproduce on or in close association with particular resources (e.g. host plants), changing resource distributions could affect the success of mate finding. We examine how mate-finding behaviours in an herbivorous insect mediate the impact of changing host plant spatial distribution. We tracked movements of 84 Melissa blue butterflies (Lycaeides melissa) in the Great Basin of western North America. Track data revealed sex differences in movement: males spent more time moving fast and females more time moving slowly; males moved more ballistically and females moved more diffusely. These differences vary quantitatively, but not qualitatively, between environments with contrasting resource distributions.From these data we created and parameterised a computer model of male–female encounters and used it to examine implications of changes to the patchiness and abundance of host plants. We use the cumulative encounter time between each simulated male–female pair as a proxy for mating success, thus allowing for the consideration of different female behaviours. The simulations suggest observed movement parameters exist in a trade-off between individuals maximising the number of potential mates they encounter and the probability that each encounter leads to mating success. Increasing host plant abundance decreases encounter rates thus encouraging males to be more diffuse to compensate. Changing the local resource density, i.e. increasing host plant patchiness, accentuated these trade-offs: by decreasing cumulative encounter time in resource rich environments and increasing it in resource sparse ones. Thus we see that both spatial resource geometry at multiple scales and plasticity in male movement strategies are important factors to consider when seeking to understand population reproductive behaviour, for example when assessing ecological impact of development, determining range boundaries and slowing invasions or outbreaks.     

Interpatch movement of the red milkweed beetle,Tetraopes tetraophthalmus: individual responses to patch size and isolation

Individual movement patterns and the effects of host plant patch size and isolation on patch occupancy were examined for red milkweed beetles, Tetraopes tetraophthalmus, residing in a heterogeneous landscape. Male beetles were found to move both more often and farther between host plant patches than female beetles, and this difference affected the patterns of patch occupancy observed. Overall, unoccupied milkweed patches were smaller and more isolated than patches occupied by beetles. Patches uninhabited by females tended to be more isolated, but not necessarily smaller, than patches with female beetles, indicating that females may be affected more by patch isolation than patch size. Presence of male beetles on patches showed a stronger response to patch size than to patch isolation. Differences in movement between males and females illustrate the need for demographically based dispersal data. Comparisons of Tetraopes interpatch movement patterns between landscapes composed of patches of different size revealed that landscapes with overall smaller patches may have greater rates of interpatch movement.     

Host frass as arrestant chemicals in locating host Mythimna separata by the tachinid fly Exorista japonica

We investigated the response of the tachinid fly, Exorista japonica (Townsend), to host frass or its extracts in order to clarify the host location mechanisms of female flies in a potential host habitat. Host searching time in a patch and the number of patch visits were analyzed by using a frass-containing patch which was excreted by host larvae, Mythimna separata (Walker) (Lepidoptera: Noctuidae), and patches to which host frass extracts were applied. E. japonica females were arrested in response to the host-frass-containing patch after contacting the frass with their front tarsi, thereby spending most of the time to search the patch and to revisit the host-frass patch. While host-searching time in the patch by the females was longest at their first visit of a patch with host frass, searching time decreased with successive visits. The female flies also exhibited area-restricted searching with methanol extracts of the host frass. Area-restricted searching activity increased with the concentration of host-frass extract, i.e., total searching time in the patch and the number of patch visits varied in a dose-dependent manner. E. japonica females likely employ chemicals in host frass as arrestants in host location.     

Sex-different response in growth traits to resource heterogeneity explains male-biased sex ratio

In dioecious plants, differences in growth traits between sexes in a response to micro-environmental heterogeneity may affect sex ratio bias and spatial distributions. Here, we examined sex ratios, stem growth traits and spatial distribution patterns in the dioecious clonal shrub Aucuba japonica var. borealis, in stands with varying light intensities. We found that male stems were significantly more decumbent (lower height/length ratio) but female stems were upright (higher height/length ratio). Moreover, we found sex-different response in stem density (no. of stems per unit area) along a light intensity gradient; in males the stem density increased with increases in canopy openness, but not in females. The higher sensitivity of males in increasing stem density to light intensity correlated with male-biased sex ratio; fine-scale sex ratio was strongly male-biased as canopy openness increased. There were also differences between sexes in spatial distributions of stems. Spatial segregation of sexes and male patches occupying larger areas than female patches might result from vigorous growth of males under well-lit environments. In summary, females and males showed different growth responses to environmental variation, and this seemed to be one of possible causes for the sex-differential spatial distributions and locally biased sex ratios.     

HOST-PLANT-INDUCED ASSORTATIVE MATING IN ENCHENOPA TREEHOPPERS

The hypothesis tested here is whether extrinsic host-plant-induced life-history timing and mating biology promote assortative mating along host-plant lines. In the arboreal, univoltine Enchenopa treehopper system, host plants mediate the timing and synchronization of egg hatch. The result is a uniform age structure with a restricted mating window during which females mate once. Enchenopa on host plants that differ in phenology have asynchronous life histories and mating windows, suggesting that temporal differences may promote assortative mating. To test this hypothesis, egg hatch of Enchenopa from the same host-plant species was manipulated to produce continuous adult age-classes. Under experimental conditions with no spatial barriers, mating occurred between individuals similar in age. The mechanism promoting this assortative mating is differential mortality in males and females, such that few males are still alive when females in successive age-classes mate. Such host-plant-induced assortative mating is viewed as an effective mechanism to protect the integrity of gene pools from migrants, permitting selection for host-plant-adapted genotypes and speciation.     

Host finding by Uscana lariophaga (Hymenoptera: Trichogrammatidae) in stored cowpea: the effect of distance, time interval, host patch size and spatial orientation

Host finding and parasitization by Uscana lariophaga Steffan, a potential biocontrol agent of the storage pest Callosobruchus maculatus (Fabricius), were investigated in stored cowpea. Host finding was shown to be a function of distance, time, host patch size and the spatial position of U. lariophaga relative to the host patch. Uscana lariophaga females were able to find hosts up to 75 cm horizontal distance from the host patch, which was the largest distance tested. The probability that a host patch was found when an individual U. lariophaga female was released at 2.5 cm horizontal distance from the host patch ranged from 0.6 after 2 h foraging time to 0.9 after 8 h foraging time. At 10 cm from the host patch, host finding probability ranged from 0.2 to 0.45 at these respective foraging times. Finding probabilities doubled compared to horizontal distances when U. lariophaga was released below the host patch, and halved when it was released above the host patch. The negative geotaxic response was shown not to be an artefact of the release method. The median net displacement rate in the direction of the host patch was two beans per hour (1.4 cm h(-1)). The results suggest that U. lariophaga females start searching for hosts regardless of the quality of the olfactory information they receive. Additional observations indicated that U. lariophaga is adapted to a host with a patchy distribution, which implies that host finding over larger distances is relevant for U. lariophaga.     

Sex‐related spatial patterns of Poa ligularis in relation to shrub patch occurrence in northern Patagonia

Abstract. Poa ligularis is a dioecious species and a valuable forage plant which is widespread in the arid steppe of northern Patagonia (Argentina). The vegetation in these areas consists of a system of perennial plant patches alternating with bare soil areas defining contrasting micro‐environments. We hypothesized that (1) male and female individuals of P. ligularis are spatially segregated in different micro‐environments, (2) the intensity of spatial segregation of sexes depends on plant structure and (3) spatial segregation of sexes is enhanced by competitive interactions between the sexes within the vegetation patches. We analysed the spatial distribution of female and male individuals in relation to the spatial pattern of vegetation in two areas differing in their vegetation structure. The location of P. ligularis within patches where either male, female or both sexes occurred was also analysed. The results indicate that different patterns of spatial distribution of sexes of P. ligularis may be found at the community level depending on the dominant life forms and geometric structure of plant patches. Where patches are of a lower height, with a high internal patch cover, individuals of both sexes are concentrated within patch canopies. In sites characterized by large, tall patches and less internal patch cover suitable microsites for female and male P. ligularis occur both within and outside the patch with males located at further distances from the patch edge. Where the patch is large and tall enough to allow the establishment of males and females at relatively high numbers, males occupy the patch periphery or even colonize the interpatch bare soil. These spatial patterns are consistent with selective traits in which females better tolerate intraspecific competition than males, while males tolerate wider fluctuations in the physical environment (soil moisture, nitrogen availability, wind intensity, etc.).     

Consequences for a host–parasitoid interaction of host-plant aggregation, isolation, and phenology

Abstract.  1. Spatial habitat structure can influence the likelihood of patch colonisation by dispersing individuals, and this likelihood may differ according to trophic position, potentially leading to a refuge from parasitism for hosts.
2. Whether habitat patch size, isolation, and host-plant heterogeneity differentially affected host and parasitoid abundance, and parasitism rates was tested using a tri-trophic thistle–herbivore–parasitoid system.
3.  Cirsium palustre thistles ( n = 240) were transplanted in 24 blocks replicated in two sites, creating a range of habitat patch sizes at increasing distance from a pre-existing source population. Plant architecture and phenological stage were measured for each plant and the numbers of the herbivore Tephritis conura and parasitoid Pteromalus elevatus recorded.
4. Mean herbivore numbers per plant increased with host-plant density per patch, but parasitoid numbers and parasitism rates were unaffected. Patch distance from the source population did not influence insect abundance or parasitism rates. Parasitoid abundance was positively correlated with host insect number, and parasitism rates were negatively density dependent. Host-plant phenological stage was positively correlated with herbivore and parasitoid abundance, and parasitism rates at both patch and host-plant scales.
5. The differential response between herbivore and parasitoid to host-plant density did not lead to a spatial refuge but may have contributed to the observed parasitism rates being negatively density dependent. Heterogeneity in patch quality, mediated by variation in host-plant phenology, was more important than spatial habitat structure for both the herbivore and parasitoid populations, and for parasitism rates.     

Effects of host-plant quality on male two-spotted spider mite (Acari: Tetranychidae) mate location and guarding behavior

Effects of host-plant quality on two-spotted spider mite,Tetranychus urticae Koch, mate location and guarding behaviors were described using a no-choice bioassay. Males and quiescent deutonymphs were collected from lima bean leaves of one of two host qualities. High-chlorophyll (HC) leaves had been infested with spider mites for 6–10 days, while low-chlorophyll (LC) leaves had been infested for>21 days. Three parameters of maleT. urticae guarding behavior were quantified: approach arrestment, and arrestment duration. HC males approached quiescent deutonymphs more often than did LC males, even though host quality of females had no effect on male approach frequency. HC males were arrested more frequently by HC quiescent deutonymphs than were LC males, while LC males were arrested more often by LC females than were HC males. However, a different pattern was observed for arrestment duration. HC males were arrested for twice as long by LC quiescent deutonymphs than by HC females, while the LC-male arrestment durations elicited by HC and LC females did not differ. These results show that host-plant quality affectsT. urticae intersexual communication, in terms of both the female signal and the male response. Whether the differing male responses observed in this study indicate alternativeT. urticae mating strategies or are incidental by-products of host-induced physiological changes remains to be determined.     

Interplant movement by pear psylla (Homoptera: Psyllidae): Effects of sex ratio and reproductive status

Sticky trap catch of pear psylla,Cacopsylla pyricola Foerster, is male biased during the reproductive generations, but not the diapausing generation. In cage studies, we monitored movement by male and female pear psylla between host plants, and tested whether reproductive and diapausing psylla exhibit similar rates of movement. We also experimentally varied sex ratio to determine whether sex ratio affected movement. Male-biased sex ratios prompted increased movement off of the original host by male psylla of the reproductive generations; no such effect was noted for diapausing insects. We interpret these results to indicate that male movements increased under male-biased conditions due to mate-searching activities. There was also evidence in two experiments that severely male-biased sex ratios prompted movement off of the original host plant by reproductive females; this effect may have been due to harassment of ovipositing females by males.     

The effects of foods consumed after adult eclosion on the mate-searching behavior and feeding preferences of the white-spotted longicorn beetle Anoplophora malasiaca (Coleoptera: Cerambycidae)

The effects of changes in host plants on the mate-searching behavior and feeding preferences of the white-spotted longicorn beetle Anoplophora malasiaca (Thomson) (Coleoptera: Cerambycidae) were examined. All individuals were raised on the same artificial diet until they became pupae. Analysis of the mate-searching behavior of the males showed that many more newly emerged males were attracted to the odor of the artificial diet than to an unbaited control. We prepared three groups of beetles and fed each group on different host plants for one week. The host plants used included the following: an artificial diet (containing Morus alba Linné), Citrus unshiu Marc. branches, and Vaccinium spp. branches. The mate-searching behavior of the males changed in relation to the plant supplied for feeding. Simultaneously, the preference among the three host plants was tested. The newly emerged males preferred the artificial diet. After a week of feeding on one of the three plants, however, the adult males selected and consumed significantly more of the plant that they had just experienced than the other plants. These results suggest that the male mate-location cue can be acquired after adult eclosion. In addition, the male beetles are capable of changing their host-plant preference. If they do so, they use different odor cues for mate location. Newly emerged A. malasiaca females showed no preference for their first choice of food among the three host plants presented, whereas the consumption was significantly larger on C. unshiu branches. After one week of feeding on different host plants, females chose their host plant after the adult stage as well as C. unshiu, but consumed mostly C. unshiu. These results suggest that the food preferences of females are different from those of males. The behavior of females may not be affected by chemical signals from their original host-plant species (as pupae) or from the host-plant species acquired when they emerge as adults.     

Male orientation to trail sex pheromones in parasitoid wasps: does the spatial distribution of virgin females matter?

We studied male locomotory response to trails and patches of sex pheromone (left respectively by free-ranging females and females constrained to stay on a small area) in the two parasitoids Aphelinus asychis (Hymenoptera: Aphelinidae) and Trichogramma brassicae (Hymenoptera: Trichogrammatidae). Under the hypothesis that the spatial distribution of virgin females differs between these species (scattered among host plants in A. asychis, gregarious at emergence sites in T. brassicae), we predicted that male locomotory response to their sex pheromones should also differ: A. asychis males should follow pheromone trails on plants in order to encounter the females along these trails, whereas T. brassicae males should stay on pheromone patches, at emergence sites, and mate the females on these patches. Using an improved video-tracking system, we found that males of both species respond to conspecific sex pheromone trails and patches, but that the response does not differ much between species. Males released on marked substrates walked in a more convoluted pattern (i.e. higher path fractal dimension and higher number of crossings within tracks) than males released on unmarked substrates. On pheromone patches, males turned persistently in the same direction when leaving the patch, which explains a higher number of visits on marked patches than on unmarked patches, and possibly, higher track convolution on pheromone trails. Contrary to our hypothesis, male A. asychis did not follow female trails more accurately than male T. brassicae, and male T. brassicae did not stay longer on pheromone patches than male A. asychis. We argue that these discrepancies between our predictions and the observed responses originates from discrepancies between the assumed spatial distribution of virgin females and their actual distribution in the wild.     

The role of aggregation in the response of Mexican bean beetles to host-plant density

Summary According to the resource concentration hypothesis, specialized insect herbivores predominantly attack host plants growing in pure, large, and/or dense stands because they are more likely to find and less likely to leave such stands. This study examines movement of a herbivorous beetle, Epilachna varivestis, in an attempt to understand why the beetle's numbers per plant increase with plant density. I studied immigration into, emigration from, and movement within host patches by following movements of individually marked beetles, while experimentally varying host-plant density. In order to study the effect of conspecifics on movement, I varied the number of beetles released on the same plant.The probability of intrapatch movement decreased, and the probability of emigration increased when plant density was reduced. Both immigration rate and intrapatch movement were strongly affected by presence or absence of conspecifics. Beetles were much more likely to stop at plants with a high number of beetles, compared to plants without beetles.On the basis of these results I advance a model that provides a mechanistic explanation of why dense patches should acquire high herbivore loads in the E. varivestis-garden bean system. Movement in Mexican bean beetles is strongly aggregative, and in dense patches frequent intrapatch movement provides ample opportunity for beetle aggregations to build up. In sparse patches, however, intrapatch movement is virtually absent and such aggregations are less likely to arise.     

Extinction-colonization dynamics and host-plant choice in butterfly metapopulations

Species living in highly fragmented landscapes often occur as metapopulations with frequent population turnover. Turnover rate is known to depend on ecological factors, such as population size and connectivity, but it may also be influenced by the phenotypic and genotypic composition of populations. The Glanville fritillary butterfly (Melitaea cinxia) in Finland uses two host-plant species that vary in their relative abundances among distinct habitat patches (dry meadows) in a large network of approximately 1,700 patches. We found no effect of host species use on local extinction. In contrast, population establishment was strongly influenced by the match between the host species composition of an empty habitat patch and the relative host use by larvae in previous years in the habitat patches that were well connected to the target patch. This "colonization effect" could be due to spatially variable plant acceptability or resistance or to spatially variable insect oviposition preference or larval performance. We show that spatial variation in adult oviposition preference occurs at the relevant spatial scale and that the other possible causes of the colonization effect can be discounted. We conclude that the colonization effect is generated by host preference influencing the movement patterns of ovipositing females. Migrant females with dissimilar host preferences have different perceptions of relative patch quality, which influences their likelihood of colonizing patches with particular host composition.     

Host‐searching responses to herbivory‐associated chemical information and patch use depend on mating status of female solitary parasitoid wasps

1. In a tritrophic interaction system consisting of plants, herbivores, and their parasitoids, chemicals released from plants after herbivory are known to play important roles for many female parasitoids to find their hosts efficiently. On the plant side, chemical information associated with herbivory can act as an indirect defence by attracting the natural enemies of the host herbivores. 2. However, mated and virgin females of haplodiploid parasitoids might not necessarily respond to such chemical cues in the same way. Since virgin females can produce only sons, they might refrain from searching for hosts to invest eggs until copulation, in order to produce both sexes. 3. Here, we investigated differential host‐searching behaviours shown by mated and virgin females in the solitary parasitoid wasp, Cotesia vestalis, in response to herbivory‐associated chemical information from cruciferous plants infested by their host larvae, Plutella xylostella. 4. Mated females showed a significantly higher flight preference for host‐infested plants over intact plants, while no preference was observed with virgin females. Mated females also showed more intensive antennal searching and ovipositor probing behaviours to leaf squares with wounds caused by hosts than did virgin females. Furthermore, mated females stayed longer in host patches with higher parasitism rates than virgin females. 5. These results indicate that mating status of C. vestalis females clearly influences their host‐searching behaviour in response to herbivory‐associated chemical information and patch exploitation. Female parasitoids seem to forage for hosts depending on their own physiological condition in a tritrophic system.     

Population dynamics of a phytophagous lady-beetle,Epilachna vigintioctopunctata (Fabricius), living in spatio-temporally heterogeneous habitats. III. Effects of habitat structure on population dynamics

Temporal changes in the population size of a phytophagous lady-beetle were analyzed to identify mechanisms affecting lady-beetle population dynamics at different spatial scales. The study area (15 ha) included 18 habitat patches. The major host plants were potato for first generation larvae and eggplant for second generation larvae. The habitat patches were classified into three groups according to the major host plants in each patch: P-E patches (both host plants available), P patches (potato only), and E patches (eggplant only). The winter disappearance of adults in the whole study area, and larval mortality in E patches were apparently the most important factors disturbing the overall population density. Density-dependent movement of females appeared to have the greatest stabilizing effect on the yearly fluctuation of population density. Rate of increase of female adults from the first to the second generation,R, was generally higher on eggplants in E patches than in P-E patches because the adult density of the first generation was much higher in P-E patches. The yearly fluctuation of adult density in each generation tended to be less in patches with all habitat components necessary for the full life cycle (P-E patches). However, such patches were not favorable for first generation females, as indicated by the lower rate of increase from the first to the second generation. The density and stability of lady-beetle populations is discussed in relation to habitat structure.     

The potential for sexual selection in males: Effect of sex ratio and spatiotemporal distribution of receptive females

Summary An animal mating system characterized by male-male competition and active searching for sexually receptive females was modelled to study how varying sex ratio and spatiotemporal distribution of receptive females can affect the variance in male mating success (i. e. potential for sexual selection) in males. The temporal distribution of female receptivity periods appeared to be the variable that had the most pronounced effect on the potential for sexual selection in males. The potential for sexual selection increased monotonically as the degree of asynchrony of female reproduction increased. Female spatial distribution and sex ratio were important only when female reproduction was asynchronous. Then, the potential for sexual selection in males was at its peak when females were overdispersed in space and the sex ratio was female biased. Some of the results derived from the model analysis contrast with predictions from previous studies. The deviating results are most likely caused by different assumptions about modes of mate acquisition in males.     

Role of host-plant stimuli in sexual behaviour of small ermine moths (Yponomeuta)

ABSTRACT. 1. Studies involving experiments with olfactometers and field experiments have shown that the release of sex-pheromone by females of Yponomeuta spp. is stimulated by the presence of host plants.
2. While in Y.evonymellus L. less females emitted pheromone in the absence of host plants, in Y.padellus L. the initiation of pheromone release was delayed.
3. Females of Y.evonymellus, Y.cagnagellus Hüb., Y.padellus and Y.plumbellus D. & S. all preferred a place with host-plant odour to one with non-host odours for sex-pheromone release.
4. Male preference for host-plant odour was demonstrated in the laboratory only for Y.cagnagellus.
5. Trap catches in the field indicated a reinforcement of female attractiveness by host-plant stimuli.     

Geographical variation in temporal and spatial vocalization patterns of male harbour seals in the mating season

In the aquatically mating harbour seal, Phoca vitulina, oestrous females show marked differences in spatial and temporal distribution between geographical areas. This suggests that the males' display behaviour may also vary between areas. We recorded male vocalizations in two areas, the Moray Firth and Orkney, U.K. In the Moray Firth, females haul out on a few intertidal sandbars and travel along predictable routes to forage at sea. In Orkney, female haul out sites are much less influenced by tidal availability and females are much more dispersed. In the Moray Firth, males vocalized only during a short mating season, from 1 July to 12 August. Vocalizations varied significantly with the tide, the peak at high tide clearly coinciding with the period when most females were in the water. In contrast, vocalizations in Orkney were significantly related to both tidal and diel patterns. We suggest that the timing of male vocalizations reflects differences in female availability between sites. In the inner Moray Firth, vocalizations were heard throughout the females' range, whereas vocalizations in Orkney were heard only in two discrete areas. However, at both sites the density of vocalizing males was highest in narrow channels and/or along predictable female travel routes. Therefore, males clearly adapt their temporal and spatial behaviour patterns to variations in female distribution and density. These results suggest that male mating strategies in aquatically mating pinnipeds are more variable than was previously envisaged. Copyright 1999 The Association for the Study of Animal Behaviour.     

Spatial distributions of male and female strawberry poison frogs and their relation to female reproductive resources

In many species with a resource-based mating system, males defend resources to increase their attractiveness to females. In the strawberry poison frog, Dendrobates pumilio, suitable tadpole-rearing sites appear to be a limited resource for females. Territorial males have been suggested to defend tadpole-rearing sites to increase their access to females. In this study we investigate the spatial association between tadpole-rearing sites and the sexes as well as the spatial association of males and females. If strawberry poison frogs have resource defense polygyny, we expect males and females to be associated with tadpole-rearing sites and that females will deposit their offspring in tadpole-rearing sites inside the territories of their mates. To test this hypothesis, home range and core area sizes were calculated for both sexes and the association patterns were compared in two areas that differed in their abundance of tadpole-rearing sites. Home ranges and core areas of females were much larger than male home ranges. Females showed a clumped distribution in the vicinity of tadpole-rearing sites. Males were not clumped and were less associated with tadpole-rearing sites. Females generally did not use tadpole-rearing sites in the territory of their mates and we therefore conclude that males did not defend tadpole-rearing sites for females. Our data are consistent with the general assumption that female distribution is influenced by resource distribution and that male distribution depends on female distribution. Nevertheless, the distribution of D. pumilio females was also influenced by male spacing patterns. Males probably initially establish their core areas where female density is high and then females move among territories to sample males. Males compete vigorously for places with high female density, the defense of which is likely important for enhancing their mating success. In general, the spacing patterns did not differ between populations but the sex ratio was strongly female biased in the habitat with more tadpole-rearing sites, reflecting the direct reliance of females on these resources.