Prey selection by a spider wasp,Batozonellus lacerticida (Hymenoptera: Pompilidae): Effects of seasonal variation in prey species,size and density

Prey selection by a spider wasp,Batozonellus lacerticida, was investigated at the riverside of the Toyohira River in Misumai, Sapporo, Hokkaido over 7 years from 1981 to 1987. Seventeen species of araneid spiders were found in the study area and six of them were hunted by the wasps. Most (97.3%) of the 223 prey records obtained were of three species,Araneus macacus, A. marmoreus andA. pinguis. Nesting activity of the wasps continued until mid-August in 1981, 1982 and 1986, but stopped in July in 1984, 1985 and 1987. The length of nesting period was related to whether the wasps successfully switched prey species fromA. macacus toA. marmoreus andA. pinguis andA. pinguis or not Switching seemed to occur at the phase when density ofA. macacus remained high, depending on the density of largerA. marmorus andA. pinguis. This switching had large effects on species composition and size distribution of actual prey. Chesson's index α calculated at every 10 day period revealed that the wasps preferredA. macacus to the two other species and large prey to medium and small ones. The effect of prey density on preference depended on whetherA. macacus was present or absent. In the presence of it there were only slight differences in preference among various conditions of prey density. However, whenA. macacus was absent, densities of bothA. marmoreus andA. pinguis had some complementary effects on preference. The importance of prey size selection by pompilid wasps, and implications of density effects on preference are discussed.     

Ants and plant size shape the structure of the arthropod community of Hirtella myrmecophila, an Amazonian ant-plant

Abstract.  1. To investigate the influence of plant size and the presence of resident ants on the arthropod community associated with the ant-plant Hirtella myrmecophila , ants ( Allomerus octoarticulatus ) were experimentally removed from plants of various sizes, with a second group of similar-sized plants remaining as controls. The abundance and diversity of arthropods on these plants were determined at 45-day intervals for 7 months. Collected arthropods were assigned into one of three predetermined groups: herbivores, spiders (mostly web-building spiders), or 'tourists' (mostly non-resident ants, hemipterans, dipterans, or scarabeid beetles).
2. Ant removal had a positive influence on the abundance and diversity of herbivores, but no measurable effects on the abundance and diversity of 'tourists', while the effect on spider abundance was significant only when data from Dipoena sp. nov. were removed from the analysis. While A. octoarticulatus had a negative effect on most spiders, it favoured Dipoena sp. nov., which was found to be a specialist predator of A. octoarticulatus .
3. Plant size positively affected the abundance and diversity of 'tourists' in plants with and without ants, but for herbivores and spiders this effect was dependent on ant treatment. In ant-maintained plants the per-plant abundance of herbivores and spiders was independent of plant size, whereas in ant-removed plants it was not.
4. These results suggest that A. octoarticulatus affects all arthropods found on its host plant, except those presenting an occasional and temporary association with the plant, and that the magnitude of ant effects on the susceptible guilds increases as plant size increases.     

Spider abundance and diversity in apple orchards under three insect pest management programmes in Washington State,U.S.A.

1 Many apple growers in Washington State, U.S.A. use mating disruption (MD) for control of codling moth, Cydia pomonella (Linnaeus). Fewer applications of synthetic, broad‐spectrum insecticides are made in MD orchards than in orchards under conventional (C) management. Spider abundance and diversity in MD, C and certified organic (O) orchards were compared. Spiders inhabiting the trees (arboreal), the understory vegetation, and the ground surface were studied. 2 Total arboreal spider density and total understory spider density were significantly higher in O orchards than in MD and C orchards. Many species occurred in both the trees and the understory. 3 Arboreal, visually orientated, hunting spiders and arboreal ambushers/runners were significantly more abundant in O orchards compared to C and MD orchards. Visual hunters were significantly more abundant in MD compared to C orchards. Numbers of spiders in two other guilds (web‐makers and nocturnal hunters) showed no statistical differences with respect to orchard management type. 4 The highest density of ground surface‐dwelling spiders occurred in one of the O orchards. Two C orchards had higher densities than any MD orchard. Ground surface species were distinct from those in the understory and the trees. 5 With one exception, an orchard's arboreal fauna was most similar to that of another orchard under the same type of pest management. Three exceptions were noted among comparisons of the understory faunas. The ground surface‐dwelling fauna of one O orchard was distinctive, whereas that in the second O orchard was similar to the C and MD orchards. 6 Reduced use of synthetic, broad‐spectrum insecticides in MD orchards did not result in arboreal spider densities comparable to those found in O orchards. A contributing factor may be that all species were univoltine. Spider populations may thus be severely reduced by even a small number of synthetic, broad‐spectrum insecticide applications and the time required for recovery may be lengthy.     

The evolution of cryptic spider silk: a behavioral test

Phylogenetic patterns of change in spider silk coloration provideinsight into the selective pressures directing evolution ofsilks. Trends toward evolution of silks with low reflectanceof ultraviolet (UV) light suggest that reduced UV reflectancemay be an adaptation to reduce visibility of webs to insectprey. However, a test of the visibility of primitive and derivedspider silks is lacking. Several genera of orb-weaving spidersinclude conspicuous designs of silk, called "stabilimenta,"at the center of their webs. Due to their large size, stabilimentapresent signals that insects can use to avoid webs. Unlikeother silks in the orb web, which reflect little UV light,evolutionarily derived stabilimentum silk retains a bright UV reflectance. But, unlike primitive silks, stabilimentum silkalso reflects large amounts of blue and green light. We comparedthe visibility of primitive tarantula silks and derived stabilimentumsilks to insects by using the ability of honey bees to learnto forage at targets of spider silk. We found that the uniquespectral properties of stabilimentum silk render it crypticto insects and that primitive silks are more visible to bees.Our findings support a hypothesis that the coloration of stabilimentumsilk is an adaptation to reduce the ability of insects to avoidwebs and that ancient biases in the color vision of insectshave acted upon the evolution of spider silk coloration throughsensory drive. But our findings question the emphasis on UVreflectance alone for visibility of spider silks to insects.     

A test of the hypothesis that a pathway of intraguild predation limits densities of a wolf spider

Abstract.  1. The high number of potential predatory interactions among the many arthropod generalist predators in terrestrial food webs makes exhaustive testing of interaction strengths by field experiments unfeasible. Thus, correlative patterns and laboratory observations of behaviour often form the basis of inferences about the strength of interaction pathways involving generalist predators (intraguild predation).
2. Previous research has revealed a negative correlation between survival of juvenile wolf spiders of the genus Schizocosa (Lycosidae) and densities of another abundant spider family, the Gnaphosidae.
3. Feeding trials in laboratory microcosms with a leaf-litter substrate revealed that gnaphosids prey on juvenile Schizocosa in a structurally complex habitat.
4. Gnaphosid densities were manipulated in two different field experiments, each conducted in a different year, in order to test directly the hypothesis that intraguild predation by gnaphosids limits densities of juvenile Schizocosa .
5. Reducing numbers of gnaphosids, and doubling their numbers to two times the mean natural density, had no impact on the survival of juvenile Schizocosa in either field experiment. This finding suggests that correlative patterns in nature and feeding trials in the laboratory may at times provide deceptively simple and potentially misleading generalisations about the strengths of interaction pathways in complex networks of generalist predators.     

Payoffs and strategies in territorial contests: ESS analyses of two ecotypes of the spiderAgelenopsis aperta

Summary Game-theoretic analyses were completed on the territorial contest behavior of two populations of a desert spider that exhibit markedly different levels of within-species competition. Numerical payoff matrices were constructed from field data collected on the behavior and demography of each population. Payoffs were expressed in terms of expected future egg production. Three behavior patterns that a spider might exhibit following assessment of its weight relative to that of its opponent and the value of the site were considered: withdraw, display, or escalate. The model predicts for the more harsh grassland habitat an evolutionarily stable strategy (ESS) that makes ownership decisive in settling contests between opponents with small weight differences, whereas it otherwise assigns victory to the heavier opponent. Whereas the empirical data collected for this grassland population closely approximates the predicted ESS, that for a population occupying a more favorable riparian habitat deviates significantly. The ESS prediction for this latter population is that an intruding spider will withdraw from a contest if it is similar in weight to the web-owner. Withdrawal is common in this population, but so are display and threat and these actions were not predicted. We hypothesize that gene flow from surrounding habitats is preventing the riparian population from completely adapting to its local environment.     

Spatial and temporal variation in a suite of life-history traits in two species of wolf spider

Abstract.  1. Life-history traits and density were assayed in seven populations of two sympatric species of wolf spider for three consecutive years. The goal of the study was to quantify population dynamics and its relation to spatial and temporal life-history variation.
2. Adult female body size and fecundity varied significantly, among field sites and among years, in both species. Female spiders of both species differed in mean relative reproductive effort among sites, but not among years. The size of offspring was invariable, with no significant differences due to site or year.
3. All populations of both species tended to either decrease or increase in density during a given year and this was tightly correlated with changes in prey consumption rates.
4. Since life-history patterns are determined primarily by selection, it is concluded that size at sexual maturity for females is phenotypically plastic and responds to changes in prey availability. Offspring size however is not plastic and it is likely that other selection forces have determined offspring size. Temporal fluctuations in population size are correlated over a large area relative to dispersal capabilities for these species and conservation efforts for invertebrates must take this into consideration.     

Pennycress double‐cropping does not negatively impact spider diversity

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Relating shading levels and distance from natural vegetation with hemipteran pests and predators occurrence on coffee

The study was carried out to determine the effect of shading levels and/or distance from the natural alpine vegetation (NAV) on the occurrence of two insect functional groups: hemipteran herbivores and generalist predators in farmers' coffee fields in the Mt. Elgon region of Uganda. Three distance categories from the NAV, that is (i) 0–250 m; (ii) 250–1000 m and (iii) 1000–1500 m, were used to demarcate farms in the first stage of selection, and within each distance category, three levels of shading, that is (i) no shade, (ii) moderate shade and (iii) full shade, were used for final selection of coffee farms for the study. A total of 90 individually owned coffee fields were studied; 30 for each distance category, of which 10 represented each shading level. In two separate rounds, inventories of scale insects (Coccus spp.), antestia bugs (Antestiopsis spp.), root mealybugs (Planococcus spp.) and aphids (Toxoptera aurantii) on coffee plants were made for the hemipteran herbivores, whereas ants (Formicidae) and spiders (Araneae) were recorded for the predatory taxa. The results showed that the interaction between distance from the NAV and shading level consistently influenced the occurrence of the insects in both functional groups. For scale insects, root mealybugs and ants, it was closest to the NAV that shading‐level effects were most discernible and generally limiting. To the contrary, the occurrence of aphids and spiders increased with the increase in the level of shading for plants furthest from the NAV. These results indicate that if inclusion of shade trees is to be a strategy in ecological pest management, the level of shading should be determined basing on the insect taxa as well as other pertinent factors in the landscape.