Ferenc Samu

Effects of agricultural diversification on the abundance, distribution, and pest control potential of spiders: a review.

A review of the literature showed that spider abundance was increased by diversification in 63% of studies. A comparison of diversification modes showed that spider abundance in the crop was increased in 33% of studies by ‘aggregated diversification’ (e.g. intercropping and non‐crop strips) and in 80% of studies by ‘interspersed diversification’ (e.g., undersowing, partial weediness, mulching and reduced tillage). It is suggested that spiders tend to remain in diversified patches and that extending the diversification throughout the whole crop (as in interspersed diversification) offers the best prospects for improving pest control. There is little evidence that spiders walk in significant numbers into fields from uncultivated field edges, but diversification at the landscape level serves to foster large multi‐species regional populations of spiders which are valuable as a source of aerial immigrants into newly planted crops. There are very few manipulative field studies where the impact of spiders on pests has been measured in diversified crops compared with undiversified controls. It is encouraging, however, that in those few studies an increased spider density resulted in improved pest control. Future work needs are identified.

ON THE NATURE OF AGROBIONT SPIDERS

Abstract Results from a 10 year survey of spiders in Hungarian arable and natural grassland habitats are cumulated in order to reveal the key characteristics of agrobiont species. We define agrobionts as species that reach high dominance in agroecosystems. The most dominant species, Pardosa agrestis, on average accounted for 40% of the total spider population in Hungarian arable fields. The presence of agrobionts led to a strong skew in arable spider community species distriibution. Regardless of the over-dominance by agrobionts, arable spider communities had a potential for very high species richness. The agrobiont segment of arable spider communities showed very little field-to-field or regional variation, i.e. the same agrobiont species occurred in all fields. Agrobionts were indicators of arable habitats, and were rare in other habitat types, but in many species preferences for specific natural habitat types could be shown. These natural habitat types were often strongly abiotically driven, frequently disturbed habitats. The life cycle of agrobionts showed synchronization with the arable crop-growing season. While many closely related non-agrobiont species had maturity and reproductive periods either earlier or later than the main crop vegetation period, agrobionts invariably reached adulthood and reproduced during that period. Association with frequently disturbed natural habitats and phenological synchronization with the annual arable disturbance regime are such traits that support the theory that agrobiont species are adapted to predictably ephemeral habitats.

Factors influencing cannibalism in the wolf spider Pardosa agrestis (Araneae, Lycosidae)

Abstract Cannibalistic tendencies are well known in spiders and may be a significant factor influencing population size. The wolf spider, Pardosa agrestis, is the dominant non-web-building spider in a wide range of central European agricultural habitats. Preliminary field observations indicated an extended reproductive period, which results in a very wide size distribution of juvenile instars. We hypothesised that if cannibalism is enhanced by differences in size, especially during periods when prey is scarce, these populations might be susceptible to cannibalism in an ecologically significant way. Laboratory studies were conducted on juvenile P. agrestis in arenas. We analysed the following specific aspects of cannibalism: (1) the effect of the weight ratio between the opponents; (2) the effect of weight per se, and (3) the role of hunger level in determining cannibalistic tendencies of spiders. The role of weight and hunger were analysed in separate experiments, in both cases by controlling for the other variable. The results showed that cannibalism was strongly positively correlated with both weight ratio and hunger, but absolute size/age of an individual could not predict the occurrence of a cannibalistic event. These experiments generated the plausible hypothesis that cannibalism might be an important phenomenon in the regulation of real populations, which should be tested specifically in future field experiments.

A spider population in flux: selection and abandonment of artificial web-sites and the importance of intraspecific interactions in Lephthyphantes tenuis (Araneae: Linyphiidae) in wheat

Lepthyphantes tenuis, a small sheet-webbuilding linyphiid spider is one of the most abundant spider species of cereal fields in Europe. In the present study we examined the process of web-site selection and web-site tenacity by adult females of this species in a winter wheat field. Spiders were selective in their choice of web-site. Different immigration rates into various manipulated web-sites, in field and laboratory, suggested that structural support and suitable micro-climate (high humidity) are the most important factors in the selection. Small holes dug in the ground were the most favoured web-sites. Web-site occupation was influenced by the presence of other conspecific spiders. Territorial contests occurred between spiders attempting to occupy the same web, these almost invariably led to the take-over of the web when the intruder was heavier. Interference, but also a certain level of tolerance, between spiders within the same web-site but in different webs was suggested by direct and indirect evidence. Many holes supported two or even three spiders in vertically stratified webs. Leaving probability of marked spiders was significantly higher in multiply occupied holes than in holes with a single web. Comparison with the results of a no-interference stochastic model showed that multiple occupancy in nature is less frequent than predicted by the model. There was further evidence for weak extra-web-interference between spiders in that multiple occupancy was even less frequent and overall occupancy was lower in web-sites which were packed close to each other. However, a level of tolerance for crowding is shown by the fact that closely packed hole colonies supported a spider density 13 times higher than in natural web-sites in the field. A marking experiment was carried out to gain information on web-site tenacity (i.e. the length of time a spider spends in a web-site) and abandonment. The average duration of tenacity was less than 2 days. A random loss function gave a good fit to the tenacity distribution and suggested that spiders abandoned web-sites randomly with a fixed leaving probability of c. 0.5. Individual webs were often used consecutively by more than one spider, and some spiders built more than one web in the same web-site. Calculations showed that abandonment is the most frequent leaving mode, while take-over by contest between spiders and disappearance due to destruction were some-what less frequent and equally likely modes of ending tenacity. It is suggested that the apparent contradiction between the selectiveness and competitiveness of spiders for web-sites and the relatively short tenacity observed can be resolved by hypothesising that spiders leave websites soon because they apply the strategy of spreading risk: spiders by frequently moving from one web-site to another distribute their reproductive efforts across several localities. This hypothesis is further supported by changes in web-site preference and ballooning behaviour at the onset of the reproductive stage in L. tenuis.

SIZE DEPENDENT INTRAGUILD PREDATION AND CANNIBALISM IN COEXISTING WOLF SPIDERS (ARANEAE, LYCOSIDAE)

Abstract Two species of wolf spider, Hogna helluo (Walckenaer 1837) and Pardosa milvina Hentz 1844 dominate the predatory community on the soil surface of agroecosystems in eastern North America. Although as adults they are very different in size, differences in phenology ensure that they overlap in size at various times during the year. In a laboratory experiment, we explored the propensity of each species to attack and kill the other wolf spider species (intraguild predation), conspecifics (cannibalism) or crickets (ordinary predation). Both spiders attacked and killed a broader size range of crickets more quickly than they approached other spiders. We found no differences in Hogna foraging on conspecifics or Pardosa, but Pardosa attacked and killed Hogna more readily than conspecifics. Because Hogna was so slow in attacking other spiders, their impact as an intraguild predator may be quite small, especially if their approach to crickets is an indication of their predatory tendencies with insects. On the other hand, Pardosa attacked and killed small Hogna as readily as crickets, which suggests they may have an influence on Hogna populations if Hogna young emerge coincident with large juvenile or adult Pardosa.

Wolf spider feeding strategies : optimality of prey consumption in Pardosa hortensis

Feeding behaviour of the wolf spider Pardosa hortensis Thorell (Araneae, Lycosidae) was studied in the laboratory. Characteristics of feeding were measured while prey availability was increased and the results were compared with the predictions of three models: the marginal value theorem (MVT), gut limitation theory (GLT) and the digestion rate limitation model (DRL). As a result of more frequent encounters with prey, the wolf spiders were able to modify their feeding behaviour so that their net energy intake rate increased substantially. Handling time decreased by 30%, and consumption rate increased by 40%. Partial consumption of prey did not occur until the spiders became nearly satiated. This indicated that spiders did not reach the optimum predicted by MVT. The most plausible mechanism for the increased efficiency was prey-stimulated digestive enzyme production as suggested in DRL. The predictions of GLT were not applicable for most of the feeding session, though gut satiation had an influence on the final stages of feeding. P. hortensis seemed to apply a “responsive but cautious” strategy: (i) spiders improved feeding efficiency on entering the higher quality habitat, but (ii) feeding times appeared to be sub-optimal and (iii) spiders were also willing to continue feeding when, as they approached satiation, the previously high efficiency could not be maintained. Such feeding behaviour optimizes long-term energy intake when food is scarce and unpredictable, which corresponds well with the known degree of natural food limitation of these animals.

Changes in arthropod population composition in IPM apple orchards under continental climatic conditions in Hungary

The effects of broad spectrum and selective insecticides on the structure of phytophagous and zoophagous communities were investigated in a controlled experiment carried out in an apple orchard in Hungary. Following the cessation of intensive use of broad spectrum insecticides and the regular application of selective insecticides (mainly insect growth regulator compounds) a characteristic succession commenced in the arthropod populations in apple orchards. The IPM strategy was developed against the arthropods occurring at a high population density in the apple orchards previously treated with broad-spectrum insecticides for many years (e.g Phyllonorycter blancardella, Leucoptera malifoliella, Panonychus ulmi). After the regular application of selective insecticides the population density of the pests Stephanitis pyri, Dyasphis devecta, Quadraspidiotus perniciosus increased, thus further improvement in the IPM program is still required.

Spider orb web as bioassay for pesticide side effects

Pesticides may affect the predatory and reproductive behaviour of beneficial arthropods short of having direct effects on their survival. We present a bioassay designed to test (ultimately in the field) such hidden effects of agrochemicals in their application concentrations. As a paradigm we chose the web‐building behaviour of the cross spider Araneus diadematus Clerck (Araneidea, Argiopidae) and we selected four commonly used pesticides: Oleo Rustica 11E (mild insecticide), Fastac (pyrethroid insecticide), Bayfidan and Sportak (fungicides). Neither fungicides nor the mild insecticide seem to affect web‐building behaviour significantly, whereas the pyrethroid insecticide suppressed web‐building frequency and severely affected web size and building accuracy.

COMPARISON OF AUTUMN AND WINTER DEVELOPMENT OF TWO WOLF SPIDER SPECIES (PARDOSA, LYCOSIDAE, ARANEAE) HAVING DIFFERENT LIFE HISTORY PATTERNS

Abstract Pardosa species do not overwinter in the adult stage in the Holarctic region, therefore penultimate instars should avoid precocious maturation in autumn. We tested how artificially increased temperature and/or lengthened light regime would affect the pre-overwintering development of two common species with different phenological patterns. Juvenile instars of Pardosa agrestis (Westring 1861) and P. hortensis (Thorell 1872) were collected in autumn from the field. The experimental spiders were held either indoor at 26 °C or outdoors at ambient temperature and were exposed either to short or to long daylength. Molting events were monitored for five months. At outdoor temperatures no spiders reached adulthood and molts of younger instars occurred more frequently at long daylength. In the indoor temperature groups all P. hortensis and the majority of P. agrestis individuals reached adulthood during the experiment. Long daylength treatment enhanced the effect of increased temperature by almost halving the time needed to reach adulthood in both species. Penultimate instars of both species needed at least 17 days to molt, while earlier instars, present only in P. agrestis, responded rapidly to higher temperature by molting. This stage dependent response suggests that earlier instars can use favorable autumnal temperatures to catch up with penultimate instars which leads to higher synchrony of developmental stages in the overwintering and spring populations.

Density of spiders (Araneae: Ctenidae) in Ivory Coast rainforests

To monitor the biodiversity quality of rainforest fragments in the eastern region of the Ivory Coast, the density of at least one important invertebrate group needs to be determined. This has been possible for nocturnal hunting spiders from the family Ctenidae using a novel combination of sampling and census techniques. The reflective characteristics of the eyes of these spiders meant that individuals were readily located using headlamps. Collections were made along transects (total length 5.5 km), adopting a distance sampling protocol. Density was estimated using distance sampling models. More than 2600 ctenids (15 species) were captured and identified. Seven of these species were present in sufficient numbers for density assessment. The combined density of these seven most common species in degraded forest fragments (0.124 m −2 ) was only half that in less disturbed forest (0.240 m −2 ). Ctenidae are a promising candidate for future monitoring of invertebrate groups in the rainforest.