Population Densities Of Spiders Research Articles

Biological control in winter: novel evidence for the importance of generalist predators

Summary The role of generalist predators in pest control has been neglected because generalists are not able to track pest populations. Generalist predators are suggested to be important in spring before specialist predators become active. Here, we show that some generalist predators are important even during winter, when the majority of arthropod pests and their enemies are dormant. We quantified the role of winter‐active generalist predators on the suppression of pear psylla during winter using a discrete nonlinear model of an intraguild predation system. To parameterize our model, we conducted a series of experiments on (i) functional responses, (ii) prey preferences and (iii) ontogenetic development and made observations on the population densities of spiders and potential prey. We ran the model for different winter scenarios, that is, for very cold and very warm winters. Synthesis and applications. We found that winter‐active predators considerably suppressed the pear psylla population. Predators exerted a stronger effect in a warmer winter than in a colder one. Orchard growers thus should avoid use of non‐selective pesticides during this period and instead aim to support the community of generalist predators. Our results suggest more attention should be given to encouraging generalist predator populations in other systems, even in periods when crops are not producing.

SPIDER FAUNA AND INFLUENCE OF TRAPPING METHOD AND FIELD MARGIN ON SPIDER POPULATION DENSITY IN SUGAR BEET FIELDS

Experiments were carried out at the experimental farm of Sakha Agricultural Research Station in 2006/07 and 2007/08 sugar beet seasons. The spiders inhabiting the sugar beet fields were surveyed, which indicated to the occurrence of 30 spider species belonging to Araneidae, Dictynidae, Dysderidae, Gnaphosidae, Linyphiidae, Lycosidae, Miturgidae, Philodromidae, Salticidae, Tetragnathidae, Theridiidae and Thomisidae. In all sugar beet plantations (August, September and October), the pitfall traps captured more spiders of families Lycosidae and Gnaphosidae (Ground-dwelling spiders) and Philodromidae and Salticidae (wandering spiders) than did D-vac machine which captured more linyphiids and araneids (orb-weaver spiders). Population density of spiders in the field margins were usually greater than that inside the field for the seven considered families. Traps used inside the field captured less spiders than those used in the margins by 32.50-82.14 % overall spider families.

Effect of weather factors on spiders (Araneida) in an Ontario meadow

In a 5-year investigation, population densities of spiders and associated arthropods were measured in a meadow by 'quick trap' and vacuum sampler. Analysis using empirical regression equations indicated that seasonal changes in spider numbers were largely explainable by weather factors, particularly cumulative temperature. Rainfall was also important, even on the moister, well vegetated part of the meadow. Much of the residual variability could be accounted for by the inclusion in the equations of other categories of arthropods such as Cicadellidae, Collembola, and Staphylinidae.

Change in population density of spiders in paddy field during winter

Change in population density of spiders in paddy field during winter

Seasonal fluctuation of spider population in paddy fields

For the purpose of understanding of spider's role in paddy fields, this study was carried out from 1966 to 1968 at Utsunomiya, Tochigi Prefecture, Japan. Popu-lation density of spiders was examined in two paddy fields of Mine and Ishii Town. Seasonal fluctuations of all spiders in two paddy fields are shown in Fig. 2 and 3. In every cases, just after the rice-planting season, numbers of spiders found in these paddy fields, were few. Then they gradually increased up to autumn. Maximum number of spiders was about 90/m2 at the Mine field (Fig. 3). By the insecticidal applications the spider population was scarcely influenced in early stage, however the number of spiders decreased in late stage of rice plants. Seasonal fluctuations of each main species are shown in Fig. 4-12. In the late stage of rice plants, the population density of main species in Ishii field where insecticides were sprayed by helicopter was low compared with that in Mine field where those were not done, but only the population density of Pirata subpiraticus kept exceptionaly high in the Ishii field as well as that in the Mine field. Many species reached maximum density in October, that is, harvesting time of rice plants. On the other hand, leaf hoppers and plant hoppers reached the maximum density in September. Therefore the spiders in late season, do not serve enough as the natural enemy of these insect pests. But the maximum den-sity of Pirata subpiraticus emerged in August, in cases in which the density around June was high to some extent. From these results, Pirata subpiraticus seems to be the most useful spider as predator to insect pests in paddy fields.