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Composition, abundance and pest control potential of spider communities in agroecosystems: a comparison of European and US studies

(1) Enzyme-linked immunosorbent assay (ELISA) was used to detect aphid consumption by predators collected from three fields of winter wheat. (2) A total of 7781 predators belonging to 105 species was tested and 81 species were found to have eaten aphids at some time during the study. (3) An unexpectedly high percentage of many species had eaten aphids when aphid density was low, early in the season. (4) The relative efficiencies of ELISA and gut dissection for detecting prey remains varied according to the species of predator tested, and secondary predation was a potential problem, especially in the former technique. (5) In general, staphylinid beetles and beetle larvae digested their aphid meal much more rapidly than carabid beetles and spiders. (6) Field data on percentage of predators positive in ELISA and on predator density were combined with laboratory data on digestion rates to give predation indices which enabled comparison of species in terms of some aspects of their probable value as aphid predators. Spiders usually had the highest predation indices, but mites and adult and larval beetles were also of significance as aphid predators. It is concluded that aphid control by polyphagous predators is likely to be widely based.

A study of feeding by polyphagous predators on cereal aphids using ELISA and gut dissection.

Summary 1. Spiders form a major component of the generalist predator fauna, potentially able to restrict pest population growth, but their populations may be food-limited under current farming regimes. This study aimed to quantify food availability to spiders in winter wheat and to determine whether spider web locations are positively associated with available food resources. 2. Mini-sticky traps (availability rate per 24 h, including prey falling from the crop) and mini-quadrats (instantaneous density on the ground by day) were used, in combination, to monitor the availability of potential prey to web-building species of money spider (Linyphiidae) in fields of winter wheat in Warwickshire, UK, 1997‐98. 3. These methods were applied to web sites of individual spiders and to non-web sites located randomly up to 30 cm away from each web. A total of 18 546 invertebrates were captured using these methods. 4. Overall, significantly more potential prey were available in web sites than in non-web sites (both on sticky traps and in quadrats). 5. Prey availability in May and July was about a third of that in June (both on sticky traps and in quadrats) and may have been below that known to be necessary for spiders to realize their maximum population growth rate. 6. The peak rate of capture of linyphiid spiders on mini-sticky traps was 0·6 trap ‐1 day ‐1 at web sites, and approximately half this value at non-web sites. Numbers of spiders captured by mini-sticky traps and mini-quadrats increased exponentially as the season progressed. The high capture frequency in relation to population density, and the differential between web and non-web sites, points to a dynamic and aggregated distribution of spiders in winter wheat, which is consistent with what is known about mate-searching and web site abandonment rates by the Linyphiidae. 7. The combination of techniques described here is recommended for monitoring prey availability in prey-enhancement programmes and may prove useful in quantitative studies of both intra- and interspecific interactions between spiders.

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Living where the food is: web location by linyphiid spiders in relation to prey availability in winter wheat

 1 Polyphagous predator densities were manipulated in a wheat crop using polythene exclusion barriers



2 The rates at which aphids fell to the ground and subsequently climbed and returned to the crop canopy were measured



3 Peak aphid density was highest when polyphagous predator density was lowest



4 Similar rates of aphid fall-off were measured in each plot, but aphid climbing rate was highest when predator densities were lowest



5 It was concluded that ground-zone predators reduced the rate at which aphids return to the crop canopy

Predation of the cereal aphid Sitobion avenae by polyphagous predators on the ground

The responses ofPterostichus melanarius Illiger,Harpalus rufipes DeGeer, andNebria brevicollis Fabricius (Coleoptera: Carabidae) to olfactory cues of prey and habitat were studied in a four-arm continuous-airflow olfactometer. The process was semiautomated using time-lapse image analysis by microcomputer. The primary constituent of the aphid alarm pheromone, (E)-β-farnesene (EBF), was synthesized and tested for a kairomonal role in prey detection by carabids. In addition, individual beetles were exposed to odors from live aphids, live collembolans, and a crude extract of wheat. All three beetle species showed evidence of olfaction.P. melanarius responded to all the odors except collembolans,H. rufipes responded to EBF and wheat, andN. brevicollis to collembolans. The use of a defence allomone as a prey-finding kairomone by certain carabid species has implications for pest management. Manipulation of predator chemical ecology by the inclusion of behavior-modifying compounds in a crop spray mix with reduced amounts of insecticide may allow for efficient aphid control with less environmental contamination.

Behavioral responses of three species of ground beetle (Coleoptera: Carabidae) to olfactory cues associated with prey and habitat

SUMMARY

The numbers of cereal aphids on the soil surface was assessed by soil samples and by counting the numbers captured in pitfall traps. A relationship was found by both methods between the density of aphids on the plants and the numbers of aphids on the soil. This relationship indicated that a higher proportion of the aphid population was on the soil surface at the lower densities on the plant. Reasons for this relationship are suggested and discussed.

K. D. Sunderland

Papers

Observations on the number of cereal aphids on the soil in relation to aphid density in winter wheat