INSECTIVOROUS ACTIVITIES OF SPIDERS IN UNITED-STATES FIELD CROPS

A survey of average spider numbers in U.S. field crops (considering a geographic range from the east to the west coast) gave an overall mean density of almost-equal-to 1 plant-dwelling spider/m2 (+/- 0.18 SEM). This value is more than 100 times lower than TURNBULL's famous 'overa ll mean value' (= 130.8/m2) computed from 37 published censuses of spi der numbers in a wide variety of environments from all over the world. Crop fields are disturbed systems whose spider numbers are drasticall y reduced by agricultural practices such as pesticide use, cultivation , harvest, etc. Small sized spider individuals (including large percen tages of immatures) numerically dominate the faunas of U.S. field crop s, and these feed primarily on tiny prey organisms (< 4 mm in length). Small web-building spiders are almost strictly insectivore (insects c onstituting > 99% of total prey). In contrst, the 'hunters' (nonweb-bu ilding spiders) that actively search the plant surface for prey, exhib it a mixed strategy of araneophagic and insectivorous foraging pattern s (insects constituting less-than-or-equal-to 90% of total prey). The aggressive 'active searchers' are highly polyphagous (compared to the small web-weavers), but can narrow their feeding niche significantly w hen a suitable prey species reaches high numbers relative to other pre y groups. The numerically dominant spider predators in U.S. field crop s feed heavily on small plant-sucking insects such as plant bugs (Miri dae), fleahoppers (Miridae), leafhoppers (Cicadellidae), treehoppers ( Membracidae), planthoppers (Delphacidae), and aphids (Aphididae), whic h is of interest from a biocontrol point of view. A typical agroecosys tem spider (i.e., striped lynx spider) may capture an average of almos t-equal-to 1 prey per rainfree day in the field (by multiplying this v alue with the overall mean spider density of 1/m2, it follows that the average prey kill may be in the order of magnitude of almost-equal-to 1 prey/m2/day). At prey densities of 100-300/m2 (as recorded in liter ature), spiders kill perhaps almost-equal-to 0.3-1% of the potential p rey per day. Extensive field and laboratory observations, experiments, and computer models conducted by research groups in different parts o f the U.S. indicate that the collective predation impact of spiders ma y contribute significantly to lower pest levels in some crop fields.