Spiders move into new habitats by ballooning on air currents, but if conditions are unfavorable,
the spiders will leave. Most spider migration into fields occurs early in the growing season, but
newly cultivated fields generally do not provide optimum conditions for spiders. Spiders do best
under conditions of high humidity and moderate temperatures. Mulch creates a more favorable
environment for the spiders by providing higher humidity and protection from temperature
extremes. Significantly higher spider densities were observed in test plots of mixed vegetables
(spinach, radish, cabbage, brussels sprouts, potatoes,
beans, corn, and tomatoes) that were
mulched with 4 inches of grass hay between rows and around the plants than in unmulched plots.
This type of habitat manipulation may also encourage ground beetles and other generalist
arthropod predators that contribute to pest reduction. Flowering plants to attract an additional
source of food for the spiders had no effect on spider numbers.
Although spiders are generalist predators, feeding on both pest and beneficial species, they can
have a significant effect on pest insect populations. Insect damage to the radish, broccoli, potato
and corn plants was significantly lower in the mulched plots, and there were fewer pest insects in
these plots than in the control plots. A separate experiment showed the ability of spiders to
control plant pests. Spiders were placed in cages over insect-infected broccoli plants. No insects
remained at the end of the test, and plant damage estimates averaged only 32% compared to the
control cages without spiders that averaged 93% damage.
Source: Riechert, S. E. and Bishop, L. 1990. Prey control by an assemblage of generalist predators: spiders in
garden test systems. Ecology 71(4): 1441-1450.
