A California grower reported that migrating wasps had effectively parasitized the aphids in his spinach fields, and he relied on the wasps rather than a final chemical spray to control aphids. These were, of course, the tiny predatory wasps escaping. Plants attracting beneficials may also attract insect pests. In making tradeoffs between damage to pests and damage to beneficials, encouraging beneficials should be a means to control pests, not an end in itself. Experiments in Michigan, for example, found increased numbers of eggs and larvae of the imported cabbage worm and the parasite Cotesia rebecula on broccoli interplanted with nectar-producing anise hyssop than when the broccoli was planted alone or beside the anise hyssop.
Spiders prefer areas with high humidity and stable temperatures.
Experiments at the University of Tennessee found a significant reduction in populations of aphids, flea beetles, blister beetles and Colorado potato beetles in plots with high spider populations. In these experiments, plots with grass mulch hay had up to ten times more spiders than bare-ground plots.