Harmful insects are represented by the few plant-eaters (such as mites, aphids, and tent caterpillars) that congregate and feed, or occur in large numbers (thrips). Most plant-eating insects and mites occur at low densities and cause minimal damage, so they can often be tolerated in the garden. For example, the caterpillars of many moth and butterfly species usually occur at low densities, and their feeding damage is usually negligible (except for that of mature larvae of some large moths, like hawk moths). These low-impact herbivores not only improve landscape diversity, but serve a valuable role in the garden as sustenance for carnivorous insects that will be needed when infestations of high-impact plant feeders occur.
So who are the “good guys” that provide free pest control for the home garden? A veritable squadron of “natural enemies” is available in Pacific Northwest garden landscapes, or present in nearby refuges (riparian areas in parks, along creeks, and the like). This native complex of natural enemies of pests, insects, and mites, occupies all natural and undisturbed habitats. The trick is to get this helpful fauna to visit your back yard (and stay), and set up defense lines against the troublesome herbivores. Strategies to encourage beneficial insects, spiders, and mites to visit and stay in your garden landscape, are discussed at the end of this manual.
First, it is important to be able to identify insect, spider, and mite allies, and to recognize who your insect friends are, in order to gauge how the “war” against pests is going in your garden. Beneficial arthropods (insects, mites, spiders, centipedes, and harvestmen) that help maintain a garden with few or no outbreaks of damaging plant pests, are either predators or parasitoids. Predatory insects and spiders hunt, attack, kill, and consume insect and mite prey, usually smaller than themselves. They are the equivalent of big cats in the jungle, and range in size from microscopic predatory mites to praying mantids that are 3 to 4 inches long. Parasitoids are usually very small parasitic insects that develop inside the bodies of pest insects, eventually killing them. Table 1 summarizes the groups of predatory and parasitic arthropods
There are about 25 families of insects that contain predatory species, and virtually all of the more than 100 families of spiders are predatory. Other arachnids like harvestmen (daddy-long-legs) and mites also contain many families which are predatory. The family groups of predators most likely to be found in Pacific Northwest gardens are summarized below.
Praying Mantids are among the largest (1 to 4 inches long) and most recognizable garden predators—and they’re not fussy about what they catch and eat. They are “sit and wait” predators that pounce on any insect that comes too close, including beneficial insects, like bees and butterflies (Figures 1 and 2). The most common species in the Pacific Northwest is the European mantid (Mantis religiosa). Praying mantids are most often seen in the garden from mid-summer to mid-autumn. After laying a number of white, hard-foam egg cases (which overwinter attached to branches and trunks), mantids are typically killed off by the first frosts of autumn. They kill and consume a good number of pests like caterpillars and flies, but their contribution to garden pest control is usually less than their larger-than-life image.
Although “bug” is often used to describe just about any insect, its correct use is reserved for the “true bugs,” an enormous group of both herbivorous and carnivorous insects that are characterized by having a syringe-like beak. Stink bugs, damsel bugs, big-eyed bugs, assassin bugs, ambush bugs, plant bugs, and minute pirate bugs may all be found in Pacific Northwest gardens feeding on plant pests like leafhoppers, scale insects, thrips, aphids, psyllids, whiteflies, mites, and small caterpillars. Predatory true bugs are all generalist feeders and may eat some beneficial insects, but their positive impact on garden pests far outweighs this negative aspect.