The broad diversity of local conditions that permit California growers to produce the broadest range of agricultural products anywhere in the United States also yield the nation's richest diversity of insects. While no one has added up the number of known insect species in California and new species are still being discovered, one conservative estimate is that there are about 27,000 insect species in California. That is roughly 30% of the estimated total for all of North America north of Mexico.

California is often likened to a biological island isolated by the Pacific Ocean on the west and by the deserts on the east. High mountain ranges further inhibit animal and plant dispersal into and out of the state. California's complex topography and its climatic gradient — which ranges from the cool, moist north coast to hot, dry southern desert — have subdivided the state into a number of biotic provinces. Each of these provinces has a fauna comprising some widespread species and many of more limited distribution.

Additionally, over geologic time, the state's climate has oscillated between subtropical and glacial extremes, causing repeated redistributions of organisms as they tracked the changing climate zones. Some insect populations adapted to the new conditions, while others became extinct or survived in isolated refugia. As a result, California contains a multitude of insects adapted to narrowly defined local conditions and isolated by geographic and ecological barriers.

Insect diversity is also closely tied to that of plants because about half of all insect species feed on plants. Moreover, many insects depend entirely on just one or a few plant species. Herbivorous insects are often further specialized to feed primarily on one or a few plant tissues: leaves, stems, trunks, roots, flowers, seeds, or fluids. In addition to external feeders, there are leaf miners, borers, sap suckers, and gallmakers. A plant may be a host to many different insects in as many different ways.

The link between insect and plant diversity is perhaps best exemplified by bees, which are intimately associated with the evolution and diversification of flowering plants. California has over 1,600 native bee species, twice as many as occur east of the Mississippi River and nearly half of the total (3,500) for North America north of Mexico. About half of California's bee species gather pollen from only one or a few related plant species. The imported European honeybee, which is used to pollinate 47 of California's agricultural crops (worth $1.5 billion a year), adversely affects native species by competing for their floral resources. Ironically, the impending invasion of the Africanized honeybee may lead to diminished use of honeybees and increased use of native pollinators. Increased public fear of honeybees and the possibility of Africanized bees “taking over” a commercial hive may lead to greater restrictions of pollination services.

Scores of other exotic insects have also made themselves at home in California, often to the detriment of farmers, homeowners, and California's native insect fauna and flora. The alien invaders affect native species directly through competition and indirectly through the increased use of pesticides for their control. Such nonnative insects as the codling moth, silverleaf whitefly, and Mediterranean fruit fly can cause serious economic losses, entailing use of broad-spectrum pesticides for their control. Pesticide use may upset otherwise balanced natural communities by eliminating beneficial predators that normally control potential pests. These “secondary” pests can reach damaging levels and so can require further pesticide use.

There are approximately as many predatory insects as there are herbivorous species. These include mantids and other generalists and a great many host-specific parasitoid flies and wasps. However, the host specialization, which makes native parasitoids effective regulators of native prey, renders them less effective regulators of exotic pests. Scale insects, mealy bugs, whiteflies and many other exotic pests are controlled effectively by their imported natural enemies. The enlightened practice of Integrated Pest Management seeks to conserve beneficial predators and parasitoids while minimizing pesticide use. Hedgerows, roadside and ditchbank vegetation, and riparian corridors can provide reservoirs of beneficial insect diversity to recolonize agricultural fields following pesticide use or other disturbances.

The natural fertility of California's soils owes much to a third class of invertebrates, the soil-building recyclers. A large contingent of woodboring beetles, termites and scavengers is responsible for breaking down the organic matter in dead wood, fallen leaves, animal wastes and carcasses. This organic matter is further broken down by fungi and microbes into the basic nutrients utilized by plants. The recyclers do not fare well under prevailing agricultural practices, which entail frequent soil disturbance (plowing, discing) and minimal return of raw organic matter to the soil. Nor can the relatively slow action of the soil-building recyclers keep pace with modern agricultural crop production, which often relies on fire to reduce crop residues and fertilizer supplements to make up for nutrient loss. Fire may not directly kill soil-dwelling recyclers, but it removes nutrients they need. The chief factors that inhibit recyclers are land disturbance, monocultural production practices — as opposed to a mixed plant community — and frequent changes in the type of plants grown. Because recyclers tend to be less mobile (they often lack wings) than herbivorous and predatory organisms, they are less able to become reestablished following disturbance. Minimal tillage practices, leaving land fallow for a season, and rotational use of perennial crops such as alfalfa for an extended period can benefit recyclers.