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Africanized honey bees were first detected in California in October 1994. Since then, they have established a foothold in the Imperial Valley and have spread toward San Diego and into Palm Springs. However, their spread has been much slower than originally expected. What has slowed them? The best guess is Varroa jacobsoni, an exotic ectoparasite of honey bees recently introduced into North America. The effects of varroa on Africanized honey bees may be both a blessing and a curse; the latter is especially true if Africanized bees become resistant to varroa and commercial honey bees do not.

Invasion biology is the study of the reproduction, dispersal and ecological impact of organisms that occur outside of their native range, including exotic pests and biological control agents. Alien species cost California agriculture billions of dollars annually in control measures and crop damage. Societal trends toward increased travel and import of plant and animal products suggest that exotic species will continue to enter California. Understanding the principles of the invasion process will help to predict which species will invade, where invaders will become established, and the effects of invasions on agricultural, urban and natural environments. Improved understanding of the invasion process should contribute to public policies designed to prevent or contain invasions, especially of potential pests that are not yet in the state and whose biology is poorly described.

The elm leaf beetle, first discovered in California in the 1920s, quickly became one of the state's major urban tree pests. In the past 15 years, monitoring methods have become integral to the design of the Integrated Pest Management program for the elm leaf beetle (ELB). A sampling protocol has been developed that can successfully predict ELB damage based on the presence or absence of egg clusters. A monitoring program based on this sampling technique may allow managers to direct control efforts to only those trees requiring treatment, thus avoiding unnecessary environmental and economic costs. Chemical insecticides are still a temporary solution to the problem, but increasing concern for human and environmental health has stimulated the pursuit of nonchemical approaches. Releases of egg parasitoids have been largely unsuccessful over the past 12 years. An effort is currently under way in Sacramento to improve the Integrated Pest Management program based on monitoring, spot treatments with injected chemical insecticides, foliar application of Bacillus thuringiensis and the release of a new strain of egg parasitoid from Granada, Spain.

The vast movement of people and agricultural products between distant geographical regions has created unprecedented opportunities for introducing plant viruses and the insects that carry them (vectors) to new areas. Outbreaks of new viruses may be favored in these agroecosystems by crop susceptibility, the presence of particular weeds and certain agricultural practices. In some cases, conditions in these ecosystems may be ideal for the emergence of altered plant viruses and new virus/vector relationships. This may result in the appearance of insect-transmitted plant viruses in crops and regions where they have not been seen before. Because plant viruses and their insect vectors are intimately linked, the status of both must be considered in formulating strategies to prevent or slow their introduction, as well as to manage any invasions. To illustrate these points we highlight two situations that could threaten California agriculture. First, a devastating plant virus, tomato yellow leaf curl geminivirus, is not present in California, but an insect (the silverleaf whitefly) that transmits it is present. Second, the brown citrus aphid is not present in California, but a citrus virus (citrus tristeza closterovirus) that this insect efficiently spreads, is present.

When clouds of whiteflies swarmed through California's desert agricultural areas in the fall of 1991, they were initially identified as a new strain of the sweetpotato whitefly, Bemisia tabaci. The previously known strain was called “A” or “cotton,” while the new strain was called “B”, “Florida” or “poinsettia.” Since then, research has shown that this new pest is actually a distinct species, Bemisia argentifolii (Bellows & Perring), and is known as the silverleaf whitefly. Since its introduction in the United States, the silverleaf whitefly has cost more $2 billion in crop loss and damage, and pest control. The silverleaf whitefly is exceptional in its ability to colonize a great variety of crops, weeds and ornamentals. Southern California's diverse crops, high temperatures, and low rainfall help sustain whitefly populations at high levels, even during the winter months. The level of infestation of crops attained by silverleaf whitefly populations is driven by the insect's biological traits, the crops grown and the inadequacy of pest controls.

The first reported case of a non-native termite species being introduced and established in California was the Formosan subterranean termite, which was discovered in San Diego County in 1992. Because this termite can exact tremendous damage within a relatively short time, the affected area was defined and an attempt was made to eradicate it. Intensive baiting with the insect growth regulator hexaflumeron over 12 months appears to have eliminated the original infestation. New infestations have recently been discovered in and around homes about 1/4 mile away, and winged Formosan subterranean termites have been caught nearly 3/4 mile away. Measurements of workers and soldiers and dating of damage suggest that the new colonies are 6 to 8 years old, and that new colonies established from winged reproductives before baiting took effect at the original site. If left unabated, this pest may become increasingly more serious in California.

Cracks can play a major role in water advance and infiltration in a cracking soil. Water flowing directly into subsurface cracks dominated the cumulative intake for the preirrigation at a site in the San Joaquin Valley. Differences in furrow inflow rates had little effect on cumulative infiltration for preirrigation. However, for subsequent irrigations, different furrow inflow rates significantly affected cumulative Infiltration. Crack flow was a significant factor in cumulative infiltration for the crop irrigations. Uniformity of water advance among furrows was high for the preirrigation but was less for the crop irrigations. A comparison of surge irrigation and continuous-flow furrow irrigation with furrow lengths of about 1/4 mile and 1/2 mile showed little difference in cumulative infiltration.

In northwestern California, the dusky-footed wood rat is a primary reservoir of Lyme disease (LD) spirochetes and an important host of three tick species that collectively transmit the causative agents of LD, human granulocytic ehrlichiosis, Rocky Mountain spotted fever and tularemia. It also is infested by two flea species that can transmit the agent of plague. We refined a method of applying pesticides to the host using liquid permethrin-treated bait tubes, and found it to be highly effective for reducing populations of these ticks on wood rats in brushlands. Although permethrin products are not currently registered for this use, this approach offers a promising tool for controlling arthropod-borne diseases in the western United States.