2008 Annual Report

UC Exotic/Invasive Pests and Diseases Research Program

Due to funding reductions, the UC Exotic/Invasive Pests and Diseases Research Program has ended. The program was funded through a Special Research Grant from USDA.

Summaries of project reports from the program are online at the UC IPM Web site.

The following highlights one of the research projects funded by the program.

Predatory mite, Neoseiulus californicus. Photo by Jack Kelly Clark.

Know your natural enemy

UC researchers have found that the species of predatory mites that predominate can vary quite a bit between and within crops. With the aid of a new taxonomic key, the job of identifying which mite species is the most common in a crop can improve the chances of having a successful biological control program.

Predatory mites feed on other mites and small insects such as thrips and are known to be important natural enemies in many crops including subtropical trees, stone fruits, nuts, vines, berries and field crops. They are most common when their prey is most common. For citrus and avocados, they are commonly seen in the spring and fall; for vines and stone fruits, they also are easily found during the early summer and fall.

"Predatory mites are one of the most successful commercially available biological control agents," explained entomologist Beth Grafton-Cardwell, lead scientist for the project funded by the UC Exotic/Invasive Pests and Diseases Research Program. "However, there are many species, and they can have different feeding habits. The more specialized predatory mite species tend to exert greater control of pests than the more general feeders, because they are focused on one prey species.”

Grafton-Cardwell, IPM specialist and research entomologist, said, "During 2005 to 2007, we identified the species of predatory mites collected in 11 major agricultural crops and developed a classification key to recognize them. Trained personnel can use the key to determine which species of predatory mite is most common in a crop, and this can assist with pest management decisions."

As part of the grant, nine UC farm advisors and graduate students collected and identified more than 9,000 predatory mites from grape, avocado, cherimoya, citrus, pear, peach, plum, almond, walnut, strawberry and caneberry fields and orchards.  

The general trends in the collections are 1) Subtropical fruits (citrus, avocado, and cherimoya) had the most generalist predatory mites that feed on mites, thrips, pollen and leaf sap; 2) strawberry and caneberries had the most specialized predatory mites that attack only webspinning spider mites; and 3) other crops had a wide variety of predatory mites that varied in their feeding habits.

Scientists also found that predatory mite species varied by region. For example, in vineyards, there was much more variety in predatory mites found in the north coast region (17 species found) than in the San Joaquin Valley (5 species), or the south coast region (10 species).

The study indicates that the western predatory mite, which has been traditionally thought to be the major predator in trees and vines, is not always the most important species. In addition to the western predatory mite, 21 species of predatory mites were found in these crops from 2005 to 2007.

“Knowledge of the dominant predatory mite species increases the potential for success in controlling the pest, and this could help to reduce unnecessary pesticide applications,” said Grafton-Cardwell.

UC IPM Advisor Lucia Varela surveyed grapes in four North Coast counties for four years as part of the study. She found great diversity with an average of two to seven species of predatory mites per vineyard surveyed. "This has stimulated new questions for research on the role these species play in suppressing spider mites and other pests. What is important is to conserve and not disrupt this diversity," she said.

Jim McMurtry, UC Riverside emeritus professor of entomology, and Grafton-Cardwell produced the taxonomic key. David Haviland, UCCE Kern County farm advisor, assisted them in using the key to teach an annual course to pest control advisors, farm advisors, extension specialists, graduate students and insectary staff.

“It's always a privilege to be part of the extension continuum at work,” said Haviland. “Industry and UC participants in each of the courses were able to benefit from Dr. McMurtry’s career-long expertise in predatory mites and Dr. Grafton-Cardwell’s efforts to organize his information. The knowledge participants received from the course, coupled with the long-term nature of the key, will have lasting effects on mite management in California.”

The taxonomic key to the mites is soon to be submitted for peer review and publication by UC ANR. The key requires the use of a phase-contrast microscope and so will be for use by professionals who have access to this type of equipment.

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