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Open Access Publications from the University of California


California Agriculture is a quarterly peer-reviewed journal reporting research, reviews and news on California’s agricultural, natural and human resources.

Volume 70, Issue 1, 2016

Climate smart agriculture for California

Issue cover
Cover Caption: Complete contents, including news and editorial featuresFruit and nut trees, like these in a Yolo County walnut orchard, need a certain amount of cold weather each year for proper foliage and flower development. As the climate warms, declining annual chill hours — the total number of hours when the temperature falls between 32°F and 45°F — may change where crops are grown. Photo by Will Suckow

Research and Review Articles

Modeling the effects of local climate change on crop acreage

The impacts of climate change on agriculture depend on local conditions and crops grown. For instance, warmer winter temperatures in a given area would reduce chill hours, potentially cutting yields for some crops but extending the growing season for others. Using a century of climate data and six decades of acreage data, we established quantitative economic relationships between the evolution of local climate and acreage of 12 important crops in Yolo County. We then used the historical trend in climate change to project future crop acreages in the county. Only marginal changes in acreage in 2050 were projected for tree and vine crops there, in part because chill hours, although lower, remained above critical values. Walnuts were the most vulnerable tree crop, and the projections indicated some cultivars might be marginal in years with particularly warm winters. Processing tomato acreage might increase, due to a longer growing season, and also alfalfa acreage, if water availability and other factors remain constant.

Biological control program is being developed for brown marmorated stink bug

Brown marmorated stink bug (BMSB) is an invasive, polyphagous pest that has been detected in 42 U.S. states. In 2010, it caused millions of dollars in crop damages to apple growers on the East Coast, where it arrived from Asia during the 1990s. In 2002, BMSB was reported in California; since then, it has been detected in 28 counties and is established in at least nine counties. Although this pest has not yet been found on commercial crops in the state, detections of BMSB in commercial orchards have been documented in Oregon and Washington. Proactive research in California has joined national efforts led by U.S. Department of Agriculture researchers to develop a classical biological control program for BMSB. A study is under way to determine potential non-target effects of a specialist egg parasitoid, Trissolcus japonicus (Hymenoptera: Platygastridae), imported from Beijing, China, part of the home range of BMSB. In addition, the role of BMSB natural enemies residing in California is being assessed. A review of the recent research outlines the possible opportunities for reducing the threat BMSB poses to California.

Phenology of spotted wing drosophila in the San Joaquin Valley varies by season, crop and nearby vegetation

The spotted wing drosophila, first detected in California in 2008, has become a major insect pest in caneberries and sweet cherries, causing commercial crop losses. Managing it is challenging because it has many other hosts, including riparian and backyard fruit plantings, and it increases rapidly, with generations overlapping one another. In our study we monitored trap captures in two parts of the San Joaquin Valley, within sweet cherry orchards and in nearby locations. Captures of adult flies showed two main periods of activity — spring and fall — and low captures in the winter (except for citrus and evergreen riparian areas) and summer. On many occasions during the year, trap captures were higher outside of the cherry orchards than within them. Additionally, early in the season, when decisions about control programs are being made, the sex ratio of captured flies in cherries was strongly female-biased. The results suggest that during the weeks leading up to harvest growers should experiment by placing traps in different environments surrounding their orchards to determine SWD activity and potential pest pressure locally, and monitor for both male and female flies.

Sticky traps saturate with navel orangeworm in a nonlinear fashion

Trapping is an essential tool used to decide the need for and/or timing of an insecticide application. The assumption is that the information is accurate, but accuracy is dependent on trap reliability and efficacy. One factor that affects reliability is trap saturation, defined as the measurable decrease in trap capture due to reduced trapping effectiveness caused by the accumulation of insects already in a trap. In this study, we used unmated female navel orangeworm (NOW, Amyelois transitella (Walker)) as sex pheromone baits in wing traps that varied by color and glue/trapping surface in order to evaluate saturation thresholds and quantify trap effectiveness. Effectiveness decreased in each type of sticky trap as the number of insects caught increased, because of the accumulation of scales and insect bodies on the glue surface. The continued accumulation of insects further reduced trap capture, and this decrease in capture could be described by a regression using a power transformation. The resulting saturation equations that we calculated will help pest control advisers and growers interpret their trap data by better estimating the relationship between the number of males trapped versus those that visited the trap.

Management of blue gum eucalyptus in California requires region-specific consideration

Blue gum eucalyptus (Eucalyptus globulus) is a large tree native to Australia that was widely planted throughout California for reforestation, building and timber, but in some areas has spread beyond its planted borders and substantially altered wildlands. Due to its fast growth, large size and reproductive potential, blue gum's impacts on native vegetation, wildlife and ecosystem processes are of concern, particularly in areas with reliable year-round rainfall or fog, where it is most likely to spread. Depending on levels of invasion and rate of spread, blue gum may have negative, positive or neutral impacts on fire regimes, water and nutrient availability, understory vegetation and higher trophic levels. Additional research on the abiotic and biotic impacts of blue gum, quantitative estimates of area covered by blue gum, and weed risk assessments that allow for region-specific climatic information and management goals to be incorporated are needed to guide management of blue gum populations.