One of the most promising aspects of herbarium-based phenological data is the potential to expand our taxonomic and geographic sampling of phenological research. For example, thevast collections of specimens from species-rich tropical and subtropical biomes could be used to greatly enhance phenological research in these regions where field based phenological data, especially on the timescale of recent climate change, are often limited. Herbarium data could also be used to investigate the extent to which species may no longer be phenologically responding to a warming climate. Most of the planet has experienced record breaking temperatures in recent years and plants have largely responded with advanced phenology. However, it is possible that winter temperatures may become too warm for plant species to meet their winter chilling requirements, causing a delay in leafing out and flowering. This hypothesis could be tested using specimens collected in especially warm versus cold years. Another exciting area of future research is the integration of herbarium data with other sources of phenological data . Data can sometimes be discovered through historical records and photographic collections but these are often limited in geographic and temporal coverage.
For contemporary phenological data, pot with drainage holes researchers are turning to expanding citizen science networks to provide enormous numbers of phenological observations over huge geographic areas . These datasets could be combined to greatly increase the spatial density of observations as well as to validate the results of herbarium-based phenological data. In addition, the continued development of remote sensing technology offers another source of phenological data that can be integrated with herbarium-based data. For example, ecosystem models based on remote sensing data are often limited in their predictive ability because of a lack of long-term, species-level phenological data. Herbarium-based phenological estimates, which have been found to agree with broader phenological estimates based on Landsat and MODIS satellite data, could provide the necessary species-specific data to improve these models. Herbarium specimen data combined with data concerning other, associated species may help answer another pressing phenological question: is climate change leading to ecological mismatches among organisms at different trophic levels? Due to large annual variations in climate and organismal phenology, robust evidence for ecological mismatches has been notoriously difficult to identify. As an example of a potential way forward, Bertin used herbarium specimens to compare peak flowering phenology with ruby-throated hummingbird migrations. Herbarium specimens may also be examined for other traits that contribute to fitness and interact with phenology, such as herbivory, frost damage, flower size, or fruit set.
Finally, herbarium specimens can be used to estimate changes in abundance and distribution, allowing researchers to estimate the influence of phenological sensitivity on local or regional species loss. Despite the potential for herbarium specimens to vastly expand our understanding of plant phenology – as well as other fundamental aspects of plant biology – the value of collections remains threatened by declines in institutional investment, basic research funding, and the intensity of collection of new specimens in recent decades. It is vital that these trends be reversed to preserve the value of herbarium collections as unique records of phenological change. To this end, digitization is not a means to replace physical specimens but rather an opportunity to expand access to and interest in these important collections. Physical specimens will continue to play an important role in herbarium-based phenological research and, perhaps more importantly, may contribute to research opportunities we have not yet imagined.Organic strawberry production has become big business in California, generating more than $17 million in sales annually on over 1,200 acres—nearly 5% of California’s total strawberry acreage. But as producers have found, growing this specialty crop without conventional pesticides requires a new toolbox of pest and disease control techniques. For the past five years, researchers from the Center for Agroecology and Sustainable Food Systems have been refining the use of trap crops in organic strawberry systems as a way to limit damage from the western tarnished plant bug and boost populations of the pest’s natural enemies.
A serious pest native to California’s central coast, WTPB feeds on developing strawberries, causing gnarled, “cat-faced” berries with enlarged, straw-colored seeds. These damaged fruit can’t be sold on the fresh market. Although some organically acceptable sprays exist to treat WTPB, they’re expensive and relatively ineffective.A broad range of winter weeds in central coastal California, including wild radish, mustards, chickweed, lupine and other legumes, and knotweed, offer a winter food source for WTPB. As the rainy season tapers off in the spring and wild vegetation dries out, the WTPB adults move to flowering crops, including strawberries, and begin feeding. Trap crops planted along the edges of crop fields or within the field have the potential to limit WTPB damage by offering the pests a food source they prefer over the crop itself. “That’s the definition of a trap crop—that it’s a preferred host or food source for the insect you’re targeting when compared with the main crop,” says Sean L. Swezey, the Center’s associate director, and director of the UC Sustainable Agriculture Research and Education Program. Trap crops can also serve as habitat for beneficial insects, which can supplement pest control efforts.Once attracted to the trap crop, pests must be managed so that they don’t eventually disperse into the fields and damage the crop you’re trying to protect. Conventional growers can use a pesticide spray on the trap crops, but that’s not an option for organic growers. However, tractor-mounted vacuum units known as “bug vacs” are one of the tools available for organic systems. “I worked on research of the original proprietary bug vacs for the strawberry industry back in the late 1980s,” recalls Swezey. “But back then we were using more of a shotgun approach, vacuuming all of the crop fields, which in a way was equivalent to using a pesticide because it affected all the insects in the fields—both pests and beneficials. This seemed to me to be as non-selective as an insecticide application.” Swezey and Larry Eddings, president of Pacific Gold Farms, speculated that by concentrating the pests in one place, an effective trap crop could be managed with bug vacs, thus eliminating the need for growers to run vacuum units across their entire strawberry plantings. If effective, the approach would not only decrease WTPB damage to the strawberry crop, but would save time and energy by cutting down on the area that needed to be vacuumed, and would conserve populations of beneficial insects in the crops. In 2002 and 2003 the Center research team of Swezey and research assistants Janet Bryer and Diego Nieto worked with Eddings and his staff at a Pacific Gold Farms site in Prunedale to test their theory. Grants from the Organic Farming Research Foundation and the US Department of Agriculture’s Western Sustainable Agriculture Research and Education program supported the work.Using a hand-held suction device, Bryer and Nieto collected insect samples in the trap crop plantings weekly beginning in January 2003. The samples were then frozen and insects were identified and counted under a dissecting microscope. They also monitored insects in row 1 of the strawberry plantings using the same technique. The radish trap crop flowered from February through the end of May, when it was removed.
The alfalfa trap crop began flowering in mid April and continued to flower through September. On April 11, large pot with drainage collaborators from Pacific Gold Farm began vacuuming the beds and trap crops with a tractor-mounted unit that includes three rectangular vacuum collectors that generate a suction of approximately 28 miles/hour . Operators drove the tractor at 1.2 miles per hour when vacuuming the rows, passing over the strawberry canopy at canopy height once a week, and over the alfalfa trap crop row two days a week each week through the season . In mid April, in addition to monitoring the trap crops, Bryer and Nieto began monitoring insects in strawberry rows 1, 2, 4, 8, and 16. They also examined berries from four randomly selected clusters of four strawberry plants ; each week, developing berries that showed signs of distinct WTPB damage were counted and removed, while undamaged berries were counted once they matured.Adult WTPB were first found in the radish trap crop vegetation on January 7, and in the alfalfa trap crop in mid April, when it began to flower. Based on a heat unit accumulation model1 initiated when the first adult was found on January 7, the researchers predicted that a second-generation adult would not mature until July 19 at the earliest; therefore, the WTPB adults found any time before this date had migrated to the crop . This result suggests that there is a six-month period during which migrant WTPB adults are attracted to trap crop vegetation at the edge of strawberry fields. Figure 1 shows total accumulation of WTPB in the unvacuumed trap crop treatments and the adjacent row of strawberries. Significantly more WTPB were found in the alfalfa than in either the radish trap crop or row 1 of strawberries. For seven weeks in April and May, when both the radish and alfalfa trap crops attracted adult WTPB or nymphs hatched in the vegetation, and when the grower was conducting commercial field vacuuming treatments, alfalfa attracted or retained over 7 times more WTPB than the radish trap crop. Although it flowers and matures somewhat later in the spring, alfalfa was a significantly more effective trap crop for WTPB. This result has management implications for central coast growers. “We’d experimented with a variety of trap crops through the years, including radish, mustard, alyssum, and other flowering annuals and perennials,” says Swezey. “But we’ve found that the radish and some of the other crops can become difficult to deal with once they begin to die back in the summer. Given the results of this study, which show that alfalfa is far more effective at attracting WTPB, we are focusing on alfalfa.” Because heavy spring rains often continue through April, tractor-mounted vacuum management of a trap crop can only begin in early May, when muddy conditions have diminished. This is an optimum time to begin alfalfa trap crop vacuuming. Pattern of WTPB Numbers and Strawberry Damage by Treatment and Row In June, weekly, tractor-mounted vacuuming of the alfalfa trap crop reduced total WTPB by 70% compared to the unvacuumed trap crop . The vacuumed trap crop treatment had the same accumulated WTPB as either the whole-field vacuuming treatment or the untreated control. In contrast, the unvacuumed trap crop consistently accumulated higher numbers of WTPB in strawberry rows 1, 2, 4, and 8. There were no differences among treatments at row 16, indicating that the trap crop’s effect on WTPB numbers ended somewhere between rows 8 and 16. Why the total WTPB numbers in the untreated control were consistently low in June is not clear. It’s possible that whole-field vacuuming in the commercial fields surrounding this experiment lowered the general level of WTPB in the small test plots. Movement or “sinking” of WTPB to nearby trap crops could also explain the low numbers in the control plots. As shown in figure 3 , the vacuumed trap crop treatment had a significantly lower percentage of damaged strawberries than either the whole field vacuuming or the untreated control .As I look over the array of topics in this issue of The Cultivar, I’m reminded of the many efforts that the Center has undertaken to help growers improve their farming and marketing operations. One of those efforts is our ongoing research on production practices that minimize pest damage without the use of synthetic pesticides. To that end, the Center’s associate director Sean Swezey and Center researchers Janet Bryer and Diego Nieto have been working for the past several years with commercial organic strawberry growers to refine the use of trap crops in their production systems. See the cover story for a progress report on this research. In another project aimed at serving local growers, we’ve teamed with UC Cooperative Extension researchers to study the potential for growing blueberries organically on California’s central coast. A blueberry trial planted on the Center’s farm last fall will generate information on the best-performing high bush varieties of this potentially lucrative niche crop . This spring we held a field day to introduce the project to the local farming and gardening communities, and we look forward to future field day opportunities as this variety trial progresses. Also addressed in this issue is the often vexing challenge of gopher control.