Carol Miles
Washington State University
Pullman, Washington
Carrot fly (Psila rosae), a devastating pest of carrots, causes the greatest impact on a diversified organic small farm in southwest Washington. On the twelve acre farm, 45 different crops are grown and marketed through a 100-member Community Supported Agriculture group, the local farmers' market, and a self-serve farm stand. This farm typifies small organic farms in southwest Washington that rely on diversification of both crops and marketing strategies. Historically, carrots were one of the farm's major crops. However in recent years, carrot rust fly damage has resulted in 50-60 percent annual loss of marketable carrots. The carrot rust fly deposits its eggs at the base of the carrot plant. When the larvae hatch, they migrate down the soil profile along the carrot root and penetrate into the root. Larval feeding results in bore holes and a rust-colored grass. Row covers have been the only effective alternative for control, however the covers interfere significantly with crop maintenance.
On-farm research involves growers in all stages of a research issue. Growers identify a problem, decide on a strategy to be tested, work with the researcher to implement the experiment, participate in data collection, and evaluate results. If the technique being tested is effective,
adoption can be very rapid because, through participation in the experiment, the grower already has first-hand experience in managing and adapting the new technique to current farm practices. Additionally, neighboring farmers are exposed to the experiment, making adoption more rapidly widespread. Significance to this project, the organic growers in southwest Washington had never worked with Cooperative Extension before and never viewed Extension as a resource for alternative pest management strategies.
With all these factors in mind, it was necessary to find an appropriate technique that would accommodate the practices of organic farming that offered a high level of carrot rust fly control. Access to the university library, located 400 miles away, would have proven a huge barrier if it were not for the internet. Working through the internet, it was possible to connect not only with the university library, but also with national agricultural data bases. The utilization of the internet has proven an invaluable tool for agricultural extension agents because it vastly enhances information availability and dispersal. A literature search via the internet quickly provided several management strategies for carrot rust fly control. One technique, the intercropping of harbinger strand medic (Medicago litoralis) with carrots, had been tested in Sweden and seemed the most feasible for the situation in southwest Washington.
In collaboration, the grower and the agent designed a randomized complete block experiment with two treatments, four replications and four planting dates. Soil type was a Chehalis silt loam, Cumulic Ultic Haploxeroll. Treatments were the control (no intercrop) and the medic intercrop; plots measured two beds wide by 9.1 meters long. In Sweden, medic replaced the center row of carrots in the three-row bed. This greatly reduced total carrot yields, first by reducing carrot acreage by one third, and second due to strong competition between the carrots and the medic. To reduce yield impact, medic was sown in Southwest Washington between the carrot beds, at the rate of 10 lbs/A, approximately three days after each carrot planting.
In 1995, carrots were seeded on May 26, June 5, June 16, and June 29. In 1996, carrots were seeded on May 26, June 5, June 15, and June 30. The control plots were mechanically cultivated six and ten weeks after planting. At the same time, the intercropped plots were hand weeded. In 1996, the first medic seeding was inadvertently mechanically cultivated six weeks after planting, therefore medic was reseeded in these plots June 22. In 1995, carrots were harvested on September 27, October 4, and October 21, respectively. The fourth carrot planting was not harvested due to saturated soil conditions. Carrots were harvested from a 1.5m length of bed in the center of each plot, a total row length of 4.5 meters. At each of the three harvests in 1995, carrots were sorted into three categories: undamaged; marketable damaged; and unmarketable damaged. No data is available for 1996 as carrots have not yet been harvested.
In 1995, the medic intercrop reduced unmarketable carrot yield (kg/plot) by 50%, 45%, and 20% at the three harvest dates, respectively. However, this difference was significant only for the first harvest. Damaged marketable yield was significantly greater in the intercrop for the third harvest. In all three plantings, total marketable yield (marketable yield + damaged marketable yield) was 20% greater when the intercrop was present, however this difference was significant only for the third harvest.
In the medic intercrop, yield (kg/plot) of marketable carrots was generally not increased even though yield of unmarketable carrots tended to be lower. This lack of significant increase in marketable yield may be due to reduced carrot size in the intercrop. In 1995, the medic intercrop was vigorous and it may have competed with the carrots. In 1996, in addition to weighing, carrots in each of the three categories will also be counted to determine if the medic is reducing carrot yield by competition.
Current research in Sweden indicates medic interferes with the host-plant finding and oviposition behavior of the carrot rust fly. To maximize the effectiveness of intercropping for carrot rust fly control, it is necessary that the intercrop covers the area between carrot beds as quickly as possible. To minimize the effect of intercropping on carrot yields, it is necessary to limit the competition between the intercrop and the carrots. Placing the intercrop between the beds of carrots appears to be sufficient to reduce carrot rust fly damage. However, it may be necessary to mow the intercrop during the growing season to reduce the competition between the intercrop and carrots.
In 1996, the experiment is being repeated at the request of the grower and with funding from a USDA -Sustainable Agriculture Research and Education grower grant. On July 23, 1996, a field day was held at the on-farm experiment site. The president of the company which supplied the medic seed traveled from Australia to view the experiment and to participate in the field day. Fifteen growers and agency personnel from the area attended the field day which generated more interest and test sites.
Literature Cited
Coppock, L. J., F. E. Makell, and R. Gair. 1975. Attempts at cultural control of carrot fly damage to carrots in East Anglia. Plant Pathology, 24, 97-101.
Davidson, R., A. Antonelli, and Louis Getzin. 1989. Carrot rust fly. WSU Extension Bulletin 0921.
Finch, S.. 1993. Integrated pest management of the cabbage root fly and the carrot fly.
Crop Protection, 12:6, 423-430.
Perrin, R. M., and M. L. Phillips. 1978. Some effects of mixed cropping in the population dynamics of insect pests. Entomologia experimentals et applicata, 24, 585-593.
Ramert, B.. 1993. Mulching with grass and bark and intercropping with Medicago litoralis against carrot fly (Psila rosae(F.)). Biological Agriculture and Horticulture, 9:2, 125-133.
Ramert, B. Intercropping as a strategy for reducing damage to carrots caused by the carrot fly, Psila rosae (F.). Unpublished.
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