Mount Vernon Northwestern Washington Research and Extension Center

Vegetable Research and Extension

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Carrot Rust Fly Control

Intercropping in Carrots for Rust Fly Control

Carol Miles, Ph.D.,WSU Extension, Leslie Zenz, Research Assistant, Betsie DeWreede, Owner, Independence Valley Farm, and Julie Puhich, Owner, Common Ground CSA


Carrot rust fly (Psila rosae) is a devastating pest of carrots in southwest Washington (Figure 1)

Photo of carrot rust fly   Figure 1. Carrot rust fly (See enlarged picture) Photograph by Art Antonelli, WSU.

To cause damage to the carrot, the carrot rust fly must deposit her eggs at the base of the carrot plant. When the larvae hatch, they migrate down the soil profile along the carrot root and then penetrate it as they feed. Larval feeding results in surface tunneling,bore holes and rust colored frass, rendering the carrot unmarketable (Figure 2). 

Photo off carrot rust fly damage to carrots   Figure 2. Carrot rust fly damage to carrots (See enlarged picture).

Organic and alternative growers who do not use chemical pesticides have very few options for control of this devastating pest. Floating row covers are used as an alternative and effective means of barrier control, however, this method interferes significantly with crop maintenance and is cost prohibitive when applied to larger plantings of carrots (Figure 3).

Photo of row cover on carrots   Figure 3. Row cover on carrots (See enlarged picture).

Establishing an On-Farm Research Site

In 1996, WSU Extension agent, Carol Miles, contacted Betsie DeWreede, an organic grower, with the suggestion of doing an on-farm research project. Organic farmers in the area had never before worked with Extension or considered Extension to be a resource for alternative pest management strategies. This proposal was the perfect opportunity for enhancing the relationship between farmers and Washington State University.

Considering that the greatest pest problem on Betsie's farm is the carrot rust fly, causing 15–20% annual loss to one of her major crops, it was decided that this would be the focus of the research project. Carol and Betsie set out to find an appropriate method for experimentation. A search on the internet provided an interesting study conducted in Sweden on intercropping as a control for carrot rust fly. An intercrop of Harbinger strand medic (Medicago litoralis) was planted in the carrot bed as the center row to deter the rust fly. Current Swedish research indicates medic interferes with the host-plant finding and oviposition behavior in the carrot rust fly. Using an intercrop was easily adapted to organic farming practices and presented itself as the most feasible solution for the situation in southwest Washington.

Experimental Procedure

In collaboration, Carol and Betsie 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 30–feet long (Figure 4).

Photo of medic plots in carrot field   Figure 4. Medic plots in carrot field (See enlarged picture). 
Photo of medic in alleys between carrot   Figure 5. Medic in alleys between carrot beds ( See enlarged picture).

The medic intercrop reduced non-marketable carrot yields (lbs/plot) by 50%, 45%, and 20% at the three harvest dates, respectively (Table 1). However, this difference was significant only at Harvest 1. Damaged marketable yields were significantly greater in the intercrop at Harvest 3. Marketable carrot yields tended to be higher at all three harvest dates when the medic intercrop was present but in no case were these differences significant.

In 1996, themedic intercrop did not grow as well as the previous year, and in most cases the area between the carrot beds was not completely covered by the time carrots were harvested. The medic intercrop reduced non-marketable carrot yields by 37% and 19% at Harvests 3 and 4, respectively, however these differences were not significant (Table 2). Damaged marketable yields were the same with and without the medic intercrop at all four harvest dates. Marketable carrot yields (weight) tended to be higher at all four harvest dates when the medic intercrop was present, but in no case were these differences significant. The number of marketable carrots also tended to be higher in the intercrop plots though these differences were not significant (Table 3). The number of damaged marketable and non-marketable carrots were the same with and without the medic intercrop at all four harvest dates.

Placing the medic intercrop between the beds of carrots rather than within the carrot bed eliminated competition between the medic and carrots. Both years, the medic intercrop resulted in a trend towards greater marketable carrot yields and reduced non-marketable carrot yields, however these differences were generally not significant. In 1995 and 1996 the yield differences, when expressed in percentages, appeared quite significant. However, variability between replications was so great that these differences were not statistically significant. For future testing, more replications of the treatments at each planting date are recommended to enhance statistical analysis. In 1996, poor medic stands through out the season likely reduced the influence of the medic intercrop on the carrot rust fly. Poor medic stands may have been due to a combination of rockier soils, poor nodulation, and cooler seasonal temperatures.


Results of this study from 1995 were published in the Washington Tilth Journal of Organic and Sustainable Agriculture. On July 23, 1996, a field day event was held at the carrot-medic research plots at Independence Valley Farm (Figure 6).

Photo of field day at Independence Valley   Figure 6. Field day at Independence Valley Farm (See enlarged picture).

Brian Hedt, president of Revell Seeds, who supplied the medic seed for the project, 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. The field day generated greater interest in medic as a forage crop than as an alternative method for carrot rust fly control. As a result, in 1997 conducted medic variety trials to test suitability and adaptability of medic to our region as an alternative forage crop. Betsie DeWreede and two other farmers in the area were grower-cooperators for the medic forage trial.

Preliminary results of the medic-carrot intercropping experiment, entitled On-Farm Research: A Case Study – Intercropping in Carrots for Rust Fly Control, were presented in poster format by Carol Miles at the National Small Farm Conference in Nashville Tennessee, September 10-13, 1996. Betsie DeWreede and Carol Miles co-presented a workshop session, On-Farm Research – Medic and Carrot Rust Fly Control, at the Washington Tilth Conference, November 9-10, 1996 at Leavenworth, Washington. Approximately 30 participants learned about the research project and how to conduct an on-farm trial. Leslie Zenz, Research Assistant, also presented our poster, On-Farm Research: A Case Study – Intercropping in Carrots for Rust Fly Control, at the Tilth Conference, and more than 200 participants viewed our display and many discussed the procedure and results with us.


This researchwas funded in 1996 by a Western Region USDA Sustainable Agriculture Research and Education (SARE) Farmer/Rancher Research Grant. For more information about the SARE Grant program, write to Utah State University – UMC-2300, Logan, UT 84322-2300 or email

All photographs (except Figure 1) were taken by Carol Miles and may not be reproduced without her consent. The photograph in Figure 1 was taken by Art Antonelli, WSU Entomologist.


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Davidson, R., A. Antonelli, and L. 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., andM. L. Phillips. 1978. Some effects of mixed cropping in the population dynamics of insect pests. Entomologia experimentalis et applicata, 24, 585-593.

Rambert, 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.

Rambert, B.. Intercropping as a strategy for reducing damage to carrots caused by the carrot fly, Psila rosea (F.). Unpublished.

Rambert, B., and B. Ekbom. 1996. Intercropping as a management strategy against carrot rust fly (Diptera: Psilidae): A test of enemies and resource concentration hypothesis. Population Ecology, 1093-1100.

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