Graduation date: 2007
In Oregon’s northern Willamette Valley, cabbage maggot (CM), Delia radicum (L.) (Diptera: Anthomyiidae) often renders Brassica root crops unmarketable. Scheduled insecticides are the only current control. Studies were conducted to: 1) describe and characterize spring emergence of CM flies and their seasonal flight activity relative to degree-day (DD) accumulations; 2) measure impact of CM by assessing crop damage; 3) define influence of seasonal planting and harvest dates relative to root damage; 4) identify the occurrence of ovipositional activity in relation to time of season, age of crop, distribution in field; and 5) study relationships among flight activity, oviposition, and crop damage.
Commercial rutabaga and turnip fields were studied from 2001 through 2005. Spring emergence from overwintering puparia was monitored using emergence cages. A bimodal spring emergence pattern for CM was observed: approximately 70% of the overwintering population peaked in late March. A second smaller peak was observed at the end of May.
The mean DD accumulations at 10, 50, and 95% of spring emergence using a lower and upper developmental threshold of 4.3°C and 30°C beginning January 1 had corresponding DD values of 200 ± 50.2 (8 March), 330 ± 22.2 (4 April) and 762 ± 60.1 (28 May), respectively. Spring flight patterns, monitored with yellow water traps, mirrored the bimodal emergence pattern but with an apparent lag.
Rutabaga crops sustained 1.7x higher damage levels than did turnip crops. Damage caused by CM was significantly greater (37-52%) in spring crops planted prior to an accumulated 900 DD, than in summer crops planted after 900 degree-days or in fields planted in the fall after 1500 DD. Maggot damage is abated by planting after the spring flight (> 900 DD) and harvesting before the relative peak of fall flight (< 2600 DD).
Fields documented with less than an average of 100 flies collected in a water trap over a crop’s duration in the field, had the lowest sum of percent weekly egg assessments (38.6 ± 6.1; based on 7 weekly egg level assessments), and the least amount of root infestation by CM (< 20% per 60 root samples). Fields with greater than 100 flies per trap over crop duration in the field had the greatest sum frequency of plants with eggs (95.6 ± 6.7), and the most CM root infestation (> 20%).
Oviposition significantly increased in fields at 30.9 (± 1.1) days after seeding. Large plants with > 5 leaves, root diameter of > 6 mm, and increased crop canopy were more heavily infested with CM eggs than smaller plants. A strong relationship was seen between the frequency of older plants with eggs (> 9 leaves, >19 mm root size and partially-closed canopy) and crop damage at harvest (r2 = 0.83). Egg incidence was significantly higher (46%) on plants located on the outside periphery of fields than on plants located in the field centers.
Spring monitoring of emergence and flight arrival, and density of D. radicum allows useful predictions of oviposition and crop damage. Appropriate oviposition assessment, combined with timely planting and harvesting schedules, and use of egg thresholds to time treatments could greatly reduce maggot populations.