The Use of Straw Mulch to Enhance Predator Populations Along with Biopesticides to Control Onion Thrips in Dry Bulb Onions
The alternative approach to controlling thrips was tested again this year on two red and one yellow onion varieties. Colossal sized bulbs were significantly higher in Redwing and Vacquero and super colossals were higher in Vacquero. There was a trend toward higher overall yields in both varieties. Flamenco was susceptible to plate rot (Fusarium oxysporum) which appeared in the trials and made stands and yield erratic, so statistical differences did not appear in that data, although the trend was toward higher yields with the alternative program.
All of the plots were infected late in the season with the Iris Yellow Spot virus (IYSV), a new disease of onions throughout the world. The virus is spread by onion thrips, so this trial provided the opportunity to evaluate the alternative program for spread of the virus. There was a significant reduction in the severity of the virus with the alternative program, suggesting a secondary benefit from the alternative control strategy for controlling thrips.
The objectives in 2004 were:
1. Test the alternative strategy in small plots at the Malheur Experiment Station.
2. Get the bio-insecticide Success (spinosad) registered for onions so that commercial growers could use it.
3. Try the alternative strategy on at least one commercial grower’s field.
Three onion varieties were selected for comparison of the alternative strategy. The red varieties were Flamenco and Redwing, and the yellow variety was Vacquero. These varieties give a spectrum of resistance to onion thrips injury ranging from susceptible for Flamenco to fairly resistant for Vacquero with Redwing being intermediate. Unfortunately, there was a high degree of plate rot (fusarium oxysporum) that appeared in the trial area and severely reduced the plant population in the susceptible variety Flamenco. Because of the erratic stands, there were no yield differences for Flamenco using the alternative control strategy, although there was a trend for higher yields with the alternative program. The larger and more valuable onion bulbs in the colossal and super colossal classes were significantly increased with the alternative program in the Redwing and Vacquero varieties. Overall yields were increased with the Vacquero but were not significantly different in Redwing.
An EPA section 18 emergency exemption was applied for to use Success (spinosad) for bulb onions. That section 18 was granted, but not until it was too late to use the product on a commercial field.
The Iris Yellow Spot Virus (IYSV) is a relatively new disease of onion production areas. It has been detected in the production areas of Idaho, Oregon, Utah, Colorado, Washington, and Georgia. It is vectored by the onion thrips and, because onion thrips are so difficult to control, controlling the virus by controlling thrips has proved futile. The trials in 2004 received a late season infection of the virus, not enough to impact yields but enough to be able to evaluate the treatments for effectiveness in controlling the disease. There was a significant reduction in the amount of disease in the alternative treatments compared to the standard insecticide treatment or the untreated check.
Impacts and Contributions/Outcomes
No impacts will be observed until the program is adopted by commercial growers. Now that Success has been granted a section 18 emergency registration, it should be relatively simple to get the registration granted again for the 2005 season. The potential for impact is great, both to control onion and western flower thrips and to suppress the impact from the Iris Yellow Spot Virus (IYSV).
Malheur County Onion Growers Association
701 SW 5th Ave
Ontario, OR 97914