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ARS Home » Pacific West Area » Corvallis, Oregon » Forage Seed and Cereal Research Unit » Research » Publications at this Location » Publication #245709
Title: Straw Mulch and Reduced-Risk Pesticide Impacts upon Thrips and Iris yellow spot virus on Western-Grown Onions

Author
item SCHWARTZ, HOWARD - Colorado State University
item Gent, David - Dave
item FICHTNER, SCOTT - Western Farm Service
item HAMMON, ROBERT - Colorado State University
item CRANSHAW, WHITNEY - Colorado State University
item MAHAFFEY, LINDA - Colorado State University
item CAMPER, MATT - Colorado State University
item OTTO, KRISTEN - Colorado State University
item MCMILLAN, MARK - Colorado State University

Submitted to: Southwestern Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2009
Publication Date: 3/1/2009
Citation: Schwartz, H.F., Gent, D.H., Fichtner, S.M., Hammon, R., Cranshaw, W.S., Mahaffey, L., Camper, M., Otto, K., Mcmillan, M. 2009. Straw Mulch and Reduced-Risk Pesticide Impacts upon Thrips and Iris yellow spot virus on Western-Grown Onions. Southwestern Entomologist. 34(1):13-29.

Interpretive Summary: Iris yellow spot virus (IYSV) and its thrips vector are yield limiting pests of onion throughout the western U.S. Insecticides often are used heavily for management of the vector. In experiments conducted in Colorado during 2005 to 2007, straw mulch applied to the center of onion beds reduced thrips populations up to 33% when compared to non-treated plots of transplanted onions. The addition of conventional insecticides was associated with 12 to 27% higher cumulative thrips-days compared to the untreated control in two experiments. In contrast, a reduced-risk insecticide program had lower cumulative thrips-days on both bare soil and straw mulch compared to untreated controls. Yield and jumbo yield were increased by straw mulch compared to bare soil treatments. This study also documented for the first time an association between thrips populations and the incidence of iris yellow spot. Effective long-term management of thrips and iris yellow spot will depend on a multi-faceted approach that integrates host resistance, modified cultural practices such as straw mulching and irrigation scheduling, and judicious use of reduced-risk pesticides.

Technical Abstract: Iris yellow spot virus (IYSV) and its thrips vector, Thrips tabaci Lindeman, are yield limiting pests of onion throughout the western U.S. In experiments conducted in Colorado during 2005 to 2007, straw mulch applied to the center of onion beds reduced early to mid-bulb growth stage thrips populations up to 33% when compared to non-treated plots of transplanted onions. Cumulative thrips-days indicated that straw mulch significantly reduced populations by 10 to 20% compared with bare soil control plots in these studies. The addition of conventional insecticides (methomyl alternated with lambda-cyhalothrin) was associated with 12 to 27% higher cumulative thrips-days compared to the untreated control in two experiments. In contrast, a reduced-risk insecticide program (spinosad alternated with azadirachtin) had lower cumulative thrips-days on both bare soil (15%) and straw mulch (36%) compared to untreated controls. Enhanced thrips control generally persisted during mid-season and may have contributed to reduced stress from thrips feeding damage and reduced IYSV incidence and/or severity during the early to mid-bulb stages of plant growth. Total yield and jumbo yield were increased up to 13 % and 18 % by straw mulch compared to bare soil treatments among the individual experiments. Peak thrips populations recorded on commercial red onion plants in an insecticide trial during 2004 were positively correlated with the incidence of iris yellow spot 40 days (R2 = 0.5864, P = 0.0060) and 54 days (R2 = 0.6086, P = 0.0046) later, indicating that thrips suppression may provide some control of the disease. Effective long-term management of thrips and iris yellow spot in onion crop systems will depend on a multi-faceted approach that integrates host resistance, modified cultural practices such as straw mulching and irrigation scheduling, and judicious use of reduced-risk pesticides.