Reducing incidence of iris yellow spot virus in onion fields by targeting its thrips vector early in the season

Iris yellow spot virus (IYSV) epidemics are a major problem in western New York onion fields. IYSV is transmitted by onion thrips, Thrips tabaci Lindeman, and its management relies on controlling T. tabaci infestations using a season-long insecticide program. Thrips management begins with two applications of a spirotetramat-based insecticide, which is systemic and highly effective against T. tabaci eggs and larvae, but not adults. Because viruliferous adults that colonize onion fields early in the season may not be impacted by this strategy, we hypothesized that methomyl applied before and with spirotetramat applications would control adults and reduce IYSV levels long enough to prevent yield loss. Each year the incidence of IYSV was evaluated twice during the season in pairs of onion fields in which one field in each pair received applications of methomyl early in the season (referred to as “treatment” fields) and the other did not (referred to as “control” fields). Marketable bulb yields also were compared between the two treatments. Results indicated that IYSV incidence in treatment fields was significantly lower (65 to 68%) than incidence in control fields, and incidence was higher in the second sampling period (62 to 85%) than in the first. Marketable yield was 23% higher in treatment fields than control fields, but the difference was not statistically significant. Applying insecticides early in the season during T. tabaci adult colonization may reduce levels of IYSV long enough to either prevent or minimize yield loss.