What’s 1+ 1+ 1= ? As applied entomologists, we often hope for the answer being ‘3.5’ or at least ‘3’. We aim for pest management programs that are at least equal to or greater than the sum of their parts. So if we combine host plant resistance, a cultural control, and a chemical control, we desperately hope that this will produce an ideal pest management program, resilient against even the worst outbreak. In this paper, we did just that: combined onion cultivars (host plant resistance) with different nitrogen regimes (cultural control) and insecticide programs (chemical control). Warning: hopes and dreams were dashed. 😉
Figure 1: Mean densities of onion thrips during the season in onion cultivars with varying susceptibility to onion thrips, ‘Avalon’ (top), ‘Delgado’ (middle), and ‘Bradley’ (bottom) in 2015 and 2016. Densities of larvae are shown for plots that were either not treated with insecticides (control) or treated following either a standard or threshold-based insecticide program, whereas densities of adults are shown for plots pooled across all insecticide treatments. Studies were conducted in commercial fields near Elba, NY. Monitoring thrips densities began when onions had 4-5 leaves, and concluded near harvest. Standard and action threshold-based insecticide programs were initiated on 15 July 2015 and 5 July 2016. Insecticide applications were made weekly in the standard program and only when thrips densities were ≥1 larva/leaf in the action threshold program. See Table 2 for average number of insecticide applications.
Abstract
Onion thrips (Thrips tabaci) is a significant pest of onion worldwide, causing both direct and indirect damage to the crop. Integrated pest management of onion thrips should improve profitability and sustainability of onion production. Promising management approaches include reducing nitrogen application rates, using thrips-resistant cultivars and implementing action threshold-based insecticide programs. However, the impact of these integrated pest management approaches on thrips densities and damage, crop yield, and thrips-associated plant diseases like iris yellow spot (IYS) (caused by Iris yellow spot virus) and bacterial center rot (caused by Pantoea agglomerans and P. ananatis) remains largely unknown. In a two-year field trial in New York, combinations of varying levels of nitrogen applied at planting (67, 101 and 140 kg ha−1) and different insecticide programs (standard weekly insecticide program and action threshold-based insecticide program) were evaluated for onion thrips management in onion cultivars that had moderate resistance (‘Avalon’), low resistance (‘Delgado’) and no resistance (‘Bradley’) to onion thrips. Results indicated that regardless of cultivar, nitrogen did not affect larval thrips densities, onion yields, IYS or bacterial center rot. Across all cultivars, insecticide use (both programs) significantly reduced larval thrips densities and damage, IYS severity and incidence, and increased onion yield. Insecticide use did not consistently affect the incidence of bacterial center rot. Both insecticide programs reduced onion thrips larval densities by 60–81% relative to the untreated control, but the action threshold-based application program used 2.8 fewer applications than the standard program. ‘Avalon’ had low thrips densities and IYS disease, but required the same number of insecticide applications as ‘Bradley’. Onion yields in both insecticide programs were statistically similar in both years, and bulb weights averaged 10–54% more than those in the untreated control. Our results indicated that growers can reduce nitrogen levels at planting and insecticide use without compromising control of either onion thrips or IYS disease or onion bulb yields.
