Determination of beginning of bulb enlargement time in shallot, Allium cepa var aggregatum for managing onion thrips (Thrips tabaci)

Although knowledge of bulbing time is important in developing effective insect pest management strategy and for predicting maturity time, such information has not yet been generated on shallots grown under field condition. Thus, this study was conducted for four consecutive years (2012–2015) at Debre Zeit, Ethiopia, to determine the onset of bulb enlargement time in shallot. Cultivars Minjar and Huruta were planted at the rate of 13 plants per m2 on ridged plots of 2×3.2m2. Each variety was replicated 12 times. From each plot five shoots were randomly selected and the number of onion thrips (Thrips tabaci) was counted in situ. Then the sample plants were carefully uprooted and neck diameter was measured using vernier digital caliper at the point where the leaf sheath turns to green color, while bulb diameter was measured approximately 1cm above the basal plate (root base). These sample plants were also used to determine bulb and shoot dry weight. In all the seasons and on both cultivars bulb enlargement began about 46–49days after sprouting, which is equivalent to 753–769° days. At this stage bulblets “push each other apart”. However, the number of days required to reach bulb ratio of ≥2.0 was variable between cultivars and across seasons. Thus, on Minjar bulb ratio became greater or equal to two 64 (981.75° days), 85 (1307.5° days), 74 (1128.5° days) and 81 (1285.15° days) days after sprouting in 2012, 2013, 2014 and 2015 seasons, respectively. The corresponding values for Huruta were 64 (981.75° days), 95 (1469.5° days), 89 (1312.0° days) and 81 (1285.15° days) days. Therefore, we suggest bulblet split time, which is the time at which the bulblets begin to “push each other apart,” as reliable indicator of onset of bulb formation in shallot rather than the arbitrary bulb ratio of 2.0. The intensity of onion thrips was very low throughout the growing season and across years. But, depending up on seasons, it tends to peak one to three weeks later than the onset of bulb enlargement. Peak onion thrips intensity was recorded when the bulb ratio reached 2.0 or when the bulb dry weight to shoot dry weight equals 1.5.