Rheological Properties and Postharvest Behavior of Onion (Allium cepa L. 'Superex') Subjected to Different Preharvest Treatments to Control Armyworm [Spodoptera exigua (Hubner Lepidoptera: Noctuidae

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Heavy reliance on synthetic pesticides has led to insecticide resistance of pests such as the onion armyworm, leading to the use of alternative strategies such as biopesticides. While studies on the efficacy of these biopesticides on pest control and yield exist, their subsequent effects on the quality and postharvest behavior of bulb onions during cold storage need to be determined. Allium cepa L. ‘Superex’ yellow type onion plants were subjected to preharvest treatments of microbial (NPV crude), botanical (neem oil), synthetic insecticide (chlorfenapyr), and Farmer’s Practice (control) for the control of onion armyworm (Spodoptera exigua (Hubner)). Bulbs were then harvested at commercial maturity, cured at ambient condition (33.6°C, 48.5% RH) for 11 d, then stored at 4.7 ± 0.6°C, 91.7 ± 1.6% RH for 5 mo. Rheological properties of the bulbs after curing and prior to storage did not vary significantly across treatments. Stiffness, however, decreased by 50 − 70% during storage, with bulbs from NPV crude having higher values. Weight loss during the 5-month storage ranged from 10 − 12%. Pungency of the bulb increased with increasing duration of cold storage concomitant with the onset of internal sprouting on the 5th mo. Bulbs preharvest-treated with NPV crude had the highest pungency of 4.85 μmol g-1FW on the third month and increased to 6.72 μmol g-1FW on the 4th mo. The preharvest treatments nor the storage duration did not affect the total soluble solids content of the bulbs which ranged from 6.9 − 7.1°Brix. Internal sprouting occurred on the 5th mo at 75 − 100% but external sprouting was very low at 0 − 1.0%. At ambient condition post-storage, it took 45 − 63 d for bulbs to exhibit external sprouting after being taken out of cold storage on the 4th mo. After 5 mo of cold storage, bulbs sprouted after 6 − 19 d at ambient condition. New root growth occurred on the 4th mo in all stored bulbs regardless of the preharvest treatment. Disease incidence during the 5-month storage was 0 − 1.1%. The recommended storage is 4 mo for the yellow type ‘Superex’ onions with bulbs having a shelf life of almost 2 mo at ambient retail condition (33.6°C, 48.5% RH).