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

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 degrees C, 48.5% RH) for 11 days, then stored at 4.7 +/- 0.6 degrees C, 91.7 +/- 1.6% RH for 5 months. 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 five-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 fifth month. Bulbs preharvest-treated with NPV crude had the highest pungency of 4.85 mu mol g(-1)FW on the third month and increased to 6.72 mu mol g(-1)FW on the fourth month. 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 degrees Brix. Internal sprouting occurred on the fifth month at 75 - 100% but external sprouting was very low at 0 - 1.0%. At ambient condition post-storage, it took 45 - 63 days for bulbs to exhibit external sprouting after being taken out of cold storage on the fourth month. After five months of cold storage, bulbs sprouted after 6 - 19 days at ambient condition. New root growth occurred on the fourth month in all stored bulbs regardless of the preharvest treatment. Disease incidence during the five-month storage was 0 - 1.1%. The recommended storage is four months for the yellow type 'Superex' onions with bulbs having a shelf life of almost two months at ambient retail condition (33.6 degrees C, 48.5% RH).