Ninhydrin staining and quantification of aphid honeydew

B. juncea leaves were infested with 100 individuals each of L. erysimi (LE) and A. craccivora (AC) in independent experiments. Honeydews were collected on the 3MM whatman paper discs in Petri dishes (90 × 15 mm), placed under infested B. juncea leaves. Similar arrangement was replicated for uninfested B. juncea leaves which served as controls. After 24 h of infestation, whatman papers were collected and soaked in 0.1% (w/v) ninhydrin prepared in acetone and dried at 65 °C in a hot air oven for 30 min²⁴. The whatman paper discs with purple colour spots were scanned for documentation. For quantifying the aphid honeydews, the paper discs were cut into pieces and the stains were extracted in 10 ml of 90% (v/v) methanol for 1 h with periodical shaking. The samples were centrifuged at 6000 rpm for 5 min. and the supernatants were measured at 500 nm against 90% methanol as blank²⁴.