The pattern of a whitefly season
Whitefly numbers in cotton rarely explode without warning. Adults appear on the underside of upper leaves from 50–60 days after sowing, and populations build over two to three weeks before honeydew and sooty mould become visible.
The economic threshold commonly followed is 6–8 adults per leaf on the top three leaves. Scouting twice a week from day 50 costs an hour and saves a panic.
Why nymphs decide the outcome
Adult knockdown looks satisfying and changes little. The nymphs fixed on the underside of leaves are the next generation, and a spray that only removes adults invites a rebound within ten days.
This is why combination chemistry pairing an insect growth regulator (against eggs and nymphs) with a knockdown partner (against adults) — such as Pyriproxyfen 5% + Fenpropathrin 15% EC at 300 ml per acre — is designed the way it is. Coverage of the leaf underside, with adequate water volume, matters as much as the product.
Cultural pressure points
Avoid excess nitrogen in mid-season — lush, soft growth is whitefly habitat. Keep field borders free of alternate hosts, and avoid repeated sprays of broad-spectrum products early in the season that remove the parasitoids doing free work.
Note
Doses stated are indicative label rates. Always confirm against the product label before spraying. Use agrochemicals responsibly. Read the label before use.