A Spray Calendar That Stopped Working
Selvaraj grows 2 ha of cotton (variety CO 14) in Coimbatore. At 45 days after sowing, leafhopper nymphs average 15 per three leaves, leaves are cupped and covered with sooty mould, and few ladybird beetles remain. He has sprayed monocrotophos 36 SL six times at 8-day intervals, spending ₹9,000, and the last spray was three days ago. He plans to mix monocrotophos with a pyrethroid and spray again tomorrow because the hoppers are still active. The field is bordered by a vegetable plot and a coconut garden. He has heard that neem oil and yellow sticky traps are 'only for organic farmers'. He asks why the sprays are failing, whether he should spray tomorrow, and what he should do for the rest of the season.
1. Why have the repeated monocrotophos sprays failed to control leafhopper?
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The failure is consistent with selection for resistance and disruption of natural control. Six sprays of the same insecticide at 8-day intervals expose successive leafhopper generations to monocrotophos; surviving females pass resistance traits to the next generation, so the population rebounds despite spraying. The sprays also kill ladybird beetles and other predators that normally suppress hoppers, as shown by the few beetles remaining. Early-season hopper flare-up is common after broad-spectrum sprays. The 8-day interval is shorter than needed for a reasoned rotation and may also be inefficient because eggs and nymphs sheltered under leaves are not all contacted. The sooty mould is a consequence of honeydew from the hopper population, not evidence that a stronger mixture is needed.
2. Should he mix monocrotophos with a pyrethroid and spray tomorrow? Justify your advice.
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No. Spraying tomorrow would repeat the same mistake and add a pyrethroid, which is likely to kill more natural enemies and may select for cross-resistance. The last spray was only three days ago, so residues are still present. The population is above the common action threshold, but the response should be a rotated, selective product rather than a tank mixture chosen in frustration. If the field is above threshold, use a labelled selective insecticide with a different mode of action, such as flonicamid or diafenthiuron, directed at the underside of leaves, and follow the label dose. Do not mix unrelated products without advice. Install yellow sticky traps at about 25 per hectare to monitor and mass-trap adults, and conserve remaining predators.
3. Outline a season-long plan to bring the hopper population back under control without escalating costs.
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First, adopt an action threshold instead of a calendar: treat when nymphs reach about 2 per leaf or damage is increasing, not every 8 days. Rotate insecticide groups so that no mode of action is used twice in succession, and avoid pyrethroids and repeated monocrotophos. Use neem oil 3% or neem-based formulations in the early season when populations are low; these are not 'only organic' and fit conventional cotton. Keep yellow sticky traps and inspect weekly. Avoid broad-spectrum sprays that flare secondary pests. Remove or manage alternate hosts around the field, and keep a border of flowering plants where possible to support predators. Record each spray, date and product so that resistance management can be checked. The aim is to reduce spray frequency and cost while preserving natural enemies.