Mustard Aphid, Pollinators and a Farmer Reaching for a Strong Spray
A farmer in Mainpuri has 1.0 ha of mustard (var. Rohini) sown on 12 October at 5 kg/ha. On 5 January, about 55 days after sowing, the crop is at late pre-flowering, temperature is 16–22°C with 60% humidity, and roughly 30% of terminal shoots carry colonies of Lipaphis erysimi, with honeydew and early sooty mould on the lower leaves. He applied 50 kg N/ha at sowing and 50 kg N/ha at first irrigation; the crop is dark green and succulent. He has monocrotophos 36 SL and imidacloprid 17.8 SL in his store, a knapsack sprayer, and two boxes of Apis mellifera at the field edge that he uses for pollination and honey. A neighbour sprayed twice last season and the aphids returned within a week. He wants one strong spray now, before flowering, to "finish the pest".
1. Decide whether a spray is justified at this point, using an economic threshold, natural-enemy evidence and the field's own history.
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Use a threshold instead of a percentage. The working economic threshold for mustard aphid is about 20–25 aphids per 10 cm of terminal shoot (roughly 50–60 aphids per plant) at the pre-flowering to flowering stage. Thirty per cent of shoots carrying colonies is a distribution figure, not a severity figure; the decision needs the colony size on those shoots. If colonies are small and mostly on lower leaves, the field is below threshold and should be re-scouted in 3–4 days.
Look for the reason behind resurgence. Broad-spectrum sprays such as monocrotophos kill coccinellids, syrphid larvae and Chrysoperla, which normally hold aphids down. With the predators gone and immigrant winged aphids arriving continuously, colonies rebuild within a week - exactly the neighbour's experience. Repeated sprays also select for resistance.
Check the crop, not only the insect. Two 50 kg N doses have produced dark, succulent growth, which is genuinely more attractive to aphids. Part of the cure is nutritional: stop further nitrogen and let the crop harden.
Judgement: do not spray on 5 January on this evidence. Scout twice weekly, record aphids per 10 cm shoot and predator counts, and keep the spray decision ready for the flowering stage if the threshold is crossed.
2. If the threshold is crossed next week, what would you recommend, keeping the honeybee boxes and resistance management in view?
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Choose selective chemistry and a bee-safe window. Monocrotophos is highly toxic to bees and to natural enemies, and repeated use invites resistance; imidacloprid is also hazardous to bees during bloom, although it is effective against aphids. The safe sequence is:
- Before flowering, if the threshold is crossed, use thiamethoxam 25% WG at 100 g/ha or imidacloprid 17.8% SL at 100–125 ml/ha, sprayed in the evening when bees have stopped foraging, with 500 L water per hectare for full terminal coverage.
- From the start of flowering, stop neonicotinoids. Shift to a selective aphicide such as flonicamid 50% WG at 60 g/ha or pymetrozine, which spare bees and most predators.
- Never tank-mix two aphicides "for strength"; alternate modes of action between seasons if a second spray becomes necessary, and do not exceed two applications of the same group.
Practical safeguards. Move the hives to the field edge opposite the spraying direction and cover them during the spray day; warn neighbouring beekeepers. Observe the label waiting period before harvest, and note that mustard is harvested about 110–120 days after sowing, so a mid-February spray still needs checking against that interval.
Support the predators. Keep a border strip of coriander or berseem as refuge, avoid pyrethroid sprays, and release Chrysoperla if available.
3. What agronomic and monitoring changes would you build into the next mustard season to reduce dependence on insecticide?
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Make the crop less attractive and the field easier to scout.
- Cap nitrogen at about 60–80 kg/ha in two splits, with the second dose at first irrigation and none after flowering; the dark, succulent canopy in this field is a direct result of 100 kg N/ha.
- Sow on time (early to mid-October) so that the crop passes its vulnerable bud stage before aphid populations peak in January.
- Keep plant population uniform - a dense, lodged crop holds humidity and suits aphids.
- Grow a mustard-based intercropping or border pattern, for example mustard with gram or a coriander border, to increase predator and pollinator activity.
Monitor systematically. Put out yellow sticky traps and check 10 random plants per hectare twice a week from 40 days after sowing, recording aphids per 10 cm shoot and the number of coccinellid larvae. Keep a written threshold chart in the field.
Use IPM in order: tolerant variety, timely sowing, balanced nutrition, trap and border crops, predator conservation, and only then a selective aphicide in the evening. Budgeting for scouting and one selective spray usually costs less than two broad-spectrum sprays plus the honey lost from disturbed colonies - and it avoids the resistance spiral the neighbour is already in.