Yam Scale
Aspidiella hartii (Cockerell) [Aspidiotus hartii]
📖 Description
Yam Scale (Aspidiella hartii) is a serious pest of stored yam tubers and growing yam plants across the Indian subcontinent and tropical Africa. Particularly severe in the major yam-growing belts of Kerala, Tamil Nadu, Andhra Pradesh, Odisha, West Bengal, Jharkhand, and the Northeast in India; and in West Africa (Nigeria, Ghana, Côte d’Ivoire — major yam-producing region). India is a major yam producer with growing belts in Kerala, Tamil Nadu, Andhra Pradesh, Odisha, West Bengal, Jharkhand, and the Northeast. Yams (Dioscorea spp.) are a staple food in many tropical regions — particularly West Africa where yam is a major staple feeding over 100 million people. The pest is unusual among yam pests because (a) it is an armoured scale insect (family Diaspididae) — the female body is completely hidden under a protective waxy scale, making it difficult to control with contact insecticides (the scale must be penetrated to reach the insect), (b) the pest is most damaging in stored yam tubers — where it can cause 20-40% weight loss over 3-4 months of storage and 50-80% reduction in sprouting of seed tubers (directly affecting the next crop), (c) a single female lays 30-60 eggs under the protective scale, with crawlers (first instar nymphs) dispersing to settle and form new scales — crawlers are the only mobile stage, (d) the pest attacks multiple Dioscorea species and other aroids (Colocasia, Amorphophallus) — making crop rotation less effective than for host-specific pests, (e) the pest is most damaging during storage when tubers are dormant and natural enemy populations are absent — making seed tuber treatment essential for control, and (f) infested seed tubers produce weak plants with reduced tuber yield — making the pest a multi-season problem. IPM with scale-free seed tubers (most critical), seed tuber treatment (malathion or neem oil dip before storage), cool dry storage, monthly inspection and removal of infested tubers, biological control (ladybird beetles, Aphytis parasitoids), neem-based sprays, and need-based selective insecticide application (Buprofezin, Acetamiprid) provides effective control. The pest is a particular concern for Indian yam farmers because (i) India is a major yam producer and yam is a staple food in many parts of the country, (ii) the pest is most damaging in storage — where yam tubers are typically stored in traditional structures that favour scale reproduction, (iii) infested seed tubers produce weak plants — making the pest a multi-season problem affecting both storage and field crop, (iv) much Indian yam farming is done by small and marginal farmers with limited access to IPM knowledge and storage facilities, and (v) the protective waxy scale makes the pest difficult to control with contact insecticides — requiring systemic insecticides or seed tuber treatment for effective control.
⚠️ Symptoms to watch for
- Direct feeding damage: sap-sucking on stored yam tubers and on yam stems and leaves
- on stored tubers, scales cluster densely and cause characteristic 'greyish' discolouration of tuber skin, shriveling and weight loss of tubers (20-40% weight loss in severely infested stored tubers over 3-4 months), reduced sprouting of seed tubers (50-80% reduction in sprouting — directly affects next crop), on plants, yellowing and curling of leaves, stunting of plants, reduced tuber yield (20-40% reduction in severely infested crops).
- Crop loss: 20-40% tuber yield loss in severely infested crops
- 20-40% weight loss in severely infested stored tubers
- ₹30-50 crore annual loss to Indian yam industry.
- Particularly damaging to seed tuber storage — infested seed tubers produce weak plants.
🩹 Treatment
- Dip seed tubers in 0.1% malathion (1 ml/L malathion 50 EC) for 10 minutes before storage (kills scales on tuber surface — most effective treatment)
- dip seed tubers in neem oil 1% + 5 g/L soap for 10 minutes before storage (organic — deters scale settlement)
- dust seed tubers with malathion 5% dust @ 5 g/kg tubers (kills scales on tuber surface)
- spray neem oil 1% on infested plants (focus on stems and leaf undersides)
- spray dimethoate 30 EC @ 1 ml/L on infested plants (systemic — reaches scales feeding on plant sap)
- store tubers in cool dry well-ventilated conditions (reduces scale reproduction).
🌿 Organic treatments
- Dip seed tubers in neem oil 1% + 5 g/L soap for 10 minutes before storage (organic — deters scale settlement — repeat at 30-day intervals during storage)
- dip seed tubers in cow urine + neem oil emulsion for 10 minutes (traditional organic practice — repels scales)
- dust seed tubers with wood ash + 5% neem leaf powder (smallholder practice — deters scale settlement)
- apply Verticillium lecanii @ 5 g/L as tuber dip (entomopathogenic fungus — infects scales)
- apply Beauveria bassiana @ 5 g/L as tuber dip (entomopathogenic fungus)
- spray neem oil 1% on infested plants (focus on stems and leaf undersides — repeat at 15-day intervals)
- apply NSKE 5% spray on infested plants (azadirachtin-based — deters scale settlement)
- release Aphytis parasitoids @ 50/ha (parasitoid — limited commercial availability in India)
- conserve native ladybird beetles (Chilocorus, Scymnus — feed on scales)
- apply essential oil blends (clove, peppermint — repel scales)
- store tubers in cool dry well-ventilated conditions (reduces scale reproduction)
- inspect stored tubers monthly and remove severely infested tubers.
🧪 Chemical treatments
- Dip seed tubers in 0.1% malathion (1 ml/L malathion 50 EC) for 10 minutes before storage (most effective chemical treatment — kills scales on tuber surface — observe 7-day withholding period before planting), dust seed tubers with malathion 5% dust @ 5 g/kg tubers (alternative to dip — easier to apply), spray dimethoate 30 EC @ 1 ml/L on infested plants (systemic — reaches scales feeding on plant sap), spray monocrotophos 36 SL @ 0.5 ml/L on infested plants (systemic — hazardous, restricted use), spray acetamiprid 20 SP @ 0.2 g/L on infested plants (systemic — relatively safe to natural enemies), spray buprofezin 25 SC @ 1 ml/L on infested plants (insect growth regulator — kills nymphs, safe to predators).
- Note: Once scales are established on stored tubers, chemical control is difficult — prevention through seed tuber treatment is most effective.
- Rotate modes of action to slow resistance.
- Observe withholding period before consumption or planting.
🛡️ Prevention
- Use scale-free seed tubers (inspect all planting material for scale colonies — reject infested tubers — most important preventive measure)
- dip all seed tubers in 0.1% malathion or 1% neem oil + soap before storage (kills scales on tuber surface — prevents infestation of stored tubers)
- store tubers in cool dry well-ventilated conditions (below 25°C and 60% RH — slows scale reproduction)
- inspect stored tubers monthly and remove severely infested tubers (prevents spread to other tubers)
- practice crop rotation with non-hosts (cereals, pulses — breaks scale life cycle)
- destroy crop residue after harvest (removes scale colonies on plant material)
- avoid continuous yam cultivation in same field (builds up scale population)
- use balanced fertilization (avoid excess nitrogen which favours scale population buildup)
- conserve natural enemies
- (ladybird beetles — Chilocorus, Scymnus — feed on scales; Aphytis parasitoids)
- avoid broad-spectrum insecticides (kill natural enemies)
- use selective insecticides when pesticide application is necessary (Buprofezin, Acetamiprid — safe to natural enemies)
- implement Integrated Pest Management (IPM) with seed tuber treatment
- storage hygiene
- monitoring
- biological control
- and need-based selective insecticide application
- plant yam at proper spacing (reduces humidity and scale incidence)
- harvest tubers at proper maturity (over-mature crops are more susceptible to scale infestation).
🪴 Affected plants
- Yam (Dioscorea spp. — particularly Dioscorea alata — greater yam/water yam, Dioscorea esculenta — lesser yam, Dioscorea rotundata — white yam, Dioscorea cayenensis — yellow yam; major tuber crops of India and tropical Africa)
- also reported on Colocasia (taro), Amorphophallus (elephant-foot-yam), and other aroids.
- India is a major yam producer with growing belts in Kerala, Tamil Nadu, Andhra Pradesh, Odisha, West Bengal, Jharkhand, and the Northeast.
- Yams are a staple food in many tropical regions.
🦋 Natural predators
- Ladybird beetles (Chilocorus nigritus, Chilocorus circumdatus, Scymnus spp. — feed on scales — important natural control), Aphytis parasitoids (parasitise scales — limited commercial availability in India), predatory bugs (Mirid, Anthocorid — feed on crawlers), spiders (capture crawlers), entomopathogenic fungi (Verticillium lecanii, Beauveria bassiana — infect scales).
- Conservation of natural enemies is essential for sustainable scale management.
🌱 Host plants
- Dioscorea alata (greater yam/water yam)
- Dioscorea esculenta (lesser yam)
- Dioscorea rotundata (white yam)
- Dioscorea cayenensis (yellow yam)
- Colocasia esculenta (taro)
- Amorphophallus paeoniifolius (elephant-foot-yam)
- other aroids
🔄 Lifecycle
- Adult female lays 30-60 eggs under the protective waxy scale.
- Egg incubation 3-5 days.
- Crawler (first instar nymph) is mobile for 24-48 hours, then settles and forms a scale.
- Female passes through 2 nymphal instars over 20-30 days, then becomes adult.
- Male passes through 2 nymphal instars, prepupa, and pupa over 25-35 days, then emerges as a tiny winged adult (rare).
- Adult female lifespan 60-90 days (lays 30-60 eggs); adult male lifespan 2-5 days (does not feed, only mates).
- Total life cycle 25-40 days at 25-30°C; 8-10 overlapping generations per year in tropical India.
- Pest persists year-round on stored tubers and on volunteer plants.
📅 Season
- Year-round in tropical India; peaks in storage during Nov-Mar (cool dry winter — most favourable for scale reproduction on stored tubers).
- Pest activity on growing plants peaks Aug-Oct (post-monsoon — when plants are in active growth).
🔎 Appearance
- Adult female is a minute, circular, light brown to greyish scale (family Diaspididae — armoured scale insects) with a central or sub-central pale yellow exuviae (shed skin); body completely hidden under the protective waxy scale; scale is flat, circular, 1.5-2.5 mm diameter.
- Adult male is a tiny winged insect (rare).
- Crawlers (first instar nymphs) are mobile, yellowish, with legs and antennae.
- Distinct from other scales by occurrence on yam tubers and characteristic circular flat scale.
- Scales cluster on stored yam tubers and on yam stems and leaves, often completely covering the tuber surface.
💥 Damage level
- Moderate
🔁 Reproduction rate
- Fast
🗺️ Geographic regions
- Indian Subcontinent
- South Asia
- Southeast Asia
- Tropical Africa
- Cosmopolitan in Tropical and Subtropical Regions Worldwide
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