Braconid Wasp
Cotesia vestalis
Beneficial Insects Beneficial 2–3 mm (adult); 3–4 mm (cocoon cluster)
📖 Description
Cotesia vestalis (formerly Cotesia plutellae) is the most important larval endoparasitoid of the diamondback moth (Plutella xylostella) — the world’s most destructive brassica pest. The diamondback moth causes global annual losses of US$4–5 billion, and Indian brassica vegetable growers spend ₹1,500–2,000 crore per year on insecticides — much of which is now ineffective due to diamondback moth resistance. C. vestalis is the natural enemy most often credited with suppressing diamondback moth populations in unsprayed brassica fields. Indian research institutes (NBAIR Bangalore, IIHR Bangalore, IARI New Delhi) have mass-reared and released this parasitoid for over 30 years. The species is gregarious — a single host larva supports 15–30 parasitoid larvae, which emerge together and form a characteristic cocoon cluster. The lifecycle from egg to adult takes 14–18 days at 25 °C; adults live 10–20 days and females lay 200–400 eggs by parasitising 10–25 host larvae. The species was formerly placed in the genus Apanteles (as Apanteles plutellae) — older literature and many field guides still use this name.
⚠️ Symptoms to watch for
- Adult female wasp lays 15–30 eggs inside a single second- or third-instar diamondback moth larva; eggs hatch within 24 hours and larvae feed internally on the host's haemolymph and fat body; after 7–10 days, parasitoid larvae emerge through the host's body wall, spin cocoons on the leaf surface alongside the dying host, and pupate. Parasitised host larvae feed less, grow slower, and fail to pupate. The presence of characteristic yellowish cocoon clusters alongside dying diamondback moth larvae is diagnostic.
🩹 Treatment
- Conservation biological control — avoid broad-spectrum insecticides in brassica vegetable crops, especially during the seedling and head-formation stages. Use selective insecticides (Bacillus thuringiensis var. kurstaki, spinosad, emamectin benzoate, chlorantraniliprole) when pest control is needed. Plant nectar-producing companion crops (alyssum, buckwheat, mustard) to provide adult food. Maintain refuge strips of unsprayed brassicas to maintain parasitoid populations.
🌿 Organic treatments
- Conservation biological control as above. Purchase and release laboratory-reared adults at 50,000–100,000 per ha for management of diamondback moth on cabbage. Release in early morning or evening when temperatures are moderate. Release at the seedling stage (3–4 weeks after transplanting) and repeat at 14-day intervals during the active season. Avoid releasing during heavy rain or strong wind.
🧪 Chemical treatments
- Not applicable — this is a beneficial biocontrol agent, not a pest. However, avoid using broad-spectrum insecticides (organophosphates, carbamates, synthetic pyrethroids) in fields where this parasitoid is present. Use selective insecticides when pest control is needed: Bacillus thuringiensis var. kurstaki (1 g/L), spinosad 45 SC (0.3 ml/L), emamectin benzoate 5 SG (0.3 g/L), or chlorantraniliprole 18.5 SC (0.3 ml/L).
🛡️ Prevention
- Not applicable — this is a beneficial parasitoid that should be conserved, not prevented. Conservation strategies: plant diverse flowering nectar sources (alyssum, buckwheat, mustard, coriander), avoid broad-spectrum insecticides, maintain refuge strips of unsprayed brassicas, integrate with other IPM tools (Bacillus thuringiensis, spinosad). In commercial production, integrate this parasitoid with selective insecticides and other biocontrol agents (Trichogramma) for IPM of diamondback moth on brassica vegetables.
🪴 Affected plants
- Not a plant pest — parasitoid of Lepidoptera larvae. The primary host in India is the diamondback moth (Plutella xylostella) on cabbage, cauliflower, broccoli, and other brassica vegetables. Also recorded on Plutella porrectella, Crocidolomia binotalis, and other small lepidopteran larvae on brassicas.
🦋 Natural predators
- Adult wasps are preyed upon by spiders (Theridiidae, Salticidae) and predatory bugs (Anthocoridae). Cocoons are attacked by hyperparasitoids — particularly Tetrastichus sokolowskii (Eulophidae), which can parasitise 30–50 % of C. vestalis cocoons in unsprayed fields. The braconid Diadegma semiclausum competes with C. vestalis for diamondback moth larvae and is the dominant parasitoid in some regions.
🌱 Host plants
- Not host-specific to plants — parasitoid of diamondback moth larvae on cabbage, cauliflower, broccoli, Brussels sprouts, knol-khol, radish, mustard, and other brassica vegetables and oilseed crops.
🔄 Lifecycle
- Egg (1 day, laid inside host larva) → Larva (7–10 days, 3 instars, internal parasitoid) → Pupa (5–7 days, in yellowish cocoon on leaf alongside dying host) → Adult (10–20 days, females parasitise 10–25 host larvae).
- Multivoltine — 10–15 overlapping generations per year in tropical India, synchronised with diamondback moth populations.
📅 Season
- Active throughout the year in tropical India; peak activity September–March (cool dry season, when brassica vegetables are grown).
- Lowest activity during the hot summer (April–June) when brassica cultivation is minimal.
- Population dynamics closely track diamondback moth populations — high parasitoid numbers are seen 2–3 weeks after diamondback moth outbreaks.
🔎 Appearance
- Adult is a tiny, slender, black braconid wasp with clear wings and long antennae; the abdomen is dark brown to black with pale yellowish legs; the ovipositor is barely visible.
- Cocoons are the most commonly observed stage — yellowish-white, oval, 3–4 mm long, and laid in characteristic clusters of 8–20 alongside the parasitised host caterpillar on the leaf surface.
- Each cluster represents the brood from a single host larva.
💥 Damage level
- Beneficial
🔁 Reproduction rate
- Fast
🗺️ Geographic regions
- Cosmopolitan where diamondback moth occurs — recorded from all continents. In India
- common across all states where brassica vegetables are grown; particularly abundant in Karnataka (Bangalore Rural
- Kolar)
- Tamil Nadu (Nilgiris
- Krishnagiri)
- Maharashtra (Nashik
- Pune)
- West Bengal (Hooghly
- Nadia)
- Punjab (Ludhiana
- Jalandhar)
- and Himachal Pradesh (Kullu
- Shimla).
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