The cabbage moth, also known as the diamondback moth, is scientifically named *Plutella xylostella* (Lepidoptera: Plutellidae). It is a significant pest that affects a wide range of cruciferous vegetables. The moth has an extremely broad host range, with over 100 species across 45 plant families. Commonly affected crops include cabbage, broccoli, cauliflower, radishes, rape, eggplants, beans, melons, and potatoes.
**Harmful Characteristics:**
Newly hatched larvae tend to congregate on the underside of leaves, feeding on the upper epidermis. As they grow, older larvae become more destructive, creating holes in leaves. By the fourth instar, they begin to feed at night, leaving only the veins and petioles behind. Mature larvae can bore into the heart of cabbages or other brassicas, excreting waste that promotes bacterial rot, significantly reducing the crop's market value.
**Morphological Features:**
Adults measure 18–25 mm in length with a wingspan of 45–50 mm. Their forewings are gray-brown with a distinctive black-edged white line and three small white dots near the base. The hind wings are pale gray without markings.
**Eggs:**
Eggs are hemispherical, measuring about 0.6–0.7 mm in diameter. They have radial ridges on the surface and start off yellow-white before turning dark purple before hatching.
**Larvae:**
The final larval stage can reach up to 29.1 mm in length. Newly hatched larvae are dark green, while later stages vary in color from light green to dark brown. They have distinct white lines along the back and black stripes on either side. Younger larvae move like rulers due to their lack of certain leg segments.
**Pupae:**
Pupae are about 20 mm long, reddish-brown with a dark central stripe and two long spines at the rear.
**Life Cycle and Habits:**
In Tibet, the cabbage moth completes one generation per year, while in regions like Gansu, it may have 1–2 generations. In northern China, there are typically 2–3 generations annually. Overwintering occurs in the soil, often beneath host plants or weeds. Adults emerge in spring when temperatures rise above 15°C, with peak activity during the night. They are attracted to sweet substances and are less responsive to light. Females lay eggs in clusters on the undersides of leaves, producing up to 3,000 eggs in their lifetime. The eggs develop best between 23.5–26.5°C, taking 4–5 days to hatch.
Larvae feed on leaves, causing damage that increases with each instar. By the 6th instar, they consume most of the available foliage, making this the most damaging stage. The larvae prefer temperatures between 20–24.5°C and complete their development in 20–30 days. After maturing, they burrow into the topsoil to pupate, with the pupal stage lasting about 10 days, though some may remain dormant for several months.
The moth thrives in warm, humid conditions, with optimal temperatures between 18–25°C and humidity levels of 70–80%. Temperatures below 15°C or above 30°C, and humidity outside the 65–85% range, are unfavorable.
**Natural Enemies:**
Various natural enemies help control the population, including parasitic wasps such as *Trichogramma*, and predators like ladybugs, ants, and spiders. In Chongqing, certain nematodes and worms play a key role in controlling second-generation larvae.
**Control Methods:**
1. **Cultural Control:** After harvest, deep plowing and weed removal can reduce overwintering populations. Collecting and destroying infested leaves and larvae helps prevent spread.
2. **Physical Trapping:** Use of black lights or sweet-vinegar traps (mix ratio: 10:1:1:8 or 6:3:1:10) during adult emergence can effectively capture moths.
3. **Biological Control:** Apply *Bacillus thuringiensis* (Bt) before the third instar for effective control. Releasing *Trichogramma* wasps at 6–8 points per 667 square meters, 2000–3000 per point, every 5 days for 2–3 cycles, is also recommended.
4. **Chemical Control:** Apply insecticides during the third instar when larvae are most vulnerable. Rotate pesticides to avoid resistance. Ensure sufficient water volume and even spraying for maximum efficacy.
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