The adult form of the cabbage caterpillar, also known as *Pieris rapae* (L.), belongs to the order Lepidoptera and family Pieridae. Commonly referred to as the cabbage white butterfly, it primarily targets cruciferous vegetables, with over 35 host species across nine plant families. Among these, cabbages are the most commonly affected.
**Harmful Characteristics:**
The larvae feed on leaves, starting with the first and second instars that consume the leaf surface, leaving behind transparent spots. By the third instar, they begin creating holes or notches in the leaves, and severe infestations can result in nearly complete defoliation. The veins and petioles often remain intact, while larval frass contaminates the foliage, reducing both quality and marketability. Additionally, wounds caused by the larvae can lead to bacterial soft rot, increasing damage significantly.
**Morphology:**
The cabbage white butterfly is a medium-sized butterfly, measuring about 20 mm in body length and 50 mm in wingspan. It has a gray body and white wings. Female butterflies have a grayish-black tip on their wings with two prominent dark spots, while males typically have only one. The larvae are greenish in color, covered in fine, rough hairs. The final instar can grow up to 35 mm long and go through five developmental stages.
**Behavior:**
This pest has 4–5 generations per year in northern China and 8–9 in southern regions, with overlapping generations. They overwinter in sheltered areas like fences, eaves, and bark crevices near vegetable fields. In Guangzhou, wintering is less distinct. The timing of infestation varies by region—late spring to early summer (April–June) and late autumn to early winter (September–November) are peak periods around the Great Wall. In the northeast, activity peaks in July and September, while in Jiangxi, it occurs in July and August. Adults are active during the day, feeding on nectar, and rest under leaves or in grasses at night or during rainy weather. Females lay eggs on the underside of leaves, often while flying, with the highest egg counts found on broccoli and cauliflower. Young larvae tend to hang down, while older ones drop when disturbed. When mature, they spin silk threads to pupate, wrapping themselves in a cocoon before emerging as adults.
Environmental factors such as temperature, humidity, and food availability strongly influence population dynamics. Natural enemies include parasitic wasps like *Cotesia glomerata*, which attack larvae, as well as fungal pathogens and predatory insects such as ladybugs and lacewings. Optimal conditions for larvae are temperatures between 20–25°C and relative humidity around 76%. High temperatures above 32°C, low humidity below 68%, or limited food sources can lead to high larval mortality.
**Control Methods:**
1. **Field Sanitation:** After harvest, remove weeds, old plant debris, and plow the soil deeply to reduce pest populations.
2. **Biological Control:** Apply *Bacillus thuringiensis* (BT) emulsion or *Pyricularia oryzae* powder according to instructions. Infected larvae containing polyhedral granule virus can be used for spraying. Combining these with low concentrations of pyrethroid insecticides in the evening can enhance effectiveness.
3. **Chemical Control:** Apply pesticides before the second instar stage. Options include 2.5% emulsifiable concentrate diluted 1000–1500 times, or 5% suspension for heavy egg infestations. During the early hatching period, use 5% suspension diluted 1500–2000 times. For young larvae, apply 5% sodium tetrachloromethane at 4000 times dilution or 25% diflubenzuron at 800 times. These options are effective without harming natural predators or polluting the environment. Other options include 60% emulsifiable concentrate, 20% cypermethrin, 25% emulsion, 40% emulsion, 20% chloramphenicol, 20% dimethoate, or 50% phoxim, applied every 7–15 days. Ensure even coverage on both sides of the leaves and rotate different types of insecticides to prevent resistance.
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