Soybean meal is a persistent pest that affects cowpeas during the summer and autumn seasons. The larvae cause significant damage, making traditional insecticides like trichlorfon and chlorpyrifos less effective. In recent years, the Rugao Plant Protection Station has conducted extensive research on the life cycle and control methods of bean larvae in cowpea fields, developing more efficient strategies to manage infestations. For example, during 2006-2007, the technology was tested in vegetable gardens across several towns, resulting in a pod damage rate below 10% and a locust control effectiveness of over 85%.
The newly hatched larvae primarily feed on flower buds. As they develop into the second instar, they begin to chew through the corolla at night, using silk threads to pull together the receptacles, petals, and fruit branches of the affected flowers. During hot weather, cowpea flowers open in the morning and close around 10 a.m. The normally closed corollas typically fall off at night, during which most of the first- and second-instar larvae are dropped along with the corolla. A small number of larvae may remain attached to the tender pods or get trapped in the filaments, continuing to feed on the flowers.
When the flowers are no longer suitable for feeding, the larvae move to nearby pods or other floral structures. If larvae reach the third to fifth instar within the closed flowers, they can cause severe damage, leading to flower drop and negatively affecting plant growth. However, some flowers remain suspended on the fruit branches due to entanglement with filaments, allowing the larvae to continue feeding on them.
Based on these damage patterns, it's crucial to focus on protecting the buds during production. A "preventing flowers and protecting the locusts" strategy should be implemented, including manual removal of pests and the use of flowering agents. First, manually removing pests: every afternoon, silkworms found on the fruit branches and corollas still attached to the tender pods should be removed, provided it doesn't impact yield. Second, chemical control: applying insecticides with strong penetration and good efficacy, such as 1.8% avermectin or 35% octyl pyrophosphate, in the morning when the flowers are open. Spraying should occur within 6 hours after rainfall. Applications should start from the early flowering stage and continue every 6–7 days until the flowering period ends.
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