What to Do When Rapeseed Gets the Cabbage Webworm? Effective Control Methods to Teach You
Rapeseed, also called oil cabbage or bitter cabbage, Latin name: Brassica campestris L., a plant of the Brassicaceae family and Brassica genus, originally from China, with dark green stems and stalks like Chinese cabbage, a variety of Brassicaceae cabbage, with yellow flowers. In agronomy, the multiple species of plants whose seeds contain oil are collectively called rapeseed. At present, the main cultivated (variety) types of rapeseed are: Brassica rapa (campestris) L., Brassica juncea L., and Brassica napus L.
What to do when rapeseed gets the cabbage webworm?
First, let us learn about the rapeseed cabbage webworm together
Hellula undalis Fabricius, Lepidoptera, Pyralidae. Also known as cabbage heart wild borer, radish borer, cabbage borer, cabbage webworm, Chinese cabbage borer, cabbage heart-eating wild borer, heart-eating worm, heart-boring worm, heart-scraping worm. Synonyms Phalaena undalis, Oeobia undalis Fabricius. Its distribution runs from Heilongjiang and Inner Mongolia in the north to the national border in the south. Damage is severe in the south. In recent years, damage has also been relatively severe in Hebei, Shandong, and Henan.
The larvae are boring pests, damaging the heart leaves and leaf blades of vegetable seedlings. Affected seedlings stop growing or wilt and die because the growing point is destroyed, causing missing plants and broken rows. When cabbage and Chinese cabbage are affected, they cannot form heads or fold leaves, and the pest can spread soft rot, leading to reduced yield.
The adult is 7mm long, with a wingspan of 15mm, grayish brown; the forewings have 3 white transverse wavy lines, with a dark brown kidney-shaped spot in the middle bordered in white; the hindwings are grayish white. The egg is about 0.3mm long, oval, flat, with an irregular reticulate surface, light yellow when newly laid, later gradually showing red spots, and orange-yellow before hatching. The mature larva is 12-14mm long, with a black head, a light yellow body, a yellowish brown pronotum, indistinct grayish brown longitudinal stripes on the back, and hair-bearing tubercles on each segment, with 6 pairs on the mesothorax and metathorax, and on each abdominal segment 8 in the front row and 2 in the back row. The pupa is about 7mm long, yellowish brown, with wing buds reaching the rear margin of the fourth abdominal segment, 5 longitudinal lines faintly visible on the dorsal side of the abdomen, and 2 pairs of spines at the end of the abdomen, with the central pair slightly shorter and slightly curved at the tip.
In Beijing and Shandong there are 3-4 generations a year, in Shanghai and Chengdu 6-7 generations, and in Liuzhou, Guangxi 9 generations. It overwinters as mature larvae that spin silk to bind soil particles and dead leaves into a silk cocoon on the ground (a few overwinter as pupae). The following spring, the overwintering larvae enter the soil to a depth of 6-10cm and make cocoons to pupate. Adults are not strongly phototactic and have weak flight ability. Eggs are mostly laid scattered on the tender leaves of vegetable seedlings, with an average of about 200 eggs per female. The egg development period is 2-5 days. Newly hatched larvae mine leaves, with wide and short tunnels; after the 2nd instar they emerge through the leaf surface; at the 3rd instar they spin silk to bind the heart leaves and feed inside, causing the heart leaves to wither and die and preventing new heart leaves from emerging; at the 4th-5th instars they can bore from the heart leaves or petioles into the stem pith or roots, with conspicuous bore holes, fine silk outside the holes, and excreted moist frass. Affected seedlings die or the petioles rot. Larvae can move to damage 4-5 plants. The larvae mature at the 5th instar and pupate in the soil near the vegetable roots. From May to September, the larval development period is 9-16 days, and the pupal period is 4-19 days. This pest prefers high temperature and low humidity. For autumn Chinese cabbage sown in Beijing in August-September, at the 3-5 true leaf stage of the seedlings, when the temperature is about 24°C and the relative humidity is about 67%, this coincides exactly with the peak occurrence period of the larvae, and the damage is most severe.
Control methods for the rapeseed cabbage webworm
(1) Agricultural control. First, plowing the land can kill some of the overwintering larvae in the surface soil or in dead leaves and residual plants, reducing the source of the pest. Second, adjust the sowing period so that the 3-5 true leaf stage of the vegetable seedlings is staggered from the peak occurrence period of the cabbage webworm. Third, irrigate appropriately to increase field humidity, which can both suppress the pest and promote the growth of the vegetable seedlings. (2) Chemical control. Because this pest is a boring insect, spraying for control must seize the peak adult occurrence period and the larval hatching period. You can use mieshabi (21% synergized cyanide · malathion emulsifiable concentrate) or 40% fenvalerate emulsifiable concentrate at 3000 times dilution, 2.5% Kung Fu emulsifiable concentrate at 2000 times dilution, 20% miexiaoli emulsifiable concentrate or 2.5% Uranus emulsifiable concentrate at 3000 times dilution, 20% chrysanthemum · fenitrothion emulsifiable concentrate at 2000-3000 times dilution, and 10% chrysanthemum · malathion emulsifiable concentrate at 1500-2000 times dilution, all with relatively good results.