Bt (Bacillus thuringiensis)
Bacillus thuringiensis var. aizawai and Bt var. kurstakiare very effective in controlling infestations of the cabbage webworm. Bt var. kurstakiis is used at a weekly interval and a rate of 0.5/ha. This type of strategy provides effective control of the cabbage webworm. However, continuous use of Bt can induce development of resistance.
It is important to start control measures early when caterpillars are still young and have not yet penetrated plant tissue.
Bt is a naturally occurring soil bacterium that causes disease on insect pests. It is accepted as an alternative in organic farming and is considered ideal for pest management because it is host specific and is non-toxic on natural enemies and on humans.
Bt is commercially available in most agricultural suppliers. It is sold in various formulations (spray, dust, and granule) and strains (Bt tenebrionis, Btkurstaki, Bt israelensis, Bt aizawai, Bt san diego).
Products: Bt products in Kenya are sold under the following commercial names: Dipel(r), Javelin(r), Thuricide(r) and Xentari(r). They kill the cabbage webworm and do not harm beneficial insects.
Application: Bt insecticides should be applied when the first L1-larvae are appearing. Sprays may need to be applied at intervals of 5 to 7 days when populations are high. Because Bt insecticides are UV-degraded treat crops in the late afternoon.
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Spinosad(r), a broad-spectrum insecticide derived from fermentation of the naturally occurring soil bacterium Saccharpolyspora spinosa, controls many caterpillars, leafmining flies, and thrips. It is useful for the management of caterpillar pests in brassicas, including the diamondback moth, the cabbage looper and the cabbage webworm. Spinosad(r) has low activity against most beneficial insects.
Farmers in some countries produce their own homemade biopesticides by collecting diseased diamondback moth larvae (fat and white or yellowish or with fluffy mould on them), crushing them and mixing them with water in a blender. Large tissue clumps are filtered out and the liquid is sprayed onto the crop (Dobson et al, 2002).This preparation is equally against the webworm.