Cabbage Integrated Pest Management : An Ecological Guide.

Major Natural Enemies of Cabbage Insect PestsMost pathogens are too small to be seen by human eyes. Only the symptoms that insect-pathogenscause can be seen with the eyes: for example a dead insect covered with fungus spores like “hairs” or“dust” or a dead insect which is black and spills fluid out of the body.Some pathogens have been mass produced and are available commercially for use in standard sprayequipment. These products are often called biocontrol agents, microbial insecticides, microbials, bioinsecticidesor biopesticides. Some of these microbial insecticides are still experimental, others havebeen available for many years. The best known microbial insecticide is probably the bacterium Bacillusthuringiensis or Bt which is available in many different formulations. NPV is increasingly being used inAsia because it can be produced on-farm. See section 6.3.3 below.Microbial insecticides can be used together with predators and parasitoids. Beneficial insects are notusually affected directly by the use of a microbial insecticide, but some parasitoids may be affectedindirectly if parasitized hosts are killed.Below, some pathogens of cabbage insect pests are described.6.3.1 Bacillus thuringiensis (Bt).Bacillus thuringiensis (Bt) occurs naturally in the soil and on plants. However, in the field, Bt is usuallyapplied as a microbial insecticide. There are different varieties of Bt. Each Bt variety produces a proteinthat is toxic to specific groups of insects.Some of the varieties of Bt with some of their target insect groups are:Bt var. aizawai Caterpillars, including diamondback mothBt var. kurstaki CaterpillarsBt var. tenebrionis: Colorado potato beetle, elm leaf beetleBt var. israelensis Mosquito, black fly and fungus gnat larvaeBt has been available as a commercial microbial insecticide since the 1960s and is sold under varioustrade names. Since 1985, the importation of Bt in Asia has greatly increased and Bt products are nowlocally produced, for example, in Vietnam and Thailand. Bt products are generally effective and safe fornatural enemies and non-target insects and can be used until close to the day of harvest. Bt can beapplied using conventional spray equipment. Good spray coverage is essential because the bacteriamust be eaten by the insect to be effective.Formulations of Bt var. kurstaki are available for the control of many caterpillar pests of vegetables.Some of the Bt brand names are: Dipel, Javelin, Biobit, MVP, Xentari, Agree. There may be many morebrand names and they vary per country.Mode of action and symptomsThe toxin inside the bacterium is only effective when eaten in sufficient quantity by the target insect. TheBt is sprayed over the leaves and when the insect eats the leaves, it will also eat the Bt. The toxindamages and paralyzes the gut of the insect. The toxin can only affect insects that have a specific gutstructure, that’s why Bt is specific for certain insect groups.Affected larvae become inactive, stop feeding and die from the combination of starvation and damage ofthe gut by the toxins of the bacterium. The larva may have a watery excrement and the head capsulemay appear to be overly large for the body size. The larva becomes soft and dies usually within days ora week. The body turns brownish-black as it decomposes.125 Cabbage Ecological Guide - 2000