Cabbage Integrated Pest Management : An Ecological Guide.

Major Natural Enemies of Cabbage Insect PestsEffectivenessTo obtain effective control of the caterpillars, it is essential to apply Bt at the correct target species, at thesusceptible stage of development, in the right concentration, at the correct temperature and beforeinsects bore into the plants where they are protected. Young larvae are usually most susceptible.Caterpillars have to eat sufficient quantities of Bt in order to be affected and die. When they eat just alittle Bt, they may not die but their growth is retarded.ÕSunshine and Bt, not a good match...!?The Philippines highland vegetable FFS programme included a specific experiment for farmers toobserve the effect of sunlight on the efficacy of Bt products. They compared feeding and death rateswhen DBM larvae were placed on cabbage leaves which had been sprayed with Bt at different timesof day and hence received different sunlight exposures. The results showed that sunlight deactivatesBt. By discussing the results, the farmers were able to decide the best time of day to applybiopesticides, avoiding application when the sun is strongest (CABI, 1996).Bt only works at temperatures above 15 o C. Bt formulations are deactivated by sunlight. This is a reasonthat Bt is only effective for one to three days. Rain or overhead irrigation can also reduce effectiveness bywashing Bt from the leaves.Using Bt = patience?!?L It was noted that some farmers concluded after spraying Bt that “it didn’t work” because thecaterpillars were still alive. Some farmers even sprayed a chemical insecticide only one day afterapplying Bt.J However, only when looking more closely, they found that the caterpillars were actually hardlyeating anymore, they were just sitting on leaves, not moving very much. This is most important:when they stop eating, they stop damaging the crop! Bt is a stomach poison and the toxin paralysesthe stomach. Death by starving takes some time and, caterpillars will be dead after three days. Alsosee box below about testing Bt.(pers. comm. Dr. P.Ooi, 1999)ConservationBt formulations are applied like an insecticide. The Bt formulations become inactive after one to threedays. That means the bacterium inside the formulation is dead. Bt spores do not usually spread to otherinsects or cause disease outbreaks on their own. Therefore, conservation methods, as is important forpredators and parasitoids, are not relevant for Bt.Cabbage Ecological Guide - 2000126