Cabbage Integrated Pest Management : An Ecological Guide.

Major Natural Enemies of Cabbage Insect PestsConservationLike almost all other predators and parasitoids, Cotesia sp. are quickly eliminated when spraying chemicalinsecticides. When Cotesia is active in the field, avoid using insecticides when possible. Insecticideapplication may actually lead to an increase of pest insect populations because the natural enemies arekilled (see section 4.11.11 and box in section 5.1).6.2.4 Diadromus collarisHost: pupae of diamondback moth (DBM).The wasp Diadromus collaris belongs to the family of Ichneumonidae.Adult females are about 6 mm long with an obvious ovipositor.This DBM parasitoid was originally from Europe and came to Asia via Australia. It was introduced toVietnam in 1997. It is reared and has been released in the field in Dalat, a highland area in Vietnam. Therelease in Dalat is being carried out in conduction with monitoring and ecological studies by Farmer FieldSchools and farmer study groups (FAO progress report ’96 – ‘99). Initial results, however, show a lowparasitism percentage.In other studies, parasitism of pupae of 1 to 12% has been recorded (Hoffmann, 1993).Diadromus collaris complements the action of the larval parasitoid, Diadegma semiclausum. Togetherwith D. semiclausum, Diadromus collaris helps keep diamondback moth populations low, especiallywhen there is low (or no) use of chemical insecticides.6.3 PathogensPathogens are bacteria, viruses, fungi, and nematodes. Insects, like humansand plants, can be infected with pathogens which cause diseases. Insectpathogens generally kill, reduce reproduction, slow the growth or shorten thelife of a pest insect. Under certain conditions, such as high humidity or highpest populations, these pathogens can cause disease outbreaks that reducean insect population. This is why such pathogens can be considered naturalenemies of insects. Most insect pathogens are specific to certain groups ofinsects and certain life stages of the insect. Some microbial insecticides mustbe eaten by the target pest to be effective, others work when in contact with thetarget pest.Unlike chemical insecticides, microbial insecticides usually take longer to kill or weaken the target pest.J Most insect pathogens are not harmful for other beneficial insects, Jand none are toxic to humans.Pathogens are most effective when pest populations are very high. Pathogens are difficult to managebecause their presence and effectiveness strongly depends on factors like temperature and humidity.During the dry season for example, you will almost never see aphids killed by a fungus because thatfungus needs a high humidity for survival and spread.Cabbage Ecological Guide - 2000124