Cabbage Integrated Pest Management : An Ecological Guide.

Major Natural Enemies of Cabbage Insect PestsInsect viruses are not harmful for humans, animals, predators and adult parasitoids. Larval parasitoidsthat are still developing inside an insect are affected when the host insect dies due to viral infection.Mode of action and symptomsInsect viruses must be eaten by an insect to cause infection. They may also be spread from insect toinsect during mating or egg laying. In some cases, for example while searching for suitable hosts for egglaying, beneficial insects such as parasitoids may physically spread a virus through the pest population.An example is the parasitoid Cotesia sp. that can spread granulosis virus in Pieris rapae (ref. www16).Viruses enter an insect’s body through the gut. They replicate in many tissues inside the insect andinterfere with the feeding, egg laying and movements of the insect.Different viruses cause different symptoms. NPV-infected larvae may initially turn white and speckled orvery dark. Some may climb to the top of the plant, stop feeding, become limb and hang from the upperleaves or stems (“caterpillar wilt” or “tree top disease”).Insects infected with a granulosis virus (GV) may turn milky white and stop feeding. In both cases, thebody contents of the dead larvae are liquid and the skin of the insect breaks easily to release theinfectious virus parts. Death from a virus infection occurs within 3 to 8 days.EffectivenessA virus infection, either naturally occurring or applied, can seriously reduce a pest population, especiallywhen the pest population is high. Infected insects fall apart on foliage, releasing more virus. This additionalinfective material can infect more insects. Transmission of the virus through the population may takedays or weeks but, if conditions are suitable, the entire population may eventually collapse. See also boxabove.Advantages and disadvantages of NPV are indicated in table 6.3.3Table 6.3.3 : Advantages and disadvantages of NPVNPV - AdvantagesNPV - DisadvantagesHost specificEasily produced (if live hosts are available)Symptoms easily recognized in the fieldSafeMay recycle in the field (e.g. spread throughpopulations in the field)Easy to apply and evaluateHost specificSlow actingBreaks down by sunlight (becomes inactive)Large sized larvae not affectedRequires living host to produceNeeds proper storing (cool and dark)(modified from FAO Dalat report (Carner&Shepard), 1998)Conservation and productionFor viruses that occur naturally, conservation is not an issue because the circumstances in which theviruses occur can usually not be influenced very much. Naturally occurring viruses, as well as virus131 Cabbage Ecological Guide - 2000