Cabbage Integrated Pest Management : An Ecological Guide.

Ecology of Insect Pests and Natural EnemiesThere are some virus diseases known in cruciferous crops, however, in most areas in South East Asia,they are considered to be of minor importance and will therefore not be discussed in this guide.4.5 Plant compensationNot all insect feeding reduces yield. The cabbage plant is able to compensate for feeding because moreleaves and roots are produced than actually needed for head formation. Some studies even show thatyoung cabbage plants that loose the main bud (growing point) compensate for that loss by producingother shoots and when all shoots are removed but one, a normal head can develop! Studies in Vietnamfor example, showed that when the growing point of cabbage plants were removed up to 14 days aftertransplanting, the cabbage yield was still 75% of the plants were the growing point was not removed.When the growing point was removed later, up to 35 days after transplanting, the cabbage yield droppedto 55% (FAO - TOT report Vietnam, 1995)þ Plants beyond the four to five true leaf stage can tolerate up to fiftypercent defoliation until the prehead stage without yield loss.For example, studies done in Vietnam showed that cabbage plants that weredefoliated up to 50% at 7, 14, 21 and 28 days after transplanting, still gave morethan 90% yield compared to the undefoliated plants (FAO - TOT report Vietnam,1995).Low levels of insect feeding and minor disease infections do not significantly reduce yields. It is alsoimportant to remember that spraying for insects that are not causing yield loss is a waste of money andtime and it may cause needless environmental pollution.Related exercises from CABI Bioscience/FAO manual:4.A.2. Diamondback moth injury symptoms on cabbage4.A.3. Plant compensation study4.6 A pest or not a pest insect… : how to find out !Many insects can be found in a cabbage field. Not all of them can be called “pests”, in fact, very fewinsects have the potential to cause yield loss to cabbage. The few insects that do cause some yield lossin some fields in some seasons, are called “pest insects”. As the pest insects do not cause yield loss inall fields all the time, a better term to use would be“herbivores”. Herbivores do not just eat plants or suckthe plant juices, they have an additional function: theyserve as food or as a host for natural enemies.There are many potential “pest insects” that do notbuild up in populations large enough to causeeconomic yield loss. They may chew a few leaveshere and there but this does not affect the yield orquality of the cabbage. In fact, their presence keepsthe population of natural enemies alive so one couldalmost say at that time they are “beneficial”....!55 Cabbage Ecological Guide - 2000