Cabbage Integrated Pest Management : An Ecological Guide.

Major Natural Enemies of Cabbage Insect PestsRecognizing parasitoid activitiesParasitoids are difficult to spot because they are very small and they are fast flyers. However, theyleave evidence of their activities. In the case of diamondback moth (DBM) parasitoids for example,dead caterpillars or, more commonly, cocoons can be found. Often, these cocoons are wronglythought to be eggs of insect pests. A good way to study parasitoids is to collect sick insects,cocoons, round or egg-shaped masses of spiders and even insects caught in spiders webs in thefield. These can be observed in jars or clear plastic bottles. If cocoons are found next to a dead ordying caterpillar, the connection between cocoon and caterpillar can be shown. For example, a whitecocoon of Cotesia plutellae is usually found next to a dead caterpillar with a large hole indicatingwhere the parasitoid larva emerged. After emergence, the larva spins a cocoon.Many insect pest larvae of similar age could be collected to confirm ifthe parasitoid is coming out of the caterpillar, and witness the processof emerging. Showing this will help farmers understand the importanceof the cocoons.Placing cocoons inside clear plastic bottles helps to observe what emerges from the cocoon. Drawthe cocoons, the dead caterpillar and the type of adults that emerge, using color pencils (Ooi, 1999).See also box below.Most parasitoids only attack a particular life stage of one or several related species. The immatureparasitoid develops on or within a pest, feeds on body fluids and organs, and either comes out of the hostto pupate or emerges from the host as an adult. The life cycle of the pest and parasitoid can coincide, orthat of the pest may be altered by the parasitoid to accommodate its development. Parasitoids areusually grouped into several broad categories based on their development patterns. Egg parasitoidsattack the egg stage of their host, larval parasitoids attack the larvae etc.To determine if there is any parasitism and to what extent, it is often necessary to rear samples of pestinsects to see if any adult parasitoids emerge (see box below).Some parasitoids take longer to develop than their host. To study these parasitoids, it is important to beable to rear the collected egg masses or immature stages of the insects. If collected material is kept insuitable containers or cages, be sure to keep specimen for at least one month and even after it looks likeeverything has already emerged.Rearing Parasitoids: A Study ExampleAn easy method to rear parasitoids, for example of cabbage aphids, is the following:· Collect a number of cabbage leaves with large colonies of aphids on the underside.· Check, if possible with hand lens, if there are any “mummies” (swollen, dead aphids that havebeen tanned (often brownish) and hardened).· When mummies are found, place the leaf with aphids in a jar (insect zoo), together with a pieceof tissue to avoid condensation. Close the jar with a fine netting.· Monitor changes in aphids and mummies over the next few days.· Other aphids that have been parasitized, will transform into mummies in the next few days.Adult parasitoids (possibly Diaeretiella rapae, see section 6.2.1) will emerge from the mummy.After emergence, you can see the exit hole towards the end of the aphid’s abdomen.Leaves containing aphid mummies can be redistributed to help spread the parasitoids.Cabbage Ecological Guide - 2000118