Cabbage Integrated Pest Management : An Ecological Guide.

Major Natural Enemies of Cabbage Insect PestsEffectivenessLarvae of the hover fly are voracious eaters. One larva may eat up to 400 aphids during its development!On a small scale, larvae can keep aphid populations in check but it is unknown if they manage to controlaphids in large fields.Life cycle and predation efficacy hoverfly: a study exampleWhen you know how many prey the different larval instars of the hoverfly larvae eat in a day, you cancalculate the efficacy during the whole larval stage. Collect the smallest sized hoverfly larvae thatyou can find (using a wetted hair brush to handle them). Rear them in separate pots (or they mighteat each other!) and feed them with fresh aphids every day. Handle the hoverfly larvae as little aspossible. Observe the changes in size, color and shape as the larva develops and note down theduration of each larval stage. Count how many days it takes for the hoverfly to develop into a pupa.Once it has become a pupa, it stops feeding.Calculate the total number of prey consumed of one larva with the results of the previous trial (numberof prey consumed per day per life stage x number of days the life stage takes).ConservationAs for almost all natural enemies, hover flies are easily killed by broad-spectrum insecticides. Avoid useof these pesticides when possible.Adult hover flies need flowering plants to feed on. They are attracted to weedy borders and gardenplantings. Flowers have an important function in attracting hoverfly adults. See also section 4.9 onconservation of NE’s.6.1.5 Spiders - AraneaePrimary prey: aphids, mites, moths, flies and beetles, depending on the speciesof spider. They may also attack other natural enemies.Predatory stages: nymphs and adultsDescription and life cycleSpiders are not insects but belong to the order of Araneae which have 8 rather than 6 legs. There aremany species of spiders and they can be roughly divided into two main groups: spiders that hunt insearch of prey and spiders that make webs and wait for prey to be caught in the web. Both types are verycommon predators in a vegetable field and they can be very voracious. Most hunting spiders are verymobile and spend a lot of time searching for prey. Web-makers are important predators of flying insectslike adults of moths.Like many other predators, the more prey spiders can consume, the more eggs will be laid by thefemale. This allows these predators to increase their numbers when the pest population increases.The number of eggs spiders can lay varies from a few to several hundreds, depending on the species.Some spiders carry the eggs in a little sac until the young spiders hatch from the eggs (e.g. wolf spiders– Lycosidae). Others guard the location where the eggs are deposited (e.g. lynx spiders) or place the eggmass in the web or on leaves, covered with fluffy silk. Spiders may live up to 4 months, depending on thespecies.115 Cabbage Ecological Guide - 2000