Cabbage Integrated Pest Management : An Ecological Guide.

Major Natural Enemies of Cabbage Insect PestsConservationBecause young larvae are susceptible to drought, they may need a source of moisture. Adult lacewingsneed nectar or honeydew as food before egg laying and they also feed on pollen. Therefore, plantingsshould include flowering plants (e.g. at borders of the field), and a low level of aphids can be tolerated toattract and conserve lacewings.The green lacewing appears to have some natural tolerance to several chemical insecticides althoughthere may be considerable variation. Populations tolerant of pyrethroids, organophosphates, and carbarylhave been selected in the laboratory. Still, when lacewings (and other natural enemies) occur in the field,it is advisable to avoid using pesticides.6.1.4 Hover flies - SyrphidaeHover flies are also called syrphid flies or flower flies.Primary prey: aphids, small caterpillars, sometimes thrips, possibly jassid nymphs.Predatory stages: only larvae of hover flies are predators.Description and life cycleAdults of the hover fly eat pollen and nectar from flowers. Only the larvae are effective aphid predators.The adult hover flies look like bees or wasps and are usually seen near flowers. Many species havecompact, flattened bodies, large eyes and black and yellow stripes on the body. They vary in size from9 - 18 mm.The female lays single, small (about 1 mm), white eggs that lie flat on leaves or shoots near or amongaphid populations. Females can lay several hundred eggs. The larvae hatch in 2 - 3 days. The larvae aresmall maggots without legs, they look more like tiny slugs than adult hover flies. They vary in color fromcream to green to brown, depending on the species and the prey consumed. There are 3 larval instars.The larvae suck out the inside liquids of aphids and small caterpillars until only the skin remains!In about 2 weeks, the larva develops into a pupa which usually is pear-shaped and is cream, green orbrown in color. The pupa is attached to leaves or stems, sometimes in the soil.Predation efficacy hoverfly: a study exampleTo check how effective one hoverfly larva is, you can count how many prey it eats in a day. Collect afew hoverfly larvae from the field (use wetted hair brush to handle them). Put one larva per potted plantwith a known number of prey e.g. aphids on an cabbage leaf. Count how many prey is left after 24hours. Place the same number of aphids in a jar without the predator to check how quick the aphidpopulation grows when no predator is around. Use hoverfly larvae of different sizes (there are 3 larvalstages) and compare the appetites of each stage. Discuss if hoverflies can keep up with aphidpopulation growth.The period from egg to adult varies from 2 to 6 weeks, depending on the temperature, species andavailability of aphids. If there are many aphids for the hoverflies to eat, there can be more generations.Cabbage Ecological Guide - 2000114