Cabbage Integrated Pest Management : An Ecological Guide.

Ecology of Insect Pests and Natural Enemies4.3 Insect Life CyclesInsect life cycles can be complete or incomplete(gradual). In complete life cycles, or better: lifecycles with a complete metamorphosis, insectspass through the egg, larval, pupal and adultstage. A larva is a young insect that looks verydifferent from the adult. Larvae may also behavedifferently from the adults. There are generallyseveral larval stages (also called instars). Eachlarval stage is a bit larger than the previousstage, requiring a molting or shed of the outerskin between the stages.Complete life cycles can be found with moths,butterflies, beetles, flies and wasps.Complete metamorphosis : lady beetle : egg,4 larval stages, pupa, adult(from : Hoffmann, 1993)In incomplete life cycles, or better: life cycles with anincomplete metamorphosis, insects go through egg,nymph and adult stage. There are generally severalnymphal stages. A nymph is a young insect thatresembles the adult except that they lack wings andthe nymph may be colored differently than the adult.No pupal stage is present. Nymphs and adults usuallyhave similar habitats and have similar hosts. Eachnymphal stage is a bit larger than the previous stageand requires a molting or shed of the outer skin betweenthe stages. Incomplete life cycles can be foundwith bugs, grasshoppers and aphids.Incomplete metamorphosis : aflower bug : egg, 5 nymphalstages, adult(from Hoffmann, 1993)Insects’ growth rate dependents on the temperature oftheir environment. Generally, cooler temperatures resultin slower growth; higher temperatures speed up thegrowth process. If a season is hot, more generationsof an insect may occur than during a cool season.Every insect species will have its own optimum temperature for development. Some insects can live andreproduce only at lower temperatures whereas others need high temperatures. That is why you will oftenfind other insect species in the tropics than in temperate regions. This also applies for plant pathogens.Understanding how insects grow and develop will contribute to their management. Some insects areactive predators or parasitoids during only one specific stage of their life. The hoverfly larvae, for example,are voracious predators but the adults only feed on nectar from flowers. Other insects are susceptible tocertain biological or chemical insecticides during one specific stage of their life or none at all. Larvae ofleafminers for example, are only found inside plant tissue. Spraying contact insecticides (unfortunately afrequent practice) is simply a waste of money because leafminers will not be affected. Understandinginsect life cycles helps making sensible crop management decisions regarding pesticide use.53 Cabbage Ecological Guide - 2000