Cocoon Inside Cocoon

In Australia, Cabbage, Broccoli, Brussels sprouts crops are damaged by the Diamondback moth.

The DBM (short form of the name) is less than 10mm long. It is grayish with cream colored row of diamond markings on its wings. Hind wings bear long grey hairs on the tips.

The female DBM lays oval shaped cream colored eggs which are about 1 mm long, on the under surfaces of leaves. Eggs hatch into small larvae, which grow into 12 mm long worms. They feed and become mature. They spin a loose silky cocoon and undergo pupation.

The cocoon of DBM is spindle shaped and the pupa is clearly visible from outside. Development into moth depends upon temperature. If the temperature is around 28 C, metamorphosis takes about two weeks.

DBM cocoons are attacked by a parasitic wasp (Diadegma semiclausum). The wasp gets attracted to Brassica plants which are under destruction by the DBM. The plant releases chemicals from the damaged leaves, which draw the attention of the wasp.

The wasp stings the pupa of DBM and introduces an egg. The egg hatches and the immature wasp feeds on the internal contents of the pupa. The pupa is eventually killed. The moth will never emerge from the cocoon. On the other hand the wasp larva pupates by spinning its own cocoon. At this stage, it is a cocoon inside a cocoon.

In Northern Wisconsin, Forest and shade trees are defoliated by the attack of Tent caterpillars of Malacosoma Moth.

A parasitic fly, Sacrophaga aldrichi, which is slightly larger than the housefly, attacks the pupal stage of the moth. The fly injects an egg into the pupa. The egg hatches into a maggot (larva) which feeds on the internal contents of the pupa, eventually killing it.

The wasp and the fly are parasitic insects. They are also called parasitoids. They are natural enemies to moths and friendly to man, in controlling damage to his crops and hard wood trees.