In our last article we looked at the butterflies and moths. We learnt that the most destructive stage is the larvae also referred to as caterpillar. DBM is a good example of one of the most destructive caterpillars farmers have been fighting against, most notably in cabbages and other leafy crops.
Diamondback moth (DBM) (Plutella xylostella) is a common pest in the cabbage family (Crucifers).Newly hatched DBM caterpillars feed as leaf miners inside the leaf tissue. Older caterpillars feed on all plant parts of the plant. They feed on the leaf tissue leaving the upper leaf surface intact. This type of damage is called “windowing”,
Since it gives the appearance of translucent windows on the leaf. Some farmers would say that their crops resemble sieves or nets. In cases of severe infestation, entire leaves could be damaged. Caterpillar and pupae are found on damaged leaves or even in the soil surface near the crops. Older caterpillars are often found around the growing bud of young plants. Their feeding can deform the plant. DBM caterpillars also feed on stems and pods. Heavy damage results in marketable parts being contaminated with excrement, which
Makes the produce quality be poor and unacceptable by the market forces. Plants are affected in all growth stages; the seedling stage, vegetative growing stage, flowering stage and fruiting stage.
DBM attacks the growing points, inflorescence, leaves and stems. Under heavy infestations the entire plants may become riddled with holes. Cabbages develop deformed heads which encourages soft rots.
How can you control Diamondback Moth?
• Avoid planting during the hot season, particularly at the end of the dry season.
• Separate seedbed and field to reduce danger of carrying over the pest and to ensure infestation-free planting material.
• Practice proper rotations. Having a break where no crucifers at all are grown will reduce the number of diamondback moth considerably.
• Intercropping; planting rows of tomatoes alternately with rows of cabbage reduces damage but it does not prevent the attack completely. Trap crops such as mustard also reduce attacks; 15 rows of cabbage followed by mustard rows has been shown to be most effective.
• Frequent irrigation and rain reduce the mating of moths and wash off caterpillars and pupae from plant leaves.
• Prunings of healthy tomato plants can be scattered as mulch in the cabbage field because of its deterrent effect.
• Unharvested plants and crop residues are an important source of infestation. Remove and destroy all the unharvested plants from the field as well as alternate hosts and weed hosts.
• Ploughing the land and leaving it exposed to the hot sun at least one week before cultivation helps clean up sources of diamondback moth.
• Monitor twice a week. For monitoring pest populations light traps are useful. They show when populations are increasing, meaning preventive or control methods are needed.
• Encourage natural enemies by maintaining natural surroundings with plenty of breeding places, like trees and shrubs. Birds and bats feed on moths and lacewings, wasps, parasitic wasps, spiders and larvae of hoverflies eat the caterpillars of diamondback moths. The most
effective natural predator is the parasitic wasp, Diadegma semiclausum, which feeds on the larvae of the Diamond Black moth.
• Sprays at transplanting or within a few days afterwards prevents an early build-up of diamondback moth populations. The spraying liquid should be directed on the underside of the foliage and inside the head where the larvae hide.