Understanding the Diamondback moth

The National Farming

Diamond back moth (DBM) is a very important insect pest that affects mainly the leafy vegetables like, Chinese cabbage and round cabbages.
The pest has the potential to destroy 90 per cent of the cabbage leaves in favorable conditions when poorly managed. Understanding the biology, characteristics and behavior of this insect, will help protect the cucurbits from losing its marketable potential.

Distribution
The diamondback moth is probably of European origin but is now found throughout the Americas and in Europe, Southeast Asia,
Australia, and New Zealand.
Description
Egg: Diamondback moth eggs are oval and flattened, and measure 0.44 mm long and 0.26 mm wide. Eggs are yellow or pale green in color, and are deposited singly or in small groups of two to eight eggs in depressions on the surface of foliage, or occasionally on other plant parts. Females may deposit 250 to 300 eggs but average total egg production is probably 150 eggs. Development time averages 5.6 days.
Larva:
The diamondback moth has four instars.
Average and range of development time is about 4.5 (3-7), 4 (2-7), 4 (2-8), and 5 (2-10) days, respectively. Throughout their development, larvae remain quite small and active. If disturbed, they often wriggle violently, move backward, and spin down from the plant on a strand of silk. Overall length of each instar rarely exceeds 1.7, 3.5, 7.0, and 11.2 mm, respectively, for instars 1 through 4. Mean head capsule widths for these instars are about 0.16, 0.25, 0.37, and 0.61 mm.
Pupa:
Pupation occurs in a loose silk cocoon, usually formed on the lower or outer leaves. In cauliflower and broccoli, pupation may occur in the florets. The yellowish pupa is 7 to 9 mm in length. The duration of the cocoon averages about 8.5 days (range five to 15 days).
Adult:
 The adult is a small, slender, grayishbrown moth with pronounced antennae. It is about 6 mm long, and marked with a broad cream or light brown band along the back.
The band is sometimes constricted to form one or more light-colored diamonds on the back, which is the basis for the common name of this insect. The moths are weak fliers, usually flying within 2 m of the ground, and not flying long distances. However, they are readily carried by the wind.
Life cycle
Total development time from the egg to pupal stage averages 25 to 30 days, depending on weather, with a range of about 17 to 51 days.
Host plants
Diamondback moth attacks only plants in the family Cruciferae. Virtually all cruciferous vegetable crops are eaten, including broccoli, Brussels sprouts, cabbage, Chinese cabbage, cauliflower, collard, kale, kohlrabi, mustard, radish, turnip, and watercress. Not all are equally preferred, however, and collard will usually be chosen by ovipositing moths relative to cabbage. Several cruciferous weeds are important hosts, especially early in the season before cultivated crops are available.
Damage
Plant damage is caused by larval feeding. Although the larvae are very small, they can be quite numerous, resulting in complete removal of foliar tissue except for the leaf veins.
This is particularly damaging to seedlings, and may disrupt head formation in cabbage, broccoli, and cauliflower. The presence of larvae in florets can result in complete rejection of produce, even if the level of plant tissue removal is insignificant.
Natural enemies
Large larvae, prepupae, and pupae are often killed by the parasitoids Microplitis plutellae (Muesbeck) (Hymenoptera: Braconidae), Diadegma insulare (Cresson) (Hymenoptera:
Ichneumonidae), and Diadromus subtilicornis (Gravenhorst) (Hymenoptera: Ichneumonidae). All are specific on P. xylostella. Nectar produced by wildflowers is important in determining parasitism rates by D. insulare. Egg parasites are unknown. Fungi, granulosis virus, and nuclear polyhedrosis virus sometimes occur in high density diamondback moth larval populations.
Weather
A large proportion of young larvae are often killed by rainfall. However, the most important factor determining population trends is thought to be adult mortality. Adult survival was thought to be principally a function of weather, although this hypothesis has not been examined rigorously.
Sampling:
 Populations are usually monitored by making counts of larvae, or by the level of damage. When growers monitor fields and subscribe to these treatment thresholds rather than trying to prevent any insects or damage from occurring in their fields, considerably fewer insecticide applications are needed to produce a satisfactory crop.
A minimum plant sample size of 40 to 50 is recommended except for the egg stage, where 150 plants should be examined for accurate population estimates.
Insecticides:
 Protection of crucifer crops from damage often requires application of insecticide to plant foliage, sometimes as frequently as twice per week. However, resistance to insecticides is widespread, and includes most classes of insecticides including some Bacillus thuringiensis products. Rotation of insecticide classes is recommended, and the use of B. thuringiensis is considered especially important because it favors survival of parasitoids.
Even B. thuringiensis products should be rotated, and current recommendations generally suggest alternating the kurstaki and aizawa strains because resistance to these microbial insecticides occurs in some  locations.
Mixtures of chemical insecticides, or chemicals and microbials, are often recommended for diamondback moth control.
This is due partly to the widespread occurrence of resistance, but also because pest complexes often plague crucifer crops, and the insects vary in susceptibility to individual insecticides.
For specific insecticide recommendations see:
Cultural practices:
Rainfall has been identified as a major mortality factor for young larvae, so it is not surprising that crucifer crops with overhead sprinkle irrigation tend to have fewer diamondback moth larvae than drip or furrow-irrigated crops. Best results were obtained with daily evening applications. Crop diversity can influence abundance of diamondback moth.

  • Robert Kei Geno is a Unitech lecturer with agriculture. He has a Bachelor of Science in Agriculture (BSAG), Post Graduate Diploma in Agriculture (PDG) and Masters of Philosophy in Agriculture – crop protectionist, Entomologist.