Cabbage diamondback moth: a real threat to  sustainable cabbage production in Mozambique 

Over the past years, cabbage farmers across large section of Mozambique, have observed a pest called “Cabbage diamondback moth”. This pest damage has been observed mainly in the central parts of Mozambique but has spread throughout the country and now occurs in most of the cabbage producing region
Cabbage damaged by insects pests close-up. Head and leaves of cabbage in hole, eaten by larvae butterflies and caterpillars. Consequences of the invasion butterflies Pieris brassicae.

Diamondback moth is a pest of all brassica crops and is present throughout the growing season.  The diamondback moth adults are the smallest of the brassica lepidopteran pests (<1/2 inch), light brown with a yellow diamond-shaped marking, and rest with their wings folded together like a tent.  Eggs are laid singly or in small clusters. Caterpillars go through four instars and are small (< ½ inch when fully grown), light green, and appear segmented, with a forked end and pointed shape. When disturbed they wiggle vigorously and may drop off the plant on a string of silk.

Figure 1 Young Caterpillars of Diamond Black Moth eating from the bottom side of the leaves

Diamondback moth feeds on all plant foliage, rather than concentrating its feeding in the head like imported cabbageworm, feeding causes small, round holes that don’t break through the top layer of leaf tissue, leaving translucent films across holes, called “windowpane” damage. Feeding tends to be spread across the foliage and not necessarily concentrated in the head as shown below (Fig. 2) causing low yield and quality of the product. This reduces the market value of the crop.

Figure 2 Low quality cabbage head as a result of diamond black moth damage

From the high pest incidence and the rapid spread of the pest in cabbages across the country over the years, it is evident that effective control measures are needed to ensure continued sustainable production of Brassica crops by both commercial and subsistence farmers. 

Cultural Controls & Prevention

The feeding of diamondback moth on crops, it can happen so fast especially if you haven’t figure out where the source is. You can be spraying your current crops with chemicals but it’s actually in your old crop that you are not paying attention to, so you need to eliminate the host where there are multiplying by:  

  • Incorporate crop residues shortly after harvest to reduce movement to successive plantings and reduce overwintering populations.
  • Use selective materials or microbial products to spare beneficial insects that help control diamondback moth populations.
  • The use of transplants that are free of larval contamination is a key step in avoiding damage. 
  •  Floating row covers can provide a physical barrier to imported diamondback moths in small crop plantings. 
  • Rainfall has been identified as a primary mortality aspect for younger larvae, so it is no longer shocking that crucifer vegetation with overhead sprinkle irrigation have a tendency to have fewer diamondback moth larvae than drip or furrow-irrigated crops. Best results have been acquired with day by day nighttime applications.
  • Crop diversity can impact abundance of diamondback moth. Larvae usually are fewer in number, and greater closely parasitized, when crucifer crops are interplanted with any other crop or when weeds are present. This does now not always lead to discount in damage, however. Surrounding cabbage plants with two or more rows of greater desired hosts such as collard and mustard can delay or stop the dispersal of diamondback moth into cabbage crops. 

 Natural controls are frequently quite effective in preventing build ups of diamondback moth populations.

Chemical control

Effective integrated pest management (IPM) programs for diamondback moth should be designed to prevent damage. Chemical insecticides can also be effective in controlling at caterpillar stage of the pests. If pesticides are used, there is need for careful choice. If one chemical is used all time, it is likely that the pest will develop resistance to it. When applying chemical, spay in the late after noon or in the evening, when the caterpillars are actively feeding.

To this end, Prolibuds Lda is here to partner with farmers to control the pest to ensure maximum production and good quality products on the markets and to the final consumer. The most important questions that the study will aim to answer are the identity of the host causing pest multiplication and its mode of transmission and its characteristics, taken together, the information generated in this study will provide both farmers and Prolibuds Lda with the tool to develop and implement various strategies, such as improved cropping practices, producing effective barriers against the pest or appropriate pesticides to control.   

Further reading

Cartwright B, Edelson JV, Chambers C. 1987. Composite action thresholds for the control of lepidopterous pests on fresh-market cabbage in the lower Rio Grande Valley of Texas. Journal of Economic Entomology 80: 175-181.

Harcourt DG. 1955. Biology of the diamondback moth, Plutella maculipennis (Curt.) (Lepidoptera: Plutellidae), in eastern Ontario. Report of the Quebec Society for the Protection of Plants 37: 155-160.

Harcourt DG. 1957. Biology of the diamondback moth, Plutella maculipennis (Curt.) (Lepidoptera: Plutellidae), in Eastern Ontario. II. Life-history, behaviour, and host relationships. Canadian Entomologist 89: 554-564.

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