| Diamond-back
moth |
Namibian
crop pests # 40
|
| Plutella xylostella (L.) |
Order: Lepidoptera |
Family: Pluttelidae |
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Note:
This page is brought to you courtesy of Menno Keizer and Jantien
Zuurbier, who worked at the Kavango Horticultural Production and Marketing
Project. For further information please email them directly at: mennojantien@yahoo.co.uk
Caterpillar on cabbage leaf
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Imago (photo: Otto Buhl, Denmark)
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Common names:
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Diamond-back moth (DBM) (English),
Liyeya
limbimbidi (Gciriku), Ruit rug mot larwe (Afrikaans) |
Host:
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All species of Brassica (Cabbage,
Kale, Rape, Broccoli, Turnip, Cauliflower) and a wide range of wild species,
including wild mustard and wild radish. Because there are many host plants
and these plants are often grown throughout the year this contributes to
maintaining and often increasing both pest and natural enemy populations. |
Pest status:
|
Diamond-back moth is a very common
and widespread pest of cabbage, Chinese cabbage and other Brassicas spp..
Severe attacks sometimes occur, especially in hot dry weather. The pest
thrives in tropical, subtropical as well as temperate climates. Under Namibia’s
hot conditions the pest is able to develop rapidly almost throughout the
year; a generation being completed in two weeks in summer and 3 - 4 weeks
in winter. Several generations may be completed on one crop in the field,
and with this pest population numbers increase significantly. Infestations
normally commence in spring and the insects become more abundant in dry
summers. |
Description:
|
The adult moth is small, about 7
- 8 mm long when at rest, grey in colour, with three pale triangular markings
on the inner edge of each forewing, forming a triple diamond pattern when
the wings are folded over the back.
The tiny cigar-shaped white eggs
are laid, either singly or in small groups, near the midrib or a strong
vein of a leaf. Incubation period is 3-8 days depending on the environment.
The caterpillars are light brown,
about 2 mm long when they hatch, becoming green and growing to a length
of about 12 mm. They are extremely active and, when disturbed, wriggle
away rapidly and drop from the leaves on silk threads, by means of which
they later climb back onto the plants. The total larval period varies from
14-28 days.
Pupation takes place inside a gauze-like
silken cocoon about 9mm long, which is stuck to the underside of a leaf;
the pupa is greenish at first and changes to brown colour as the moth develops.
The pupal stage last from 5-10 days.
A single female moth may live for
14 days and lay more than 400 eggs. In the tropics breeding is continuous
with as many as 15 generations a year.
Note: The larvae of DBM are
often confused with larvae of the cabbage (head) borer, also called cabbage
webworm (Hellula undalis). The latter is also a common pest in cabbage
in Northern Namibia. Larvae are creamy white and the head is black. These
larvae feed by mining and boring on plant parts, and especially cause a
lot of damage to the growing points of young seedlings. |
Damage:
|
The caterpillars of the diamond-back
moth feed on the leaves, usually on the undersides, eating into the tissue,
leaving only veins and the upper epidermis, giving the leaves a windowed
appearance, which grow in size, eventually forming holes as the leaves
grow and the dead tissue tears. On young seedlings, the growing point of
the plant may be damaged, often resulting in death of the plant. In older
plants new shoots are produced and the attacked plants produce several
small heads of little value. |
Control:
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Biological control
In Namibia no work has been undertaken
to collect or identify natural enemies of DBM.
Some of the species that are known
to parisitise diamond-back moth larva are of the genera Diadegma sp. Apanteles
sp., Cotesia sp. and Tetrastichus sp. They are mainly small wasp-like insects
that lay their eggs inside the DBM larva. When the eggs hatch the small
wasp larvae feed on the DBM larvae. Birds such as chicken can be
used in the garden to control the caterpillars. However, if you leave chicken
in the garden for too long, they will start feeding on the green leaves
of the crops.
Use of botanical pesticides
Research in other countries showed that
Neem (Azadirachta indica)-based products give good control of DBM. They
are relatively harmless to natural enemies and non-toxic to warm-blooded
animals. The action of neem is relatively slow, thus the larvae may survive
for a few days after application, but their growth and feeding is inhibited
and the larvae die. Any individuals that manage to develop to the adult
stage may be deformed and/or have reduced egg-laying capacity. In Namibia
neem is still a relatively unknown product, although some seed producing
trees can be found at ADC’s in Kavango and at Mahenene RS. The Directorate
of Forestry currently promotes this multipurpose tree in community gardens
in the north. At present three different botanicals (Neem-powder, chilli/garlic
and tobacco) are tested at Mashare ADI. Contact the horticultural section
at Mashare ADI for an update on the latest results.
Cultural practices
Field hygiene is very important. Cultural
control includes keeping down weeds early in the year to reduce the egg
laying sites and at the end of a growing cycle, the cabbage-remains should
be uprooted and burned. This will avoid the carry-over of the pest to the
next Brassica crop.
Other methods that were reported
to be effective in Kenya are intercropping with plants that have a repellent
effect, such as tomato and using overhead sprinkler irrigation for 5 minutes,
just before dusk. This disturbs the moth’s activity and can also result
in run-off of the larvae, thus increasing its mortality.
Resistant varieties are not yet available,
however dark green glossy-leafed species of Brassica tend to be less susceptible.
Chemical control
In many parts of the world this insect
has developed resistance to the common insecticides and control may be
a serious problem. In Namibia spraying with Malathion (active ingredient
mercaptothion) is still found to be an effective chemical control method.
However, remember to use chemicals only as a last resort and in combination
with the methods mentioned above. |
References:
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Keizer, M., J., Zuurbier, Manual
of Integrated Pest Management, Course for Extension staff, Mashare Agricultural
Development Institute, Rundu, Namibia, April 1999.
Myburgh, A. C., Crop pests in southern
Africa, Vol. 3 Potatoes and other vegetables, Plant Protection Research
Institute, Bulletin No. 415, Pretoria, Republic of South Africa, 1993.
Varela, A.M., A.A., Seif, Crop protection
manual for Brassicas, ICIPE, Nairobi, Kenya, June 1999. (Photo by: GTZ-ICIPE
IPM Horticultural Project). |
Other pages by the same authors:
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