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Diamondback moth larva and adult |
The diamondback
moth (DBM) is a lepidopteran insect and is of economic importance in brassicas
i.e. cabbage,
broccoli,
brussel sprout and cauliflower.
Appearace: The diamond
back moth is a small, greyish brown, night fly with one-inch wingspan. When it is
at rest the moth, hold its wings together, roof like, over its back. When in
this position a pattern of three diamond shape spots can be seen along the top
of the moth's body. The small caterpillar are 3/8 inch long at maturity pointed at both ends and
range in colour from cabbage green to yellow. When disturbed, larvae rapidly
wiggle their bodies back and forth, often causing them to fall off plants.
Symptoms and effects: Diamondback
moth larvae feed on Cole crop leaves between the large veins and midribs. They
prefer to feed on the lower leaf surface, leaving the upper epidermis intact
thereby creating a “window-panning” effect. Severe feeding damage will stunt
cabbage and cauliflower heads. Larval damage to the developing buds on young cabbages
can cause heads to abort. Diamondback moth damage to root crops is generally of
little economic importance.
Life cycle: The
diamondback moth overwinters as an adult, and therefore is an early season
pest. However, cold winters increase mortality except in protected sites. In
the early spring, females lay eggs on weeds in the mustard family. The first
instar larvae mine between the leaf surfaces. After completing four larval stages,
they spin a white silk cocoon on the lower portion of the plant. There are
typically three to five generations of the diamondback moth per year.
Scouting suggestions: Scout
fields weekly throughout the season for damage. Check plants carefully, even if
no feeding damage is apparent, to look for eggs that will hatch into small
caterpillars several days to a week later. Examine the lower leaves of plants
for the larvae of each pest. Although feeding damage and faecal material are signs
of activity, it is better to rely on larvae counts to determine the level of
infestation. Caterpillars cause varying amounts of damage depending on the
maturity of the plant, so the need for treatment changes as the crop grows cabbage,
broccoli, and cauliflower in the seed bed are particularly susceptible to
damage and therefore when 10% of the plants are infested with cabbage loopers,
diamondback moths or imported cabbage worms, control is warranted. For cabbage
between transplant and cupping, the economic threshold (ET) is raised to 30%.
Once the plants have begun to cup, until early heading, if greater than 20% of
plants are infested, treatment is warranted. From early heading until harvest,
the threshold drops back to 10% to protect the market quality of the produce.
For broccoli and cauliflower between transplant and first flower or curd, the
threshold is increased to 50%.
Control: Cabbage diamond
back moth list of chemicals; Emamectin benzoate, Macten, Nemesis, Destroyer,
Steward, Missile, Avaunt, Blacket, Coragen, Ampligo, Emamectin + Lufenuron Acephate, Fipronil, Proclaim
fit/Denim fit or Delegate, Dichlovos and Belt.
NB: What is critical is to scout and
spray chemical at economic threshold level as indicated, take note of the
active ingredient of the chemicals to avoid using the same chemical because it
result in building up resistance.
by Watty Motsi, phone: +263775226414 or +263714059914, e-mail: wmts95@gmail.com
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