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Mei 2018
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What should be done?
W
hat seems to be a few yellow
larvae inside a wheat leaf in the
year 2000, was the start of a
new pest on wheat and barley.
However, researchers are not certain if it is
a pest…
Yellow leaf miner larvae were found in
wheat leaves during 2000 in the Douglas
and Prieska areas. The insect, identified as
Agromyza ocularis
(
Photo 1
), is an indige-
nous grass leaf miner fly that was originally
described from natural vegetation in the
Ceres, Giant’s Castle and Maseru areas.
Today, this fly is known on wheat and bar-
ley in almost all irrigated wheat production
areas in South Africa. Since 2016, leaf min-
ers have been present on dryland wheat in
the Western Cape, which present a differ-
ent situation to what we have encountered
thus far. Infestation in this area started in the
Heidelberg area, but spread through the
whole production area, including the Swart-
land.
The female fly lays eggs inside host plant
leaves and the larva mines inside the leaf,
causing dead leaf tissue (
Photo 2
). Leaf
mines develop firstly in the older leaves of
the plant, since eggs tend to be pushed out
of actively growing leaf tissue, giving them
no chance to survive.
Visible leaf damage on irrigated plants can
sometimes be serious, but yield loss, how-
ever, seems to be small, which renders it
difficult to establish control measures.
Strong survival
This leaf miner has several characteristics
built into its life cycle which could favour its
survival under difficult circumstances. One
is a diapause (resting stage), which could
last for up to ten months. This occurs during
the pupal stage, which is mostly present in
the first 5 cm of top soil.
About 50% of the pupae will hatch into flies
within about 23 days at a temperature of
25°C (Adendorff, 2010). The rest of the pu-
pae will stay in the soil and will hatch occa-
sionally over the next nine months.
Flies are therefore able to survive if small
numbers of volunteer wheat, alternate hosts
and natural grass become available. This
could cause several successive populations
in a wheat field, complicating control.
Infestation pattern in
Western Cape
In the Western Cape, infestation is found
early on new emerging wheat during May
and specifically when it is dry. Drought
causes reduced growth of the then very
small plants. Fly eggs, laid in these small
leaves, will hatch and larvae are able to
grow and survive.
In the early growth stages, the yield charac-
teristics (like numbers of tillers and kernels)
become fixed and damage could influence
yield potential. However, under dry condi-
tions, plant development also slows down
and damage to leaves may not have such a
big influence.
A larval cycle is completed at 25°C (con-
stant) within four to five days. However,
with fluctuating temperatures in the field,
duration could be extended up to ten days.
Mature larvae leave the plant and pupate in
the soil. The size of larvae should therefore
be accounted for when considering chemi-
cal control.
A high percentage of mature larvae may
mean that they will pupate in probably the
next 48 hours. At this stage, maximum dam-
age has already been suffered by the plant
and a chemical application will not remedy
the situation. On the other hand, if spraying
is delayed until after the larvae leave the
plant to pupate, the treatment of ‘empty’
leaves will be fruitless.
ON FARM LEVEL
Wheat / Leaf miner /
Agromyza ocularis
Integrated pest control
DR GODDY PRINSLOO,
ARC-Small Grain, Bethlehem
Graph 1: A flag leaf spray treatment reduced the percentage leaf area damage per tiller.
Graph 2: Yield increase caused by flag leaf spray treatment.