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Staying one step ahead of
the
false armyworm
T
he false armyworm,
Leucania loreyi
,
is a cosmopolitan pest that has been
known in South Africa for years,
causing minor damage to maize
and grain sorghum (
Photo 1
and
Photo 2
on page 51). However, during November
2010, a sudden outbreak occurred on wheat
and barley produced under irrigation in the
Vaalharts and Douglas areas, with at least
R8,5 m damage caused on just more than
2 000 ha.
At the same time, larvae also caused dam
age to wheat in the Sand-Vet and Viljoens
kroon irrigation areas. Following this
outbreak, producers in all the areas have
become very cautious about the pest and
regularly administer preventative insecti
cide applications during October.
A project was initiated to monitor and map
the seasonal occurrence of false armyworm
in the Northern Cape irrigation areas for the
establishment of an early warning system
for the producers. This project was con
ducted in collaboration with North-West
University and GWK.
Trapping sites were established at Doug
las, Bull Hill, Vaalharts experimental farm,
Hartswater and Viljoenskroon. Bucket fun
nel traps were used from 2013 to December
2016 and traps were emptied once every
fortnight and the moth numbers recorded.
Later, all data were converted to ten day
intervals to compare between years. The
presence of feeding signs and larvae was
also monitored through annual surveys
during October. Moth numbers, together
with information on the presence of feed
ing signs and larvae, were used to warn
producers when necessary through radio
talks, the popular media and personal com
munication.
Climate data were recorded using onsite
data recorders, but were also obtained from
ARC-Soil Climate and Water.
Connection between
climate and moth numbers
All insects are cold-blooded and tempera
ture therefore governs the speed of their
development. Each insect has a lower and
a higher temperature threshold outside of
which no development will occur. The lower
temperature threshold is used to calculate
the number of day degrees (heat units) for
insect development per day, by subtracting
the lower temperature threshold from the
mean daily temperature.
Each insect needs a certain quantity of day
degrees to complete each stage as well as
its total life cycle. This parameter is specific
for an insect species and can be determined
in the laboratory to predict when the insects
will be present in the field and when pos
sible outbreaks may occur.
False armyworm developmental rates were
determined during the mid-seventies in
Japan. Since this information is lacking in
South Africa, we decided to test how the
Japan information (
Table 1
) would fit on the
moth flight and temperature data collected
between 2013 and 2016.
From this table it is clear that to complete
the life cycle from egg to adult we need
628,2 day °C above its lower temperature
threshold of 10,3°C. The larval stage needs
367,4 day °C above its 9,2°C threshold to be
completed.
49
September 2018
Dr Goddy Prinsloo,
ARC-Small Grain, Bethlehem
STAGE
DEVELOPMENTAL ZERO
TEMPERATURE (°C)
THERMAL CONSTANT
(DAY DEGREES [°C])
Egg
10,5
78,4
Larva
9,2
367,4
Prepupa
10,6
31,0
Pupa
11,4
162,7
Egg to adult
10,3
628,2
Table 1: The developmental zero temperature and thermal constant of the
false armyworm as determined in Japan (Hirai, 1975).
Graph 1: Number of false armyworm life cycles possible in Viljoenskroon, based on mean temperature and mean moth numbers from 2013 to 2016.
Red lines indicate the length of a life cycle. Red triangle indicates critical time for larval feeding. Broken black line indicates larval cycle length.
FoCus
Integrated pest control