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September 2017
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Molecular differentiation between South African
Russian wheat aphid biotypes
– with a twist…
C
rop pests have plagued producers since the infancy of ag-
riculture. It has recently been predicted through extensive
modelling that most agriculture regions of the world will
reach pest saturation point by 2050.
Third on the top ten list of diseases and pests expected to reach
saturation point on grain crops is the aphid species complex (as a
group). This is on the back of the demand and need for higher yield-
ing crops, as more than a 50% increase in food production is required
to meet global demand of the ever-growing human population.
Global redistribution and introduction of pest insect species by and
in response to human activities has become defining. The arrival
of invasive exotic insect pests of various crops is undeniably
negative for international food security and national agricultural
sustainability.
The Russian wheat aphid (RWA), (
Diuraphis noxia
), insect species
is clonal, and all female offspring are identical to the mother aphid.
In South Africa, there is no documented evidence of male aphids
of
D. noxia
, which eliminates the possibility of sexual recombination.
Telling the difference between the biotypes is particularly diffi-
cult and the screening process is cumbersome. The biotypes are
classified according to the reaction of a set of international differ-
ential wheat lines which contain different Dn (Dn1-9, Dnx and Dny)
resistance genes.
This process takes a month to complete and is very labour inten-
sive. Recently the genome of Russian wheat aphid was published
and very limited, if any, variation was found at DNA sequence level
between different biotypes around the world.
The local context
In South Africa the Russian wheat aphid has been a significant
insect pest on wheat for decades. Russian wheat aphid was an infa-
mous insect pest of winter/facultative wheat under dryland produc-
tion, causing significant yield losses in the mid-1980s and 1990s in
South Africa.
With the steady decline in area planted to wheat since the 2000s
– especially in the Free State Province – the presence of damagingly
high Russian wheat aphid numbers in producers’ fields has become
less common and sporadic.
There is evidence in the last few seasons that certain RWASA bio-
types have acquired specific secondary host preferences and have
been found in the fields of irrigated spring wheat, as well, suggest-
ing potential biotype adaption. Importantly for now, the number of
Russian wheat aphid found in the Western Cape and irrigation pro-
duction areas remain low.
With climate change the availability of secondary hosts and the
increasing trend in global human travel, the development of new
virulent Russian wheat aphid biotypes and/or introductions into
new wheat production regions appear unavoidable. This was the
case in 2016, when Australia had its first ever incursion of an uniden-
tified Russian wheat aphid biotype.
Currently, there are four known biotypes of Russian wheat aphid in
South Africa, RWASA1 (identified in 1978), RWASA2 (identified in
2005), RWASA3 (identified in 2009) and RWASA4 (identified in 2011).
The current cultivar spectrum has good resistance levels to RWASA1
and RWASA2, with lower resistance levels to RWASA3 and limited,
if any to RWASA4.
This high resistance level to RWASA1 and RWASA2 is primarily due
to the constant investment in pre-breeding for Russian wheat aphid
host plant resistance done at ARC-Small Grain. The limited number
of resistant cultivars available to RWASA3 and RWASA4, could be
due to the shorter time that has transpired since these biotypes were
found, as resistant cultivars take many years to develop. What is
concerning, is the RWASA4 biotype is becoming more prevalent in
survey studies.
In the last five years, a number of studies across different insect
species (honey bees, termites and ants) have been successful in ex-
ploiting epigenetic variation to identify potential biomarkers. Epige-
netic variation refers to variation in the presence of methyl-groups
that are attached to the DNA of a species. In these studies meth-
ylation sensitive, molecular techniques were used to identify differ-
ences in methylation patterns attached to the DNA.
These different methylation patterns are commonly known as
biomarkers. Differences in methyl groups attached to the single
base pairs of DNA have shown to influence gene expression
epigenetically.
FOCUS
Integrated pest control
Special
DRS SCOTT SYDENHAM
and
VICKI TOLMAY,
ARC-Small Grain, Bethlehem
“
Being able to tell the
difference between the
Russian wheat aphid
biotypes quickly and
effectively will enable
scientists to spend their
time studying the ecology
and life history of the
different biotypes.
“