The Eleventh Regional Wheat Workshop For Eastern ... - Cimmyt

Integrated control using Russian wheat aphid resistant wheat cultivars - Tolmay et al.
One parasitic wasp, Aphelinus hordei, was introduced to South Africa from the Ukraine in
1991. The adult parasite is approximately Inun in length and black in color with yellow legs.
After mating, the wasp female lays an egg inside an aphid. The larva feed on the internal
organs of the aphid resulting in the aphids' death. The larvae secrete a substance that hardens
the aphids' skin turning it black in color. This blackened aphid is called a mummy and a few
days after the mummy is fonned, an adult parasitoid emerges from it. Each female wasp is
able to lay in excess of one hundred eggs during her adult life of about ten days. A. hordei
enters a resting stage similar to hibernation from April to August each year.
Of the six entomopathogenic fungal species known to infect cereal aphids under field
conditions in South Africa the entomopathoralean Pandora neoaphidis is considered the most
important. The spores of fungal pathogens coming into contact with an aphid will genninate
under favorable conditions, penetrate the aphid and cause disease. Diseased aphids become
sluggish, stop feeding and die within two to five days. Fungal growth covers the aphid
cadaver giving it a puffed brick-brown or white appearance (depending on fungal species).
The fungus then produces spores, which may come into contact with other aphids in the
vicinity to again cause infection, disease and ultimately death. The spores of certain fungal
species can be mass produced and fonnulated to produce a myco-insecticide. Presently two
indigenous fungal species are being evaluated at the SGI for use as myco-insecticides against
cereal aphids.
Chemical Control
Insecticide application for the control of RWA is predominantly used on susceptible
cultivars. More than 15 insecticides are regIstered for the control of Russian wheat aphid in
South Africa, however, application of these products could influence natural enemies of
R W A and should therefore be undertaken with care.
DISCUSSION AND CONCLUSION
Six of the 15 RW A resistant cultivars presently available ~o producers were developed at the
Small Grain Institute. The challenge facing breeders now is to ensure that the resistance
incorporated into these and other new cultivars is durable. The use of additional control
methods in the field such as natural enemies, the promotion of non-season hosts to support
natural enemy populations and judicious use of insecticides such as seed treatments on
susceptible cultivars assist in achieving this goal. The diverse nature of the South African
wheat production region lends itself to the use of different resistance genes in different
cultivars. With 19 different cultivars to chose from, 15 with RW A resistance and a possible
four to five different RWA resistance genes in use, a patchwork effect is created and it is
unlikely that continuous large tracts of cultivars with the same resistance gene will occur.
This should reduce the potential for the development of a resistance breaking biotype of the
RWA.
Another strategy is the combining of different resistance genes in the same cultivar. This
challenge is more difficult because it is not possible to differentiate between RW A resistance
sources/genes using phenotypic symptoms. Identifying plants with more than one gene is also
very difficult. In order to overcome this problem studies have been undertaken to identify the
mechanisms of resistance in specific donor lines so that they can be used to characterize the
resistance; the assumption being that different mechanisms will be expressed by different
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