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

Integrated control using Russian wheat aphid resistant wheat cultivars - Tolmay et a1.
favorable. In this way RWA are prevented from reaching economically damaging populations
thereby avoiding the necessity for curative insecticide applications.
MATERIALS AND M.ETHODS
The approach to controlling RW A in South Africa is to use host plant resistance in the form
of resistant wheat cultivars to dramatically reduce RWA populations in the field. Natural
enemies including predators, parasitoids and entomopathogenic fungi control the remaining
aphids. In circumstances where RWA populations become very large, insecticidal application
is used as a curative measure. A short description of each component included in the
integrated R W A control program follows.
Russian wheat aphid resistant cuItivars
Resistance to RWA in Triticum aestivum was first identified by Du Toit (1987), and RWA
resistance genes have since been transferred to commercial wheat cultivars through a
backcrossing program at the Small Grain Institute (SGI) (Small Grain Institute Technology
Report, 1996). The first RW A resisr.mt wheat cultivar was released in South Africa in 1992.
At present there are 15 RW A resistant cultivars available to control this aphid in the Free
State Province. Different sources of resistance were used during the development of these
cultivars and although no exact data are available it is possible that as many as four or five
different resistance genes may be employed. It is estimated that these cultivars are planted on
more than 70% of the wheat producing area. Acceptable yields have been obtained from
these cultivars (Marasas et al., 1997). A number of different sources of plant resistance to
RW A have been reported in bread wheat (Harvey and Martin, 1990; Quick et al., 1991; Smith
et al., 1991; Souza et al., 1991; Baker et al., 1992; Du Toit, 1992; Formusoh et al., 1992; Porter
et al., 1993). Initially, five sources of resistance were used by the Small Grain Institute;
namely PI 137739, PI 262660, PI 294994, Aus 22498 and CItr 2401. Backcross breeding was
used to transfer this resistance into eight well adapted South African cultivars; namely
'Tugela', 'Betta', 'Molopo', 'Karee', 'Kariega', 'Letaba', 'Molen' and 'Palmiet'. During the
backcross breeding process, plants were screened for resistance to RWA in a greenhouse
bioassay. Though a tedious method, screening plants using live aphids has proven to be a
reliable technique to identify resistant plants and will be used until such time as reliable
genetic markers become available to assist selection of plants containing the resistance
gene(s). The presence of resistance in the cultivars leads to a reduction in both the percentage
of infested tillers and the number of RWA per infested tiller. Furthermore the leaves of the
resistant cultivars do not roll closed, leaving RWA exposed on the leaf surface. This
characteristic makes it possible to use natural enemies of the RW A to support the resistant
cultivars.
Natural enemies
Natural enemies of R W A are used to support the resistant cultivars. The small popUlations of
RWA on resistant cultivars seldom cause economic damage but natural enemy control of
these aphids protects the resistance genes in the cultivars by reducing the possibility of a
resistance breaking biotype of the aphid occurring. Seven species of ladybird predators, four
species of parasitic wasps, one species of predatory fly, two species of hemipteran predators
and six species of entomopathogenic fungi are known to be natural enemies of cereal aphids ·
in South Africa (Aalbersberg et al., 1988; Hatting et al., 1999; Hatting et al., 2000; Prinsloo,
unpublished data).
191