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

Necessary to apply insecticides to Russian wheat aphid resistant cultivars? - Tolmay and Mare
the producer receives per hectare. In addition to fluctuating wheat prices and ever increasing
input costs another variable that influences the yield of resistant cultivars is the RW A infestation
level in the particular field. This is unfortunately a very unpredictable variable and the only way
farmers can determine the level of RWA infestation in their fields is by means of regular field
scouting.
Contradictory findings such as the results presented here make it difficult to give a general
answer to the question of whether it is necessary to apply insecticides to R W A resistant
cultivars. It must be remembered that genetic resistance to a pest is a relative phenomenon and
that the expression of a resistance gene will differ in different genetic backgrounds. Therefore all
resistant lines cannot and do not possess the same level of resistance. Despite this and the
variable influence of unpredictable RWA infestation levels, the benefit of using resistant
cuItivars to control insect pests such as the Russian wheat aphid is high, as farmers purchase
insect control "inside" the seed they buy. Resistant cultivars offer a dramatic increase in yield
compared to susceptible cuItivars (Maras as et al., 1997). The host plant resistance works
throughout the season and therefore complex decisions such as choosing an insecticide,
determining the correct dosage, calibrating sprayers and timing the spray application to
ensure safe and effective control are-not a problem.
Trials to determine the efficacy of genetic resistance compared to insecticide control are both
time consuming and expensive and it is impractical to attempt to collect data for each of the ever
growing number of RW A resistant cuItivars that becomes available in the market. The solution
to this dilemma probably lies in the characterization of the level of resistance in the resistant
cuItivars. A field study is presently underway , to rank resistant cultivars by their level of
resistance expression and it is hoped that this data will answer many of the questions being
asked.
'
REFERENCES
Burd, J,D. and R.L. Burton. 1992. Characterization of plant damage caused by Russian wheat aphid
(Homoptera: Aphididae). J Econ. Entomol. 85: 2017-2022.
Du Toit, F. and M.e. Walters, 1984. Damage assessment and economic threshold values for the chemical
control of the Russian wheat aphid Diuraphis noxia (Mordvilko) on winter wheat. In: Progress in
Russian wheat aphid (Diuraphis noxia Mord .) research in the Republic of South Afric:l. Proceedings of
a meeting of the Russian Aphid Task Team held at the University of the Orange Free State,
Bloemfontein 5-6 May 1982. Walters, M.e. (ed.). Technical Communication No 191, Department of
Agriculture, Republic of South Africa.
Du Toit, F. 1992. Russian wheat aphid resistance in a wheat line from the Caspian Sea area. Cer. Res. Comm.
21: 55-6\.
Kruger, G.HJ. and P.H. Hewitt. 1984. The effect of Russian wheat aphid (Diuraphis noxia) extract on
photosynthesis of isolated chloroplasts: Preliminary studies. In: Progress in Russian wheat aphid
(Diuraphis noxia Mord.) research in the Republic of South Africa. Proceedings ofa meeting of the
Russian Aphid Task Team held at the University of the Orange Free State, Bloemfontein 5-6 May
1982. Walters, M.e. (ed.). Technical Communication No 191, Department ofAgriculture, Republic of
South Africa.
Marasas, e., Anandajayasekeram, P., Tolmay, V.L., Martella, D., Purchase, J.L. and GJ. Prinsloo. 1997. Socioeconomic
impact of the Russian wheat aphid control research program. SACCARJARC Report,
SACCAR, Gaberone, Botswana. 147 pp.
Tolmay, V.L., van Lill, D. and Smith, M.F. 1997. The influence of Demeton-S-MethyI!Parathion and
Imidacloprid on the yield and quality of Russian wheat aphid resistant and susceptible wheat cultivars.
S.A.J Plant & Soil 14(3): 107-111.
Van der Westhuisen, AJ. and Z. Pretorius. 1995. Biochemical and physiological responses of resistant and
susceptible wheat to Russian wheat aphid infestation. Cer. Res. Comm. 23(3): 305-313.
185