Induced response of three wheat varieties to greenbug, Schizaphis graminum (Rondani), under laboratory conditions

Wheat (Triticum aestivum L.) is considered one of the most important food sources of the people of the world, so that about 18 percent of the calories and 19 percent of the protein needed by humans are supplied from this grain. The greenbug, Schizaphis graminum is one of the most important pests of wheat in many parts of the world, including Iran, which causes a significant damage to this crop annually. Pest pre-infestation and measuring demographic parameters is one of the most common methods to find out induced resistance in host plants. Due to the environmental and health consequences of excessive spraying of pesticides in wheat fields, use of "induced resistance" as a biorational method, has received a lot of attention in wheat integrated pest management in recent years. The main objectives of this study were to investigate the induced responses of three wheat varieties to S. graminum and also to find the most suitable pre-infestation period to activate this reaction in host varieties.

The seeds of three wheat varieties (Kohdasht, Morvarid and Aseman) were planted in growth chamber conditions (temperature of 25±2 Cº, relative humidity of 60±5% and a photoperiod of 14:10 L: D). After the plants reached the two-leaf stage, 5 one-day old female apterus aphids were placed on them and they were allowed to feed for 0, 2, 4, 6, and 8 days (the pre-infestation duration was considered as experimental treatment). After the desired pre-infestation duration, aphids were removed from the wheat plants and the plants were kept free of aphids for 48 hours. Then, the age-specific two-sex life table of aphid was made separately on the wheat plants corresponding to each variety and pre-infestation duration. 30 one-day-old first instar nymphs were used as a cohort and biological and demographic parameters of aphid were measured and compared using the paired bootstrap test.

Aseman variety had higher resistance to S. graminum compared to Morvarid and Kohdasht varieties, based on the values of demographic parameters. Due to aphid’s longer nymphal developmental time and mean generation time (T), shorter longevity and lower fecundity on Aseman variety, the intrinsic rate of increase (rm) on this variety (0.245 1/day) was significantly lower than Morvarid and Kohdasht varieties (0.348 and 0.358 1/day, respectively). In all varieties, pre-infestation to aphid reduced the values of biological and demographic parameters, but the effective pre-infestation duration was different according to the variety and studied parameter. With increasing pre-infestation duration from 0 to 8 days, the nymphal developmental time increased, while fecundity and the intrinsic rate of increase (rm) decreased significantly. In Aseman, Morvarid and Kohdasht varieties, the rm decreased from 0.245 to 0.187, from 0.348 to 0.275 and from 0.385 to 0.335 1/day, respectively, while pre-infestation duration increased from 0 to 8 days. In Aseman variety, which was relatively more resistant to S. graminum, the nymphal developmental time and the intrinsic rate increase (rm) changed significantly after a 2-days pre-infestation period, while aphid’s fecundity changed significantly after a 6-days pre-infestation duration. In Morvarid variety, nymphal developmental time, female fecundity and intrinsic rate of increase changed significantly after 6, 4 and 2-days pre-infestation durations, respectively. In all three varieties, intrinsic rate of increase (rm), as the most important demographic parameter, was changed significantly after a 2-days pre-infestation time.

The demographic response of S. graminum on the studied varieties was different and Aseman variety had higher relative resistance to wheat green aphid. In all three varieties, the pre-infestation of wheat plants with aphids led to induced resistance in them and reduced the biological and demographic potential of green aphid. Because of showing the appropriate induction reaction to S. graminum, planting Kohdasht (is recommended as a resistant variety to pathogenic fungi and suitable for cultivation in dry and semi-arid drylands) and Morvarid (a variety resistant to pathogenic fungi for planting in humid and plain areas) varieties can help to reduce the population of this aphid and increase wheat production in these areas. The results of this research can be useful in identifying resistance sources and mechanisms and implementation of integrated management of aphid in wheat fields of Iran.