THE INFLUENCE OF NITROGEN-FERTILIZER ON THE POPULATION DEVELOPMENT OFTHE CEREAL APHIDS SITOBION-AVENAE (F) AND METOPOLOPHIUM-DIRHODUM (WLK) ON FIELD-GROWN WINTER-WHEAT

The effect of nitrogen fertiliser on populations of the cereal aphids Sitobion avenae and Metopolophium dirhodum on winter wheat was investi gated in a three year field experiment. Naturally occurring aphid popu lations were monitored on three nitrogen treatments; none, nitrogen ap plication using Canopy Management guidelines (130-210 kg ha(-1)) and c onventional practice (190 kg ha(-1)). Inoculations of laboratory reare d S. avenae were used to enhance field populations on half the plots. Natural populations of M. dirhodum remained below the current UK spray threshold level of two-thirds of shoots infested at the start of flow ering, or five aphids per shoot in all years, whilst populations of S. avenae exceeded the threshold in all years. The response of the two s pecies to nitrogen differed, Significantly higher populations of M. di rhodum were recorded in both treatments which received nitrogen in all years, whilst the response of S. avenae varied between years, In 1994 and 1995 when environmental conditions favoured aphid development, hi gher populations were recorded in the two treatments which received ni trogen. In 1993 when high rainfall created unfavourable conditions, hi gher populations were recorded in the plots receiving no nitrogen. Dif ferences in peak density and cumulative aphid index of S. avenae resul ted from differences in the rate of population increase between ear em ergence and peak density on the different treatments. Populations prio r to ear emergence were higher in the plots which received nitrogen bu t the differences were not statistically significant. There was no evi dence of a difference in the timing of population decline in the diffe rent treatments, In 1993 higher levels of infection by entomopathogeni c fungi were observed in all treatments. Significantly higher levels o f infection were recorded in the treatments receiving nitrogen, which may have accounted for the lower S. avenae populations recorded. It is possible that the larger canopies recorded in these treatments produc ed conditions which favoured infection by fungi, thereby limiting aphi d population growth. The results indicate that application of nitrogen increases natural populations of M. dirhodum, and under favourable co nditions, populations of S. avenae. However, in suboptimal climatic co nditions, the application of nitrogen fertiliser can lead to lower pop ulations of S. avenae. The data also suggest that there is no consiste nt difference between a conventional and Canopy Managed approach to ni trogen fertiliser use in terms of the risk of infestation by cereal ap hids.