Effects of nitrogen and potassium in kikuyu grass on feeding by yellow sugarcane aphid

In Hawaii, infestations of yellow sugarcane aphid (YSA), Sipha flava (Forbes) (Homoptera: Aphididae) reduced growth of the forage grass, kikuyu (Pennisetum clandestinum Hochst. ex Chiov). To determine the effects of nitrogen (N) and potassium (K) on tolerance of kikuyu grass to YSA, cuttings of eight and five cultivars were grown in the greenhouse using nutrient solutions in two separate trials, respectively. The first trial was conducted during the summer of 1991 and the second during the winter of 1993. In both trials, kikuyu was grown at three N levels (0.05, 0.5 and 3.0 mM) and four K levels (0.05, 0.5 1.0 and 3.0 mM), with one cultivar comprising a block. Prior to exposure to aphids, representative plants were harvested, and shoots were analyzed for foliar nutrients. Then, plants were confined with aphids and rated visually for YSA injury. Dry matter yields and foliar N concentration increased significantly with increasing N fertilization in both trials. In the first trial, there was a significant interaction between N and K levels, in which the greatest increases of shoot and root dry matter with increasing N levels were found at the highest K level. In the second trial, K fertilization had no effect on dry weight of shoots. In both trials, foliar K concentration increased significantly with increasing K levels. Damage due to YSA tended to increase with increasing N levels, although it was unaffected by K fertilization in both trials. Thus, fertilization with increasing N resulted in greater kikuyu dry matter production, but it also tended to increase the damage caused by YSA feeding.