Effect of Nitrogen Fertilization on Host Plant Quality and on the Developmental Parameters of Asian Citrus Psyllid and its Nymphal Parasitoid Tamarixia radiat

The Asian citrus psyllid (ACP), Diaphorina citri Kumayama, 1908 (Hemiptera: Liviidae), is an invasive species in the U.S. that has been recognized as a serious threat to the citrus industry. Feeding of D. citri can cause twisting of young citrus leaves, shoot distortion, and affect the growth of young plants, but the pest is economically important because of its ability to transmit Candidatus Liberibacter asiaticus, the putative causal agent of citrus greening disease or Huanglongbing (HLB), one of the deadliest diseases known to citrus. Vector control is one of the recommended strategies to reduce the spread and incidence of HLB. The ectoparasitoid Tamarixia radiata (Waterston) (Hymenoptera: Eulophidae) is an effective biological control agent that contributes to significant reductions of D. citri populations. Reproduction and parasitism potential of T. radiata are dependent upon the quality of its psyllid nymphal host, which in turn is affected by host plant quality on which nymphs are developing. The host mediated effect of nitrogen fertilization of host plants on both D. citri and T. radiata life history parameters was tested with the goal of providing fertilization recommendations to maximize T. radiata production. Young potted orange jasmine [Murraya paniculata L. (Jack) (Sapindales: Rutaceae)] plants were each treated with 0, 7.6, 10.6, and 13.6 g of nitrogen as ammonium sulfate [(NH4)2SO4] per potted tree and infested with 10 pairs of mated D. citri adults in insect resistant cages, one week after fertilizer application. After 7 days of oviposition, adult psyllids were removed and eggs and nymphs were allowed to develop until most of them had reached the third to fourth instar stage. Half of the cages were used to evaluate the effect of nitrogen fertilization on D. citri reproduction by recording the number of adults produced. Each of the other half of cages--with orange jasmine plants infested with D. citri nymphs--received five pairs of mated T. radiata adults for the parasitism study. The numbers of emerging parasitoids were recorded upon emergence, then counted, and sexed. Nitrogen fertilization induced profuse flush shoot production in potted orange jasmine plants, with significantly more new shoots produced on fertilized plants than the untreated control. Consequently, the number of D. citri produced per plant was significantly higher on plants that received the low (7.6 g (NH4)2S04/plant) and medium (10.6 g(NH4)2S04/plant) fertilization rates, but not on the high rate of 13.6 g(NH4)2S0 4 per plant. Nitrogen fertilization also significantly increased the percentage of nymphal parasitism by T. radiata, but this occurred only for the low and medium N application rate. In addition, immature developmental time of T. radiata was shorter on nymphs feeding on fertilized plants as compared to the unfertilized ones. No significant host mediated effects of nitrogen fertilization were observed on the sex ratio of T. radiata nor the size of adults. In all treatments female T. radiata were larger than their male counterparts. Results of this study suggest that nitrogen fertilization can improve the host plant quality for D. citri through availability of more flush shoots and higher nutritional quality of phloem sap--as shown by the higher amino acid content of phloem sap and higher nitrogen level of leaf tissue--that translate into higher nymphal parasitization rates by T. radiata and faster immature development of the parasitoid on D. citri nymphs. This host-mediated effect of nitrogen fertilization on T. radiata performance and development opens avenues for improving mass production of this parasitoid for releases in biological control programs.