A study of root aphid Aploneura lentisci Pass. biology and root aphid-host interactions with perennial ryegrass/endophyte associations in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Entomology at Massey University, Manawatū, New Zealand

Abstract

The root aphid Aploneura lentisci Pass. is an underestimated, under-researched pasture pest likely to become more problematic in New Zealand if the environmental temperature and the frequency of water deficit stress increase, as predicted. The research presented here aimed at gaining first insights into its biology and interaction with plants and endophytes to promote future pest management research. For this purpose, root aphids were observed in model systems (in climate chambers, glasshouse or insectary; in empty microcentrifuge tubes or on diploid perennial ryegrass Lolium perenne L. plants grown on nutrient-enriched agar, with or without endophy Epichloë festucae var. lolii [Latch, M.J. Chr. & Samuels] C.W. Bacon & Schard of the AR1, AR37 or common-toxic CT strains). Apterous neonate offspring, the presumed main dispersal stage of A. lentisci, survived up to four weeks without food (median survival: 8 days). On endophyte-free, mature ryegrass kept at 17 to 21 °C, neonates developed to adults within three to four weeks and lived about two months, feeding mainly on young roots of first and second branching order. Taking into account lower outdoor temperatures, root aphids are thus likely to complete six to nine generations per year in the field. Adults produced 39 to 70 offspring over their lifetime. Presuming a similar nymphal mortality in the field as in the experiments, outdoor root aphid populations could theoretically multiply 23- to 45-fold at each generation. Root aphids raised on endophyte-infected, mature plants were shorter-lived than peers raised on endophyte-free plants. Most aphids on AR37-infected plants did not even reach reproductive maturity. The response to CT-infection was dependent on the plant genotype. Why AR1-infected plants frequently support larger root aphid populations than endophyte-free plants in the field could not be explained by the data collected, however. Root aphid feeding affected the root biomass but not the shoot biomass of perennial ryegrass in the experimental environment. This finding differed from previous reports. Furthermore, colour analyses suggested root aphid feeding could modify some leaf properties. More research will be required to confirm these findings and assess whether irrigation or fertilisation could mitigate root aphid yield losses in the field.