Noncrop Host Plant Associations for Oversummering of Diuraphis noxia in the State of South Australia

Diuraphis noxia, Russian wheat aphid (Hemiptera: Aphididae), established in Australia since 2016, is dependent on grasses (Poales: Poaceae) to persist in the low-rainfall Australian wheat belt, where no crops are present during summer. To identify grasses as D. noxia hosts in Australia, plants were tested in greenhouse conditions as either whole plants collected from roadsides or grown from collected seed in 2017 and 2018. To determine actual field refugia, direct grass sampling and Berlese extraction of aphids were conducted from October 2018 to May 2020 throughout Southern Australia (2,285 samples). One hundred and twentysix grass species were collected, 54 showed presence of D. noxia, of which 24 were considered host plants, including 16 species (9 Australian natives) not recorded as host plants previously. Hordeum leporinum (Link) Arcang. Poales:Poaceae and several Bromus species (Poales: Poaceae) showed the highest D. noxia detection frequency and aphid numbers, but these introduced grass species are not summer active in most of South Australia. The native Enneapogon nigricans (Poales: Poaceae) (R.Br.) is the most important summer refuge species because of its widespread distribution, summer growth, and an intermediate level of positive detections with low D. noxia populations. The late summer represents the main bottleneck for D. noxia with very few hosts available and very low D. noxia detections overall. Late summer rainfall (February) seems essential to have the main host grasses germinate for D. noxia populations to build up and potentially invade crops sown in autumn.