An Invasive Aphid Pest of Sorghum (Melanaphis Sorghi) and Its Natural Enemies Across Landscapes, Patches, and Host Plants

Abstract

Melanaphis sorghi is an invasive aphid pest of sorghum, which damages plants and reduces yields. An important aspect of sorghum IPM, a complex of natural enemies has been found suppressing M. sorghi. Densities of this aphid and its natural enemies vary considerably across sorghum growing regions of the U.S. This body of works investigated the variability through experiments and surveys of insect populations and dynamics across multiple spatial scales (landscape, patch, and plant). In Chapter II, associations between and M. sorghi, its natural enemies, and landscape structure were evaluated. More complex landscape configuration had a positive association with aphid densities. Natural enemies were similarly affected by landscape configuration, but with additional negative associations with landscape composition, mainly non-crop habitats. Melanaphis sorghi and its natural enemies were then surveyed in patches of the non-crop host Johnsongrass in Chapter III, to assess their variability between regions, habitats, and seasons. Findings indicated M. sorghi can vary across all factors. However, densities generally remained low, which was attributed to regular suppression of aphid populations. Both natural enemies and Johnsongrass characteristics seem to be important for explaining aphid densities. Melanaphis sorghi’s interactions with a co-occurring aphid and natural enemies were then evaluated at the plant scale in Chapter IV on Johnsongrass. These finding indicated natural enemy suppression varied between a lady beetle and parasitoid, with lady beetles suppressing both aphids and no M. sorghi suppression from the parasitoid. Although results indicated competition was not occurring during the laboratory experiment, an increased number of parasitized aphids from the co-occurring species and M. sorghi parasitism indicated greater parasitism in the presence of co-occurring aphid. Finally, Chapter V evaluated UAS technology to measure sorghum reflectance and estimate aphid densities on small groups of sorghum plants. This experiment demonstrated significant relationships between aphid pressure, sorghum reflectance, and yield. These weak associations indicated UAS will not replace visual sampling in the field. Potential drivers of M. sorghi variability investigated by this research, including landscape structure, alternative host patches, and co-occurring aphid prey for natural enemies seem affirms the importance of spatial effects on M. sorghi across spatial scales.