PS 22-57 - Near-continental variation in the community structure of natural enemy guilds of an invasive aphid

Abstract: Following the invasion of an invasive aphid Melanaphis sorghi in the US, a guild of parasitoids and predators quickly adapted to this new prey resource. Recent evidence indicates that these natural enemies regulate aphid densities in some but not all areas of Texas and Oklahoma. Based on exclusion studies, aphid regulation appears to be dependent on the natural enemy guild assemblage and species richness. Multi-year data for natural enemies abundance and species richness have been collected, but unfortunately, previous analyses have summed data from pre-determined large areas of Texas and Oklahoma. These studies ignored dynamics at smaller spatial scales and landscape features that predict natural enemy abundance and richness.
Our goal was to describe natural enemy guild assemblages and species richness within smaller regions of Texas and Oklahoma. We were interested in identifying landscape features within smaller regions that correlate with guild assemblages and species richness to inform conservation approaches for aphid parasitoids and predators.
Our approach to addressing our goal was to assess richness and abundance of aphid parasitoids and predators from a multi-year data set comprised of 667 one-hectare sorghum field sampling events and a total of 72,606 whole-leaf sample units collected as part nested sampling design from southern Texas to northern Oklahoma. Instead of pre-determined geographic regions, we used findings to define spatial scales/regions at which gamma diversity was consistent, then evaluated variation in alpha and beta-diversity at decreasing spatial scales.
Here we show evidence of unique compositions of natural enemy guilds within smaller regions of Texas and Oklahoma, and differences in natural enemy abundance and richness at decreasing spatial scales. These smaller regions are characterized by landscape features that may be predictive of natural enemy abundance and richness that predict aphid regulation. Interestingly, competing parasitoid species rarely co-occur at the smallest spatial scale (plant leaves) even when together in the same fields.
In conclusion this data provides empirical evidence that natural enemy guilds of the invasive aphid M. sorghi are likely local and comprised of resident populations that respond to migrating aphid colonizers each year. In addition, among highly competitive parasitoids it appears that detection of conspecifics and discrimination among small patches are potential mechanisms of niche partitioning allowing for co-existence at smaller spatial scales. Altogether, these findings support hypotheses that predict greater natural enemy species richness and abundance in more diverse landscapes and inform conservation efforts of natural enemies for regulation of invasive aphids.