Parasitism declines with distance from the site of introduction for the kudzu bug, Megacopta cribraria (F.), and depends on host density at different spatial scales

Abstract

Individuals at the frontlines of expanding populations tend to experience less parasitism than those in the core of their range. This reduction in parasitism may contribute to the rapid spread of invasive species because parasites could otherwise reduce host population growth. To better understand patterns of parasitism in the invasive population of the kudzu bug, Megacopta cribraria (F.), we examined how both the proportion of eggs parasitized and the incidence of parasitism—presence or absence of parasitism within a clutch of eggs—varied with distance from the kudzu bug’s site of introduction and with their density at two different spatial scales: at the level of sites (10–20 m2) and the level of the egg clutch (number of eggs in a clutch). Egg clutches were collected from populations along an east–west and a south-north transect of the U.S. invasion. We found that both metrics of parasitism declined with increasing host-density at the site level, which may be a general trend for the wind-dispersed parasites of invasive species. We also found that both metrics of parasitism declined with increasing distance from the site of introduction. However, since kudzu bug density tended to decrease across their invasion gradient and parasitism rates increased with decreasing kudzu bug density, the reductions in parasitism with increasing distance from site of introduction were not discernible if the effect of site-level density was not accounted for; leaving site-level density out of our models made it seem as though there was no distance effect. This finding complicates models that exhibit a “honeymoon phase”, where vanguard populations of the host suffer from little or no parasitism. In addition, we observed a small effect of clutch size on the incidence of parasitism, but no interaction among the spatial scales of density investigated. Our results underscore the importance of considering host density at different scales when seeking to understand trends in parasitism across a host’s invasive range.