Alteration of host plant quality by an omnivorous mirid bug (Dicyphus hesperus) and its implications for interactions with resident leafhoppers (Empoasca spp.)

Background/Question/Methods

Mirid bugs (Heteroptera: Miridae) are known for their capacity to alter the growth patterns of young leaves. Problems associated with leaf shape have been documented, but issues with proliferation of defensive structures such as trichomes are also possible. Should these leaves be less defended, feeding by omnivorous mirids would thereby alter host plant quality, and potentially initiate a novel pathway for tri-trophic interactions.

At Big Creek Reserve near Lucia, CA, feeding by the mirid Dicyphus hesperus on the developing leaf buds of wood mint (Lamiaceae: Stachys bullata) results in short, asymmetrically curled leaves (pers. obs.). We hypothesized that 1) these leaves would also be less structurally defended (lower trichome densities) and 2) they would incur more feeding damage by the leafhopper Empoasca sp., a resident herbivore of wood mint.

We conducted a field survey in August of 2012 in order to address these hypotheses preliminarily. We recorded leaf characteristics including length to width ratio and leaf asymmetry (measured as deviation from 1:1 ratio of leaf area on either side of the midvein). We also recorded the proportion of leaf area damaged by leafhoppers. Ordinary least squares regression was used to compare leaf characteristics to damage incurred. 

Results/Conclusions

We found a significant positive relationship between length to width ratio and trichome density (p<.01, r²=.12). Additionally, there was a strong negative trend between the degree of leaf asymmetry and trichome density. In turn, there was a significant negative relationship between trichome density and leaf damage (p<.01, r²=.15). Finally, we found a significant negative relationship between leaf length to width ratio and leaf damage (p=.02, r²=.06), as well as a strong positive trend between leaf asymmetry and damage. 

We conclude that leaf characteristics consistent with developmental alteration by D. hesperus on wood mint also correspond to more poorly defended and damaged leaves. 

As Dicyphus hesperus is an omnivore that preys on eggs and early instar nymphs of leafhoppers in addition to wood mint, we are currently trying to determine if its alteration of wood mint serves as an indirect source of food, i.e. if it is “farming” its prey. Ongoing mesocosm experiments in both the greenhouse and field are aimed to address the hypothesis that Dicyphus hesperus reaps the benefits of the developmental changes it causes in wood mint by increasing the quality and abundance of its leafhopper prey.