Host‐plant quality alters grass/forb consumption by a mixed‐feeding insect herbivore, Melanoplus bivittatus (Orthoptera: Acrididae)

Abstract

1. Factors affecting the nutritional ecology of mixed‐feeding, polyphagous herbivores are poorly understood. Mixed‐feeding herbivores do better when they consume both forb and grass species although they typically feed primarily on forbs, which are of relatively higher protein content than grasses. 2. In a field experiment, we examined the effects of nitrogen and phosphorus fertilization and associated changes in host‐plant C:N:P on proportional grass consumption by a mixed‐feeding insect herbivore, Melanoplus bivittatus, using natural abundance stable carbon isotope (12C/13C) methods. We also examined a grass‐feeding (Phoetaliotes nebrascensis) and forb‐feeding (Hesperotettix viridis) species. 3. The C isotope signatures of M. bivittatus collected from plots fertilized with nitrogen (+N), phosphorus (+P), nitrogen and phosphorus (+N+P) and no fertilizer were compared with the C isotope signatures of plants in those plots to determine the proportion of assimilated C derived from C4 grasses and C3 forbs in each plot. We also examined the relationship between M. bivittatus diets and plant C:N:P stoichiometry. 4. The proportion of grass assimilated approximately doubled in N‐fertilized treatments (39.1 ± 0.1%) compared with non‐fertilized treatments (19 ± <0.1%), an increase associated with decreased C:N and increased N:P of grasses. 5. These results indicate that mixed‐feeding M. bivittatus can selectively feed to balance C:N:P intake even when choosing between two structurally and chemically different groups of plants. 6. The strong relationship between diet selection and grass stoichiometry also suggests that plant nutrient composition may be more important than defensive chemistry in food choice.

Herbivore and pollinator responses to grassland management intensity along experimental changes in plant species richness

“…As biomass had no influence on the number of grasshopper individuals, grasshoppers appeared to be most attracted by subplots with the highest food quality due to nutrient enrichment of the plants by fertilization. Jonas and Joern (2008) also found higher feeding activity of grasshoppers on grasses and forbs when they were fertilized. Furthermore, grasshoppers in a study of Ritchie (2000) had a strong positive response to nitrogen addition, which manifested in a 30% increase in the food quality of their host plants.…”

Not just the usual suspects: Insect herbivore populations and communities are associated with multiple plant nutrients

“…Perhaps this suggests that plant N levels are most informative with respect to population abundance of specialist insect herbivores on only grasses or forbs. For generalist insect herbivores, foliar-N levels may be less useful because eating a broad diet may allow them to reach their preferred N (protein) intake by switching back and forth between high-N (forbs) and low-N (grasses) plants (Jonas and Joern 2008).…”

Multiple host use by a sap-sucking membracid: population consequences of nymphal development on primary and secondary host plant species

“…Outbreaks on fiddlewood are normally associated with defoliation of the affected stems and a general decline in host quality (Palmer et al 2004). Host plant physiological condition, in turn, may affect the oviposition rate, development time and survival of insect herbivores (Gols et al 2007;Jonas and Joern 2008;Kay et al 2007;Sipura and Tahvanainen 2000;Van Hezewijk et al 2008), and may help explain patterns of host use in particular situations.…”