Dietary Stress From Plant Secondary Metabolites Contributes to Grasshopper (Oedaleus asiaticus) Migration or Plague by Regulating Insect Insulin-Like Signaling Pathway

Abstract

Diets essentially affect the ecological distribution of insects, and may contribute to or even accelerate pest plague outbreaks. The grasshopper, Oedaleus asiaticus B-Bienko (OA), is a persistent pest occurring in northern Asian grasslands. Migration and plague of this grasshopper is tightly related to two specific food plants, Stipa krylovii Roshev and Leymus chinensis (Trin.) Tzvel. However, how these diets regulate and contribute to plague is not clearly understood. Ecological studies have shown that L. chinensis is detrimental to OA growth due to the presence of high secondary metabolites, and that S. krylovii is beneficial because of the low levels of secondary metabolites. Moreover, in field habitats consisting mainly of these two grasses, OA density has negative correlation to high secondary metabolites and a positive correlation to nutrition content for high energy demand. These two grasses act as a 'push-pull,' thus enabling the grasshopper plague. Molecular analysis showed that gene expression and protein phosphorylation level of the IGF → FOXO cascade in the insulin-like signaling pathway (ILP) of OA negatively correlated to dietary secondary metabolites. High secondary metabolites in L. chinensis down-regulates the ILP pathway that generally is detrimental to insect survival and growth, and benefits insect detoxification with high energy cost. The changed ILP could explain the poor growth of grasshoppers and fewer distributions in the presence of L. chinensis. Plants can substantially affect grasshopper gene expression, protein function, growth, and ecological distribution. Down-regulation of grasshopper ILP due to diet stress caused by high secondary metabolites containing plants, such as L. chinensis, results in poor grasshopper growth and consequently drives grasshopper migration to preferable diet, such as S. krylovii, thus contributing to grasshopper plague outbreaks.

Keywords: diet stress; gene; grasshopper; plague; plant secondary metabolites.

FIGURE 1

IGF-PI₃K-AKT-FOXO pathway of insulin signaling. The insulin-like signaling system includes different well-defined ligands, such as insulin-like growth factor (IGF), which regulate the activity of the homologous insulin receptor INSR. Insulin receptor substrate (IRS) proteins act as messenger molecule-activated receptors to signaling, and which is an important step in insulin’s action. Phosphoinositide 3-kinase (PI₃K), 3-phosphoinositide-dependent protein kinase (PDK), and protein kinase B (AKT), three major nodes downstream of IRS, and have been implicated in many of the metabolic actions of insulin. The forkhead transcription factor (FOXO) regulates transcription of genes involved in stress resistance, xenobiotic detoxification and DNA repair. FOXO is negatively regulated by insulin-like signaling when the PI₃K → AKT cascade stimulates phosphorylation of FOXO and promotes its secretion from the nucleus and inactivation in the cytosol.

FIGURE 2

Percentage (±SD, %) of nutrition components (starch, lipid, and crude protein) in the grass species L. chinensis and S. krylovii. ^∗indicates P < 0.05 (t test).

FIGURE 3

Percentage (±SD, %) of secondary metabolites (terpenoids, tannins, phenols, alkaloids, and flavonoids) in the grass species S. krylovii and L. chinensis. ^∗indicates P < 0.05 (t test).

FIGURE 4

(A) Mean survival rate of O. asiaticus from fourth instar to adult, (B) mean dry mass (mg) of adults, (C) mean developmental time (days) from fourth instar to adult, (D) growth rate (mg/day), and (E) overall performance when fed on S. krylovii (Sk) or L. chinensis (Lc). Data are mean ± SD. ^∗indicates P < 0.05 (t test).

FIGURE 5

Relationship between grasshopper overall performance and plant chemical composition. Red circles indicate total nutrition, and blue triangles indicate total secondary metabolites.

FIGURE 6

Relationship between the relative density of O. asiaticus (mean number of individuals per 100 sweep-nets) and mean above-ground biomass (g/m²) of S. krylovii (red circles) and L. chinensis (blue circles). Data combined from measurements recorded from 2011 – 2017 (N = 8 means per year), with 2011–2014 values from our published data.

FIGURE 7

Relationship between relative density of grasshopper O. asiaticus (mean number of individuals per 100 sweep-nets) and chemical traits (g/m²). Red circles indicate total nutrition, and blue circles indicate total secondary metabolites.

FIGURE 8

Relative expression (±SD) of seven genes of ILP in grasshopper, O. asiaticus, that fed on S. krylovii (Sk), and L. chinensis (Lc). ^∗indicates P < 0.05 (t test). IGF (A), insulin-like growth factor; INSR (B), homologous insulin receptor; IRS (C), insulin receptor substrate; PI₃K (D), phosphoinositide 3-kinase; PDK (E), 3-phosphoinositide-dependent protein kinase; AKT (F), protein kinase B; and FOXO (G), forkhead transcription factor.

FIGURE 9

Relationship between relative gene expression in grasshopper and chemical traits in plants (%). Red circles indicate total nutrition, and blue circles indicate total secondary metabolites. IGF (A), insulin-like growth factor; INSR (B), homologous insulin receptor; IRS (C), insulin receptor substrate; PI₃K (D), phosphoinositide 3-kinase; PDK (E), 3-phosphoinositide-dependent protein kinase; AKT (F), protein kinase B; and FOXO (G), forkhead transcription factor.

FIGURE 10

Phosphorylation level (pg/g) of four proteins of ILP in O. asiaticus that fed on S. krylovii (Sk) and L. chinensis (Lc). ^∗indicates P < 0.05 (t test). P-INSR (A), Phosphorylated homologous insulin receptor; P-IRS (B), Phosphorylated insulin receptor substrate; P-AKT (C), Phosphorylated protein kinase B; and P-FOXO (D), Phosphorylated forkhead transcription factor.

FIGURE 11

Relationships between protein phosphorylation level (pg/g) of ILP and grass chemical traits in plants (%). Red dots indicate total nutrition; blue dots indicate total secondary metabolites. P-INSR (A), Phosphorylated homologous insulin receptor; P-IRS (B), Phosphorylated insulin receptor substrate; P-AKT (C), Phosphorylated protein kinase B; and P-FOXO (D), Phosphorylated forkhead transcription factor.

FIGURE 12

Illustration of the ‘push-pull’ roles of L. chinensis and S. krylovii to O. asiaticus migration and plague.

FIGURE 13

Regulation of gene expression (A) and protein phosphorylation level (B) of ILP (IGF → FOXO cascade) during different diet stress. IGF, insulin-like growth factor; INSR, homologous insulin receptor; IRS, insulin receptor substrate; PI₃K, phosphoinositide 3-kinase; PDK, 3-phosphoinositide-dependent protein kinase; AKT, protein kinase B; and FOXO, forkhead transcription factor.