Herbicide Stress Inducesbeetle Oviposition on Red Maples

Abstract

Flatheaded borers (FHB; Chrysobothris spp.), are woodboring-beetles that lay their eggs in the bark and cambium of deciduous trees in North America. Females often target stressed host-plants for oviposition. The reason why is unknown; however, stressed plants often suffer various induced phytochemical changes that may enhance larval infestation success depending on the stressor such as induced upregulation of defenses, reallocation of nutrients, and changes to volatile organic compound (VOC) emissions. To understand attraction of FHB to specific stress-induced changes, we analyzed phytochemical changes associated with stress treatments and attractiveness maple trees to FHB. Trees were stressed by: (1) chemical stress (pelargonic acid herbicide), (2) physical stress (physically removing leaves), and (3) physical stress (removing portions of bark near the root crown). After reflush of defoliated trees, bark tissues where FHB larvae feed were analyzed for nutritional changes (carbon and nitrogen), anti-nutritive changes (polyphenols and tannins) and emissions of foliar VOCs. At the end of the growing season, trees were assessed for FHB larval presence and oviposition attempts. There were more larvae and oviposition attempts on trees stressed by herbicide application. Compared to other treatments, herbicide-stressed trees had greater nitrogen and total polyphenol concentrations. Greater nitrogen may play a role in the fitness of feeding larvae, and the greater polyphenol concentration may stimulate female oviposition in the herbicide stressed trees. Females may be able to locate the herbicide-stressed trees by using volatile cues such as increases in limonene, α-farnesene, (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT) and hexenyl acetate.

Keywords: Flatheaded borer; Insect pests; Nutritional constituents; Plant-insect interactions; Tree pest; Volatile organic compounds.

Fig. 1

Concentration of total non-structural carbohydrates (A: sugar and B: starch) in trunk tissues or red maple trees subjected to various stressors. “G.E.” = glucose equivalents. Significant differences indicated by different capital letters from Tukey’s HSD

Fig. 2

Percent nitrogen found in the trunk tissues of red maple trees subjected to various stressors. Significant differences indicated by different capital letters from Tukey’s HSD

Fig. 3

Ratio of macronutrients (carbon: nitrogen) in trunk tissues of red maple trees subjected to various stressors. Significant differences indicated by different capital letters from Tukey’s HSD

Fig. 4

Concentrations of polyphenol (A) and tannin (B) in trunk tissues of red maple trees subjected to various stressors. “G.A.E.” = gallic acid equivalents. “T.A.E.” = tannic acid equivalents. Significant differences are indicated by capital letters from Tukey’s HSD

Fig. 5

Ridgeline plot showing the percentage of limonene in control and herbicided treatments. Percentages represent the ratio of the compound in relation to the top 18 compounds used for analysis (see Methods)

Fig. 6

Ridgeline plots showing the percentage of compounds in control and herbicided treatments. Peaks indicate the number of samples that were found to have the percent (%) indicated on the x-axis so that higher peaks demonstrate a greater number of samples with that percentage of compound. In each figure, the compounds are listed from the most abundant to those minimally or not expressed. Only compounds that are statistically different between control and herbicided treatments are shown in these figures. Percentages represent the ratio of the compound in relation to the top 18 compounds used for analysis (see Methods)

Fig. 7

Principal component analysis (PCA) plot of volatile organic compounds (VOCs) found in the chemical profile of red maples in control treatments and herbicided treatments. The axes values represent the first two PCA axes (percent explained variation and percent importance of individual compounds to each axis are recorded in Tables 4 and 5, respectively). In total, 63.9% of the variation is explained in PC1 and 15.5% of the variation is explained in PC2