(A) Mean (± SEM) relative gene expression levels of group 1 (DvvGr1), group 2 (DvvGr2), and group 3 (DvvGr3) CO2 receptors after WCR larvae were fed with dsRNA-expressing bacteria targeting green fluorescent protein (GFP, herein referred to as WT), DvvGr1, DvvGr2, or DvvGr3 genes (n = 11–13). Different letters indicate significant differences of gene expression levels (p < 0.05 by one-way ANOVA with Holm’s multiple-comparisons test). (B) Mean (± SEM) CO2 levels in each L-shaped pot of the two-arm belowground olfactometers used to test the attractive and repellent effects of CO2 to WCR larvae (n = 4–8). Asterisk indicates significant differences in CO2 levels (*p < 0.05 by Student’s t test). For detailed data on CO2 levels, refer to Figure 3—figure supplement 1. (C) Mean (± SEM) proportion of WCR larvae observed in the olfactometer arms with higher CO2 levels (carbonated water side) or in control arms (distilled water side). Larvae were considered to have made a choice when they were found at a distance of 1 cm or less from the wire mesh (indicated by dashed green lines). Seven olfactometers with six larvae each were assayed (n = 7). (D) Petri plates used to test insect responses to methyl anthranilate, to Fe(III)(DIMBOA)3, and to sugars. (E) Mean (± SEM) proportion of WCR larvae observed on roots or on roots placed next to filter paper discs impregnated with methyl anthranilate, (F) on filter paper discs impregnated with buffer or with Fe(III)(DIMBOA)3, and (G) on filter paper discs impregnated with buffer or with a blend of glucose, fructose, and sucrose. For (E), 10 choice arenas with five larvae each were assayed (n = 10). Larvae were considered to have made a choice when they made physical contact with the roots or the filter paper discs. For (F, G), 20 choice arenas with six larvae each were assayed (n = 20). Asterisks indicate statistically significant differences in larval choices between treatments (***p < 0.001 by generalized linear model followed by False discovery rate (FDR)-corrected post hoc tests). Note that the number of replicates across experiments varied depending on the availability of insects. For details regarding the statistical results, refer to Supplementary file 1. Raw data are available in Figure 3—source data 1.