Oviposition preference of adult S. frugiperda female
When we checked the number of egg masses laid by S. frugiperda female on the different maize varieties in the multiple choice experiment, we found significant difference between the maize varieties (p < 0.05, n = 72, df = 64), but no significant interaction between hours of oviposition and maize variety.
Significantly fewer egg masses were laid over the 72-hour period on P3966W and LMFP but not on DEKAIB, 30Y87, SWAN 1 and V9928 (Fig. 1A). However, after 24 h of oviposition, the number of egg masses found on DEKAIB was significantly lower than those of P3966W, LMFP and V9928 varieties (Fig. 1A). Similarly, in the no-choice experiment, we found significant difference between the maize varieties but no interaction with hours of oviposition (p < 0.05, n = 72, df = 64). Across 72 h oviposition period, we observed significantly fewer egg masses (p < 0.05) of S. frugiperda on maize varieties, LMFP and SWAN 1, but not on DEKAIB, 30Y87 and P3966W (Fig. 1B). However, there was no significant difference in the number of egg masses laid across all maize varieties after 24 h of oviposition (Fig. 1B).
When we combined the egg masses over time for each maize variety in the multiple choice experiment (Fig. 1B), we found that DEKAIB and 30Y87 had significantly lower egg mass numbers than varieties, LMFP, SWAN 1 and V9928 (p < 0.05). However, similar numbers of egg mass was found on 30Y87 and P3966W (p > 0.05). Similarly, in the no-choice experiment, we found significantly lower egg mass number on DEKAIB and 30Y87 than on LMFP maize varieties (p < 0.05) (Fig. 1D).
When combining all three hybrid varieties and the OP varieties, egg mass counts only significantly differed between 24 h and 72 h in the multiple choice (Fig. 2A), with no significant difference in the number of eggs laid over 72 h in the no-choice experiment (Fig. 2A and B). In the OP varieties, we found significantly lower egg masses on the hybrid varieties after 72 h of oviposition than after 24 h both in the multiple choice and the no-choice test (p < 0.05). After 24 h of oviposition, significantly fewer egg masses were laid by S. frugiperda female on hybrid maize than on OP varieties in both experiments (p < 0.05) (Fig. 2A and B). When the cumulative egg mass counts were compared, egg masses on the hybrid varieties were significantly lower than those on the OP varieties in both multiple choice and no-choice experiments (p < 0.05) (Fig. 2C, D).
Response of Spodoptera frugiperda larvae to intact maize varieties
Olfactometer experiment with intact plants of the four maize varieties showed that significantly more S. frugiperda larvae (χ2 = 8.33; p < 0.05) chose LMFP maize variety than clean air (Fig. 3). Larvae did not show a preference to the other three tested maize varieties relative to clean air (χ2 = 0.33; p > 0.05), 30Y87 (χ2 = 0.33; p > 0.05) and DEKAIB (χ2 = 0.33; p > 0.05).
Figure 1 Mean number of egg masses (± SEM, n = 4) of Spodoptera frugiperda female. Line plots show number of egg masses on DEKAIB, 30Y87, P3966W (hybrid maize varieties), LMFP, SWAN 1 and V9928 (open pollinated varieties) over 72 h period on maize varieties in (A) multiple choice experiment and (B) no-choice experiments. * = significant difference between mean egg masses over 72 h of oviposition; ns = no significant differences between egg masses between the three time points (24, 48 and 72 h) of oviposition. Different letters under the error bars in a and b show that mean egg numbers between varieties are significantly different after 24h in multiple choice and no-choice experiments, respectively. C) Cumulative number of egg masses after 72 h of oviposition in multiple choice experiment and (D) cumulative number of egg masses in no-choice experiments. Boxes represent the lower and upper quartiles of egg masses on each maize, whiskers indicate the minimum and maximum values, middle line represents the median. Different letters above the bars in c and d indicate significant differences among maize varieties (p < 0.05)
Figure 2 Mean number of egg masses (± SEM, n = 4) of Spodoptera frugiperda female laid on two types of maize varieties. Line plots show the number of egg masses on hybrid and OP maize varieties over a 72 h period in (A) multiple choice experiment and (B) no-choice experiment. Different letters on line show significant difference between mean egg masses laid over time. * in between the error bars in A and B show that mean egg numbers are significantly different after 24 h between hybrid and OP maize varieties in multiple choice (A) and no-choice experiment (B), respectively. (C) Mean cumulative number of egg masses over 72 h oviposition period in the multiple choice experiment and (D) in the no-choice experiment. Boxes represent the lower and upper quartiles of egg masses on each maize type; whiskers indicate the minimum and maximum values, middle line represents the median. Different letters above the bars in C and D indicate significant differences between hybrid and OP varieties (p < 0.05)
Figure 3 Orientation responses of Spodoptera frugiperda larvae to odors from intact plants and clean air. Bar charts show the preferences of S. frugiperda larvae to odors from intact four maize varieties and clean air presented in a Y-tube olfactometer. Hybrid maize varieties (DEKAIB, 30Y87) are in italics, Open pollinated maize varieties (V9928, LMFP) are in bold. Significant means were separated using Chi-square (χ2) test (p < 0.05, n = 12), ns = not significant.