Differential performance and parasitism of caterpillars on maize inbred lines with distinctly different herbivore-induced volatile emissions

Abstract

Plant volatiles induced by insect feeding are known to attract natural enemies of the herbivores. Six maize inbred lines that showed distinctly different patterns of volatile emission in laboratory assays were planted in randomized plots in the Central Mexican Highlands to test their ability to recruit parasitic wasps under field conditions. The plants were artificially infested with neonate larvae of the fall armyworm Spodoptera frugiperda, and two of its main endoparasitoids, Campoletis sonorensis and Cotesia marginiventris, were released in the plots. Volatiles were collected from equally treated reference plants in the neighbourhood of the experimental field. The cumulative amount of 36 quantified volatile compounds determined for each line was in good accordance with findings from the laboratory; there was an almost 15-fold difference in total emission between the two extreme lines. We found significant differences among the lines with respect to the numbers of armyworms recovered from the plants, their average weight gain and parasitism rates. Average weight of the caterpillars was negatively correlated with the average total amount of volatiles released by the six inbred lines. However, neither total volatile emission nor any specific single compound within the blend could explain the differential parasitism rates among the lines, with the possible exception of (E)-2-hexenal for Campoletis sonorensis and methyl salicylate for Cotesia marginiventris. Herbivore-induced plant volatiles and/or correlates thereof contribute to reducing insect damage of maize plants through direct plant defence and enhanced attraction of parasitoids, alleged indirect defence. The potential to exploit these volatiles for pest control deserves to be further evaluated.

Figure 1. Arrangement of the experimental plots with the 6 maize inbred lines.

The enlargement shows the generalized location of the plants that were infested with Spodoptera frugiperda larvae inside the plot, the actual pattern being somewhat variable among the plots depending on the availability of suitable plants. The six plots to the left and the bigger plot with plants of the variety “Delprim” were used for volatile collections.

Figure 2. Release of parasitoids and infestation of plants with caterpillars.

A) Numbers of female wasps and parasitized caterpillars released over the experimental period. B) Timing of infestation with neonate Spodoptera frugiperda larvae and of the subsequent recovery of the caterpillars remaining on the infested plants as well as of the volatile collections, whereby each line represents groups of plants that were infested together.

Figure 3. Overview of the parameters assessed for six maize inbred lines arranged in order of increasing total volatile emission.

A-C) plant parameters; D-F) performance of the herbivore Spodoptera frugiperda; G-I) parasitism; J-L) natural infestation with herbivores and potential predators; n = number of plants; N = number of arthropods.

Figure 4. Performance of Spodoptera frugiperda caterpillars on the six maize inbred lines under laboratory and field conditions (r = 0.81; p = 0.0505).

Figure 5. Total volatile emission of the six maize inbred lines according to collections conducted in the laboratory (six-arm olfactometer) as compared to collections carried out in the field (r = 0.92, p = 0.0093).

Figure 6. Principal component analysis (PCA) of herbivore-induced volatile emission and of other plant parameters.

A) Biplots of a PCA of the 36 volatile compounds (for identity see Table 1) released by the individual maize plants. B) PCA of the average values for each line of the volatiles pooled into seven groups and of the other variates (A, B, D–L as in Fig. 3 and Table 3).