Constitutive and induced resistance in soybean interact to affect the performance of a herbivore and its parasitoid

• Herbivore-induced responses in soybean impair host and parasitoid development.
• Effects varied with level of constitutive resistance and herbivore feeding mode.
• The chewing herbivore had stronger inductive effects than the sucking herbivore.
• Herbivore treatments differed with low constitutive resistance, but similar with high.

This study examined how the induced defenses of soybean cultivars that vary in levels of constitutive resistance affect the developmental performance of the tobacco cutworm, Spodoptera litura, and its parasitoid, Meteorus pulchricornis. Both soybean cultivars were injured to induce defenses by either the beet armyworm, Spodoptera exigua, or by the aphid Aphis glycines. Both types of feeding on soybean plants had a negative effect on S. litura development, extending development time, reducing larva-to-pupa survival and decreasing adult body size. The impact of induction treatments increased linearly with increasing levels of constitutive resistance. Induction by the chewing herbivore was stronger than induction by the sucking herbivore in negatively affecting developmental parameters at both low and medium levels of soybean constitutive resistance, but had similar impacts on S. litura survival and adult body weight at the high level of constitutive resistance. Induction by both chewing and sucking herbivores extended parasitoid development time and decreased survival and fecundity, with effects on both development time and fecundity increasing linearly with levels of constitutive resistance, but without any effect on offspring survival. Thus, induction by the two herbivore types differed in impact on parasitoid performance depending on the level of constitutive resistance expressed in the soybean cultivar. The chewing herbivore was more effective than the sucking herbivore at low and medium levels of constitutive resistance, but there were no differences in parasitoid performance at the high level of constitutive resistance. Overall, whereas both chewing and sucking herbivores had negative effects on the development of S. litura and its parasitoid, the chewing herbivore was more effective than the sucking herbivore at low and medium levels of constitutive resistance, but similar to it at the high level.

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