Host-Plant-Mediated Effects on the Demography, Parasitism Rates and Functional Response of Bracon hebetor on Spodoptera frugiperda

Tritrophic interactions among host plants, herbivores and parasitoids can influence biological control efficacy, yet plant-mediated effects on parasitoid performance against the invasive fall armyworm, Spodoptera frugiperda, are poorly understood. We evaluated the effects of maize and castor on the life-history traits, demographic parameters, parasitism, functional response and population projection of the larval ectoparasitoid Bracon hebetor. Laboratory and greenhouse experiments used S. frugiperda larvae reared on the two host plants. Age-stage, two-sex life tables, parasitism indices, population projections and functional response models were used to compare parasitoid performance. Parasitoids developing on maize-fed larvae had significantly shorter developmental periods, higher fecundity (247.09 ± 0.41 eggs), higher intrinsic (r = 0.217 ± 0.008 day−1) and finite (λ = 1.242 ± 0.011) rates of increase and a shorter generation time (T = 20.26 ± 0.289 days) than those on castor-fed larvae, whereas the net reproductive rate (R0 = 81.54 ± 11.55) and gross reproductive rate did not differ significantly. Effective parasitism and killing rates were higher on maize-fed larvae in both laboratory and greenhouse assays; non-effective parasitism did not differ significantly. Type II functional responses were observed on both host plants, and searching efficiency tended to be higher and handling time shorter on maize-fed larvae (Holling’s disc equation: a = 1.125 vs. 0.787; Th = 0.0556 vs. 0.0817 days). Projections suggested faster parasitoid population growth and greater cumulative host killing on maize-fed larvae. These findings suggest that host-plant-mediated effects should be considered when optimizing sustainable biological control of S. frugiperda.