Maize resistance to Spodoptera frugiperda and its relationship to landrace variety, plant stage, and larval origin.
Previous studies have raised the hypothesis that laboratory-reared insects may lose their ability to induce plant defenses because induction-associated gut bacteria may be eliminated by antibiotics used in artificial diets. The present study was conducted to determine whether maize (Zea mays L., Poaceae) responses to Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae) differ when field-collected vs. laboratory-reared insects are feeding. One fifth instar from the laboratory or field was released onto a resistant or a susceptible landrace variety at vegetative stages 4 and 6 (V4 and V6), or plants remained uninfested. Five days later, damage was estimated and sections of maize leaves were fed to forth instars in a Petri dish assay to assess the effects of previous feeding on weight gain, leaf consumption, and nutritional indices of conspecifics. Initially, field-collected larvae caused more damage on V4, but not on V6 maize plants, than laboratory-reared larvae. In V6 plants, the resistant variety showed less leaf damage than the susceptible variety. In a Petri dish assay, there were no differences between effects of laboratory-reared and field-collected larvae on the measured parameters, but larvae from both sources promoted increased susceptibility in maize plants at the V4, but not the V6 stage. This was characterized by higher weight gain and higher efficiency of conversion of ingested food (ECI) (on both maize varieties), and lower metabolic cost (MC) (on one variety) when S. frugiperda larvae fed on leaves pre-infested with laboratory-reared larvae compared to leaves of uninfested plants. Conversely, S. frugiperda larvae fed on leaves pre-infested with field-collected larvae consumed a greater amount of leaf tissue (on both varieties) compared to leaves of uninfested plants. A similar experiment with neonates showed no effects of landrace variety or previous infestation on insect weight and survival, but insects obtained a higher body weight when fed on V4 compared to V6 plants. In conclusion, S. frugiperda laboratory-reared larvae were equally able as field-collected larvae to induce susceptibility in V4 maize plants, and maize plants were more resistant to the pest at the V6 than at the V4 stage.