Twins but not identical: differential plant defense induction by the fall armyworm strains

The fall armyworm (FAW) Spodoptera frugiperda, is composed of two morphologically identical but genetically different host strains with different host preferences.  In field conditions, the corn strain is mainly associated with maize, sorghum and cotton, while the rice strain is mostly associated with forage grasses and rice.  We investigated the defense response of different host plants to the feeding by the FAW strains, aiming to elucidate the factors driving their specific host preferences.  We tested the expression of defense-related genes and the activity of plant-defensive proteins in maize, rice and Bermuda grass upon feeding by the FAW strains.  Our results show that feeding by the rice strain caterpillars induces greater accumulation of proteinase inhibitors in maize than feeding by the corn strain.  In Bermuda grass, feeding by the corn strain suppressed the induction of trypsin inhibitor activity while the rice strain induced greater activity levels.  No differences in defense induction were found in rice for the two strains.  The plant defense elicitors seem to be associated with the caterpillar saliva as similar patterns of defense responses were obtained in mechanically wounded plants treated with saliva from both strains.  The levels of plant defense responses were negatively correlated with caterpillar growth indicating a fitness effect.  We conclude that specific elicitors in the saliva of the FAW stains trigger different levels of plant defense responses that affect caterpillar fitness and may influence host plant preference in field conditions.  This research was funded by the USDA, PSU College of Ag., and the ESA-Monsanto Research grant.