Evaluation of inducible defense mechanisms of annual bluegrass (Poa annua) and creeping bentgrass (Agrostis stolonifera) in response to annual bluegrass weevil (Listronotus maculicollis) herbivory

The annual bluegrass weevil (Listronotus maculicollis) (ABW) is the most devasting insect pest of short-mown annual bluegrass (Poa annua L.) and creeping bentgrass (Agrostis stolonifera), the two predominant turfgrasses on golf courses in eastern North America. Larvae are stem borers during their first three instars, though the transition to feed externally on the crown during 4^th and 5^th instars. Creeping bentgrass has been observed to express tolerance to ABW herbivory, though underlying mechanisms have not been investigated. We hypothesized that creeping bentgrass elicits an inducible defense response to ABW herbivory resulting in reduced larval survivorship. Turfgrass cores were infested with adults to obtain uniform larval stages. Foliar, stem, and root tissue samples were collected at three development stages (2^nd, 3^rd,& 4^th instars) to measure defense response over time. In the absence of larvae, no differences were detected in jasmonic acid (JA) levels, a phytohormone elicited by chewing insects, between turfgrasses. Salicylic acid (SA), a phytohormone associated with piercing-sucking insect feeding and plant pathogen infection, was significantly higher in annual bluegrass foliar and stem tissue compared to creeping bentgrass. Once challenged with larvae, SA increased in annual bluegrass foliar and stem tissue compared to plants without larval feeding. No significant differences were found in either JA or SA production with ABW development from the 2^nd to 4^th instar for either turfgrass species or tissue type. These findings suggest that defense phytohormones may play a role in annual bluegrass susceptibility to ABW herbivory but are unlikely to provide resistance in creeping bentgrass.