Germplasm and data generation
A panel of 289 diverse maize inbred lines was used for this study
(Supplemental Table 1). The panel included both FAW resistant and
susceptible lines from various breeding programs, some of which formed
association panels published previously (Warburton et al., 2013;
Flint-Garcia et al., 2005). These lines were genotyped and phenotyped as
inbreds. They were grown at the R.R. Foil Plant Science Research Center,
Mississippi State, Mississippi (MS) in 2019, 2020, and 2021. Seeds were
planted in 4 m single-row plots with 0.97 m spacing between rows and
thinned to 20 plants plot-1. The experiment was
planted in a randomized complete block design with two replications in
2019 and 2020 and three replications in 2021. Standard production
practices were followed, and supplemental irrigation was applied as
needed.
The fall armyworm neonate larvae used in this study were obtained from
the laboratory colony at the insect rearing facility of the Corn Host
Plant Resistance Research Unit (CHPRRU), USDA-ARS, Mississippi State.
The larvae were reared using artificial diet following the procedure
described by Davis (1989). Within 24 hours following the egg hatch,
first instar larvae (neonates) were mixed with corn cob grits. The
mixture was dispensed into the maize whorl utilizing a mechanical
applicator (Wiseman et al., 1980). Each individual plant was infested
with approximately 40 FAW neonates at the mid-whorl stage (V-7 of maize
growth stage). All plants within experimental plots were rated
individually to determine the leaf feeding damage caused by FAW. The
damage was visually rated at 7 days and 14 days after infestation. The
7-day and 14-day leaf feeding ratings were conducted on a 0 (no visible
leaf feeding damage) to 9 scale according to Davis et al. (1992;
Supplemental Table 2). The 7-day ratings were conducted by one person
and the 14-day ratings were conducted independently by two people.
The mean rating for each entry was calculated as an adjusted mean (Best
Linear Unbiased Estimator; BLUE) using mixed model methodology in the
PROC GLIMMIX procedure of SAS 9.4 (SAS Institute, Cary, NC) and can be
found in Supplemental Table 1. For the 7-day ratings, the individual
plants within a plot were treated as subsamples. Plot means were
calculated within years treating genotype as a fixed effect and
replication and genotype by replication as random effects. To combine
the 7-day ratings over years, genotype was treated as a fixed effect and
year, replication nested within year, year by genotype interaction, and
genotype by replication nested in year were all treated as random
effects. For the 14-day ratings two sets of individual plant ratings,
one set per rater, were used as subsamples for each plot. Plot means
were calculated similarly to the 7-day ratings, but additional model
terms were added to account for the variance due to different raters.
Within years, genotype was treated as a fixed effect and replication,
rater, rater by replication, genotype by replication, and genotype by
rater by replication were all treated as random effects. When the data
was combined across three years, genotype was treated as a fixed effect
and year, replication nested in year, rater, rater by replication nested
in year, year by genotype interaction, year by rater interaction,
genotype by replication nested in year, and rater by genotype by
replication nested in year were all treated as random effects.