Infestation of Rhyzopertha dominica (F) first instars on sound and artificially-damaged hard red winter wheat kernels

The lesser grain borer, Rhyzopertha dominica (F.), is a devastating pest of stored wheat worldwide. Mated females lay eggs loosely outside of wheat kernels, and larvae hatching from eggs (first instars) enter wheat kernels to complete immature development. The main goal of this research was to understand wheat seed infestation by first instars. Individual, sound, organic hard red winter wheat kernels in glass vials were infested with 1, 2, 3, 4, or 5 first instars at 28ºC and 65% RH. At each kernel to larval density, 50 sound kernels were used. Successful kernel infestation was verified by dissecting kernels 21 days after infestation. Across all kernels to larval densities, infestation ranged from 12 to 34%, and infestation rates were not significantly different among the densities. The germ was the preferred site of entry for first instars (41%), followed by the brush end (29%), endosperm (27%), and entry at both germ and brush ends (3%). Significant differences were only found on germ site versus entry of larvae at both germ and brush ends. In a second experiment, infestation by first instars at one larva per kernel was evaluated using sound kernels and kernels artificially-damaged with a microdrill at the germ, endosperm, and brush end. About 82-90% of artificially damaged kernels were infested by larvae in contrast with 12% for sound kernels. The germ was the preferred site of entry for larvae (90%), followed by brush end (86%), and the endosperm (80%). Mean weight loss of artificially-damaged kernels infested at the germ was highest (10 g), followed by endosperm (9 g), and brush end (7 g) in contrast with only 4 g in sound kernels. These findings indicate that R. dominica first instars preferred germ as the site of entry, and damaged wheat kernels had higher infestation rates than sound kernels.