Artificial infestation of sorghum spikelets with eggs of Stenodiplosis sorghicola (Coquillett) (Diptera: Cecidomyiidae) by water injection

Abstract

A technique for artificially infesting sorghum spikelets with eggs of sorghum midge is described and compared with natural oviposition achieved in a glasshouse cage trial. This technique was developed to facilitate antibiosis resistance related studies of midge biology, and to overcome the reduced and inconsistent oviposition achieved in sorghum lines that contain ovipositional-antixenosis resistance. Injecting an aqueous suspension of exactly two midge eggs between the glumes of individual sorghum spikelets using a micropipette produced consistent, low egg densities across five lines of varying resistance (0.8–1.2 eggs per spikelet; 50–70% infestation), while water injection of four to six eggs per spikelet in the same lines produced high and consistent egg densities (two to three eggs per spikelet; > 80% infestation). In contrast to both of the water-injection treatments, natural oviposition within the same five lines produced inconsistent egg densities, even when midge densities were adjusted to account for the variable levels of ovipositional antixenosis present in each line (one to four eggs per spikelet; 40–80% infestation). A bioassay was also conducted to determine the effect of suspending midge eggs in water on egg hatch, neonate survival and fitness. Aqueous suspensions of midge eggs stored for 4 h at room temperature produced 79% egg hatch. However, aqueous egg suspensions refrigerated at 4°C for 1–7 days reduced egg hatch (41–64%), lowered larval longevity and reduced maximum movement of neonate larvae. No eggs hatched after 14 days of refrigerated storage.