DURHAM, NORTH CAROLINA

Duke University

MANHATTAN, KANSAS

USDA ARS

Bioassays for grain weevil resistance in Tripsacum X Zea diploperennis --Eubanks, M, Throne, J One of the most serious insect pests to corn during grain storage is the maize weevil, Sitophilus zeamais Motschulsky (Coleoptera:Curculionidae). The adult female oviposits eggs into small holes she bores into the kernel. When the larvae hatch they feed on and develop inside the grain. No-choice bioassays were conducted to investigate whether Tripsacorn, a hybrid between Tripsacum dactyloides and Zea diploperennis that is resistant to corn rootworm, Diabrotica virgifera, might also possess resistance to grain weevils.

Three 35 g samples of Tripsacorn seed were equilibrated to 30 C and 75% relative humidity for 6 weeks. There were 3 control 35 g samples of Asgrow RX899 corn. At the end of the equilibration period, 5 Sitophilus zeamais adult females age 2-3 weeks were placed in each sample cage for 72 hours to lay eggs. Thirteen days after the ovipositing females were removed. Emerging adult progeny were sieved from the cages daily until no weevils had emerged for 2 weeks. No weevils emerged from the Tripsacorn, and X-ray examination of the kernels indicated that no eggs had been laid in the Tripsacorn seeds. Mean number of adults emerged from the corn controls was 30.3 (SD = 6.43).

Tripsacorn kernels are enclosed in a hard shell-like seedcoat, and it was hypothesized that the hardness of the seed was responsible for lack of weevil oviposition. In order to test this, a second experiment was conducted using ground up Tripsacorn kernels. Since maize weevil development is poor on ground seed, the sawtoothed grain beetle, Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae) was employed in this bioassay. There were twenty replicates of one Tripsacorn kernel each weighing approximately 6 mg and twenty control replicates of 2 wheat kernels each which was about equal to a single Tripsacorn kernel. EachTripsacorn kernel or group of two wheat kernels was individually crushed with an aluminum mortar and pestle, then placed in a 0.4 ml centrifuge tube with a pinhole in the lid, and equilibrated for 1 week at 30 C and 75% RH. A single egg between 0 and 24 hr old was place in each tube. Fourteen days after infestation, tubes were checked biweekly for emerging adults. Emergence rate was 11 out of 20 on the wheat controls and 12 out of 20 on Tripsacorn. This shows that Tripsacorn seed can support insect growth when ground.

Intact Tripsacorn kernels were immune to maize weevil infestation. Given the difficulty in grinding the Tripsacorn kernels and the fact that insect growth occurred on ground Tripsacorn, immunity is probably due to the hardness of the kernels.
 
 


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