Promising germplasm for rootworm resistance in maize
--Eubanks, M

In 1972, Branson and Guss (Entomol. Soc. Amer. Proc. North Central Branch 27:91-95 ) reported resistance to corn rootworm, Diabrotica virgifera LeConte, in eastern gamagrass,Tripsacum dactyloides L., a wild relative of maize. In 1993 (MNL 67:39-41) and 1994 (MNL 68:30-41), I reported evidence from petri dish and pot bioassays for rootworm resistance in maize that was crossed with a hybrid between diploid perennial teosinte, Zea diploperennis Iltis (Doebley and R. Guzman) and T. dactyloides.

Additional pot bioassays have been conducted to address the question of whether rootworm resistance is expressed in accordance with Mendelian segregation in the Z. diploperennis-T. dactyloides hybrid referred to as Tripsacorn. In the first of these, 13 Tripsacorn S1 plants, grown in 4 inch pots, were infested with 50 Western corn rootworm larvae each, 30 days after germination. Within a few days after infestation, two plants died and two more exhibited severe lodging. Two weeks after infestation, plants were removed from pots and roots were washed for examination. Roots ranged from severely eaten to traces of larval feeding and extensive root growth. Four of the plants were identified as susceptible and nine plants considered resistant/tolerant. A chi square value of 0.25 is obtained from these numbers based on expected ratios of 3:1. Such deviation would be expected 50% of the time due to chance alone. The segregation ratio approaching 3 resistant to 1 susceptible suggests Mendelian segregation for a dominant gene for resistance.

Another bioassay was conducted with a total of 20 Tripsacorn S1 plants: 16 treatment and 4 control plants. Seed was germinated on moist filter paper in petri dishes and seedlings were transferred into 4-inch pots. Infestation of 50 newly hatched Western corn rootworm larvae was at 6 weeks after planting. Plant height was measured weekly throughout the trial. Seventeen days after infestation, the plants were removed from pots and roots gently washed for examination under a microscope. Four plants had no feeding scars, 8 had minor feeding damage, and 4 had extensive feeding. Results signal homozygous dominant plants are more resistant than heterozygous plants, and homozygous recessive plants do not carry resistance.

An interesting phenomenon was observed in this bioassay. The record of plant height revealed a noticeable spike in plant growth at time of infestation when compared with control plants. Infestation appears to stimulate a growth hormone response concurrent with a defence response by infested plants carrying the resistance gene.

Another pot bioassay conducted under the same conditions as the previous one tested S1 plants of Sun Star, a new hybrid between Z. diploperennis and a diploid T. dactyloides (all previous hybrid plants tested were derived by crossing Z. diploperennis with a tetraploid Tripsacum). Out of 16 Sun Star plants, 4 were albino and died. The remaining 12 showed no feeding damage, indicating Sun Star is another promising source for rootworm resistance.

Bioassays testing Z. diploperennis-T. dactyloides hybrids, referred to as Tripsacorn and Sun Star, indicate there is a gene for resistance to corn rootworm that is inherited in accordance with Mendelian segregation. These plants provide a genetic bridge for moving rootworm resistance from Tripsacum into maize. 

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