Growth chamber bioassays to test a natural resource for corn rootworm resistance
--Eubanks, MW

T. F. Branson (Ann. Entomol. Soc. Amer. 64:861-863, 1971) reported that Tripsacum dactyloides is resistant to corn rootworm. A bridging mechanism for moving Tripsacum genes into corn has been achieved through wide cross hybrids between Tripsacum dactyloides and Zea diploperennis (M. W. Eubanks, Econ. Bot. 49:172-182, 1995). Efficacy of this genetic bridge for conferring natural rootworm resistance to corn has been demonstrated through a series of insect bioassays (M. W. Eubanks Amer. J. Bot. (suppl.):84:116, 1997; MNL 70:22-23, 1996; MNL 68:40-41, 1994). Results of insect bioassays conducted in 1998 as part of a recurrent selection program for development of isogenic corn lines that have natural rootworm resistance are reported here. The work is supported by NSF grant no. 9801386.

Thirteen Tripsacum-Z. diploperennis hybrid lines and two corn inbreds have been tested in growth chamber bioassays at the Duke University Phytotron. Included were three hybrid lines, Sun Dance, Sun Star, and 20A (a hybrid not previously tested for rootworm resistance); two corn inbred lines B73 and W64A, and ten corn X Tripsacum-Z. diploperennis hybrid lines including 97-1 X 97-5, its reciprocal cross and backcrosses of this line to 97-5 and Tripsacorn; 97-1 X 97-3; W64A X Tripsacorn and a backcross of this line to Tripsacorn; B73 X Tripsacorn and a backcross of this line to Tripsacorn, and T33 X Tripsacorn. The research design was four replicates in a randomized block. Plants were grown in 4.5-inch diameter pots with nylon cloth covering the bottom of the pots to prevent larval escape out the holes in the bottom of the pots. Out of 482 plants, 438 germinated. Eighteen of the 438 were albino seedlings and died. Of the remaining 420 plants, 303 were infested with newly hatched Western corn rootworm larvae at three weeks post germination. In replicate one, each plant was infested with 100 larvae; in replicates two and three, treatment consisted of 70 larvae per plant, and in the fourth replicate each plant was infested with 50 larvae. At the end of three weeks after infestation, all plants were harvested. The roots were carefully washed, then scored using the 1-6 Iowa rating scale (Hills and Peters ): 1 = no damage or only a few minor feeding scars; 2 = feeding scars evident, but no roots eaten off to within 1.5 inches of the plant; 3 = several roots eaten off to within 1.5 inches of the plant, but never the equivalent of an entire node of roots destroyed; 4 = one node of roots completely destroyed; 5 = two nodes of roots completely destroyed; 6 = three nodes of roots completely destroyed. Plants that have a root rating of 1 or 2 are resistant. Any live larvae were counted and placed in 1 dram glass vials containing 95% ETOH that were labeled by individual plant. After scoring the roots, twelve resistant plants with a root rating of one (highest level of resistance) were repotted in 10-inch diameter pots and transferred to the greenhouse for selfing to increase seed of resistant lines and for cross pollinating with corn to advance the recurrent selection breeding program to develop rootworm resistant corn lines. Leaf tissue from these plants was also sampled for DNA analysis. Families with highest level of resistance identified in this bioassay are (97-1 X 97-5), NC64TC, and (B73 X TC). The root rating results are reported in Table 1.

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