Combining ability of resistance leads and identification of a new resistance source for western corn rootworm (Coleoptera: Chrysomelidae) larvae in corn

The western corn rootworm (Diabrotica virgifera virgifera LeConte) is a major insect pest in continuous corn (Zen mays L.), but no viable alternatives to insecticides are available for its control. From 1994 to 1996, seven germplasm sources of corn with reduced corn rootworm damage (resistance leads) were identified. Since utilization of these sources would likely require backcrossing into elite germplasm, a diallel including the seven resistance lines and inbred lines B37 and Mo47 was formed to determine which had the best potential for contributing com rootworm resistance to progeny. Each parental line was crossed reciprocally with each of the other eight. Parental germplasm, available crosses, and resistant and susceptible checks were evaluated in a randomized complete block experiment with four replications at three central Missouri locations in 1997. Ten of the best crosses in the diallel from 1997 were selfed in a 1997-1998 winter nursery and seed from three ears for each cross (30 ears total) were grown along with checks in 1998. Plots for both years were mechanically infested with western corn rootworm eggs and roots were evaluated for corn rootworm feeding damage. Although damage to the susceptible check was less than desired in 1997 (the mean damage rating for the susceptible check was 3.17, just over the economic threshold of 3.0 on the 1 to 6 scale), 29 of the 34 crosses evaluated (combining reciprocal crosses) were significantly less damaged than susceptible check and mio [TL92A PAR 1779 60-4 x NGSDCRW1(S2)C4-15-2S2 and ((Dent x Teo)-xIADS69) x NGSDCRW1(S2)C4-15-2S2] were significantly less damaged than the resistant check. When feeding dam age data For reciprocal crosses from the 1997 trial were combined,, three of the nine parents [NGSDCRW1(S2)C4-15-2S2, TL93A-PAR1774 28-1(C3), and TL92A-PAR1779 60-4] contributed corn rootworm resistance to their respective progeny, as indicated by a significantly negative general combining ability effect (lower ratings were desirable). Overall, specific combining ability effects for these crosses were not significant, and only mio of the 22 crosses evaluated for which reciprocal crosses were available (36 were possible) were significantly different from each other In 1998, the susceptible check was significantly more damaged (3.90) than all other entries. TL92A PAR 1779 60-4 x NGSDCRW1(S2)C4-15-2S2 had significantly less damage (2.60) than all other entries including the resistant check. The specific cross, TL92A PAR 1779 60-4 x NGSDCRW1(S2)C4-15-2S2, was the best of all entries evaluated in 1997 and 1998, had significantly less damage than the resistant check both years, and represents a new source of native resistance to feeding damage caused by corn rootworm larvae.