Kenton E. Dashiell

Native Resistance to Western Corn Rootworm (Coleoptera: Chrysomelidae) Larval Feeding: Characterization and Mechanisms

ABSTRACT Seven maize, Zea mays L., genotypes selected for native resistance to western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), larval feeding damage (SUM2068, SUM2162, CRW3 (S1) C6, NSS1×CRW3 (S1) C6, PI583927, CRW2 (C5), and AR17056–16) were evaluated along with three control maize genotypes in the field for plant damage, larval recovery, adult emergence, root size, and root regrowth. Larvae recovered were further analyzed for head capsule width and dry weight and adults for dry weight. All factors evaluated with the exception of adult dry weight varied significantly among maize genotypes. Control genotypes included a highly susceptible hybrid, B37×H84, a transgenic rootworm-resistant hybrid expressing the modified Cry3A protein (MIR604), and the untransformed modern hybrid with the same genetic background as the MIR604 we used (isoline) as a second susceptible control. In general, the genotypes previously selected for resistance to western corn rootworm larval feeding had less damage, fewer larvae recovered, smaller larvae recovered, and fewer adults recovered than the susceptible controls. SUM2162 was significantly less damaged than all other native sources of resistance. Western corn rootworm larvae recovered from SUM2162 and SUM2068 were significantly smaller in terms of head capsule width and average weight than larvae recovered from all other maize genotypes, indicating that antibiosis is a mechanism of resistance for these two hybrids.

Field screening maize germplasm for resistance and tolerance to western corn rootworms (Col.: Chrysomelidae)†

Abstract:  In the hopes of lessening the current reliance on soil insecticides, developing a viable alternative for transgenic maize hybrids, and providing sustainable options for Europe, researchers recently have been developing novel maize lines that exhibit resistance and/or tolerance to corn rootworm larvae. Here we report the results of a 2‐year field experiment in a northern growing region assessing the resistance and tolerance of 10 experimental synthetic maize populations selected for varying levels of damage from western corn rootworm larvae, Diabrotica virgifera virgifera LeConte (Col.: Chrysomelidae) and four maize hybrids. Maize non‐preference, antibiosis and tolerance to rootworms was evaluated using previously established methods, including: the Iowa 1–6 root damage rating scale, root fresh weight, compensatory root growth ratings and adult rootworm emergence. Among the experimental synthetic maize populations, BS29‐11‐01 was the most susceptible, and had a mean root damage rating that was greater than the highly susceptible maize hybrid B37 × H84. This line also had the lowest mean root fresh weight and one of the lowest mean compensatory root growth ratings. In contrast, CRW8‐3 appeared to be tolerant to western corn rootworms, and had the lowest mean root damage rating, which was comparable with that of the non‐transgenic hybrid DeKalb® 46‐26.

Time-course of dinitrogen fixation of promiscuous soybean cultivars measured by the isotope dilution method

Abstract Soybean cultivars capable of nodulating with indigenous Bradyrhizobium spp. have been developed by the International Institute of Tropical Agriculture (IITA) and national programs in Africa in order to avoid artificial inoculation by resource-poor farmers in Africa. The current selection procedure for enhanced N2 fixation is based on an assessment of nodule formation which does not directly quantify the proportions of crop N derived from the atmosphere. We have monitored N accumulation patterns and N2 fixation in nine promiscuous soybean cultivars with different maturity periods, using the 15N dilution technique. Nodule development generally peaked at the early podfill stage for all cultivars except Tgx 1519-1D and Tgx 1447-2D in which it continued to increase. The proportion of crop N derived from fixation (%NDFA) ranged between 51% and 67%, 77% and 84%, and 66% and 73% at full bloom, early podfill, and physiological maturity stages, respectively. Total N accumulation increased in all soybean genotypes with increasing plant age. Significant correlations (P<0.001) were established between nodule weight and %NDFA, even though this did not explain the relationship between nodule development and N2 fixation in cultivars such as Tgx 1519-1D. Promiscuous soybean cultivars retained between 10% and 19% of total N accumulated at the final harvest, in belowground biomass. Our results indicated that these soybean cultivars can derive substantial proportions of plant N from N2 fixation in soils where compatible indigenous bradyrhizobia populations are adequate and effective. Also, we have substantiated the claims that qualitative nodulation parameters currently used to select varieties with a high N2 fixation capacity need to be validated with other measurements of N2 fixation.

Conventional Screening Overlooks Resistance Sources: Rootworm Damage of Diverse Inbred Lines and Their B73 Hybrids is Unrelated

ABSTRACT The western corn rootworm, Diabrotica virgifera virgifera (LeConte), is a major pest of maize, Zea mays L., in the United States and Europe, and it is likely to increase in importance as a trend toward increased nonrotated maize favors larger rootworm populations. Options for rootworm management in nonrotated maize in Europe and in nontransgenic “refuge” areas in countries that permit transgenic maize are limited to insecticides. Development of additional options for growers would be helpful. Screening maize germplasm (e.g., landraces, populations, inbreds) for native resistance to western corn rootworm is labor-intensive and is usually conducted on unfinished germplasm and not on hybrid materials. However, we have recently observed that topcrossed (hybrid) materials tend to have reduced western corn rootworm damage. To formally test whether rootworm damage to inbreds and associated hybrids were correlated, we evaluated 25 diverse inbred lines and their B73 hybrids for western corn rootworm damage in seven environments. Overall, hybrids had significantly less damage than inbreds, but unfortunately, the correlation between inbreds and hybrids was not significant. These findings have important implications regarding screening germplasm for western corn rootworm resistance, namely, that inbred materials and perhaps populations should be topcrossed to form hybrid materials before screening for western corn rootworm damage to ensure that valuable sources of resistance to western corn rootworm are not missed during the screening process.

Evaluating Tripsacum-introgressed maize germplasm after infestation with western corn rootworms (Coleoptera: Chrysomelidae).

Maize (Zea mays L.) is a valuable commodity throughout the world, but corn rootworms (Chrysomelidae: Diabrotica spp.) often cause economic damage and increase production costs. Current rootworm management strategies have limitations, and in order to create viable management alternatives, researchers have been developing novel maize lines using Eastern gamagrass (Tripsacum dactyloides L.) germplasm, a wild relative of maize that is resistant to rootworms. Ten maize Tripsacum‐introgressed inbred lines derived from recurrent selection of crosses with gamagrass and teosinte (Zea diploperennis Iltis) recombinants and two public inbred lines were assessed for susceptibility to western corn rootworm (Diabrotica virgifera virgifera LeConte) and yield in a two‐year field study. Two experimental maize inbred lines, SDG11 and SDG20, had mean root damage ratings that were significantly lower than the susceptible public line B73. Two other experimental maize inbred lines, SDG12 and SDG6, appeared tolerant to rootworm damage because they exhibited yield increases after rootworm infestation in both years. In the majority of cases, mean yield per plant of experimental maize lines used in yield analyses was equal to or exceeded that of the public inbred lines B73 and W64A. Our study indicates that there is potential to use Tripsacum‐introgressed maize germplasm in breeding programs to enhance plant resistance and/or tolerance to corn rootworms, although further research on insect resistance and agronomic potential of this germplasm needs to be conducted in F1 hybrids.

Assessing larval rootworm behaviour after contacting maize roots : impact of germplasm, rootworm species and diapause status

Current methods of screening maize (Zea mays L.) germplasm for susceptibility or resistance to corn rootworms (Coleoptera: Chrysomelidae) rely primarily on information from large‐scale field experiments. Due to labour and cost constraints associated with field trials, alternative evaluation methods are desirable. We used a previously developed behavioural bioassay to: (1) investigate the host search behaviour of rootworm larvae after contact with 14 maize genotypes, (2) compare the behaviour of non‐diapausing Diabrotica virgifera virgifera LeConte, diapausing D. v. virgifera, and diapausing D. barberi Smith & Lawrence and (3) determine if this technique can be used to separate susceptible vs. resistant maize genotypes. The majority of rootworm larvae engaged in intensive (local search) behaviour after exposure to maize roots, whereas larvae continued to exhibit extensive (ranging) behaviour after contact with negative controls. Even though a transgenic hybrid with resistance to D. v. virgifera was included in analyses, quantitative path measurements were similar among genotypes and only differed between specific maize lines and controls. Notably, there were differences in host search behaviour among rootworm groups, with non‐diapausing D. v. virgifera having more convoluted paths and engaging in intensive search more frequently than diapausing rootworms. Correlations between larval path measurements and historic root damage ratings were not significant, although there were weak positive correlations between historic adult emergence densities and measures of path linearity. However, due to the lack of significant behavioural differences among maize lines with a range of susceptibility levels, we concluded that this bioassay is not useful in screening maize germplasm for rootworm resistance.

Distribution and Characteristics of Bradyrhizobium Spp. Nodulating African Soybeans

The size and effectiveness of indigenous rhizobia populations influence the legume yield response to inoculation. Some African soybean varieties (e.g. TGx lines) were developed to nodulate with indigenous Bradyrhizobium spp. as well as B. japonicum ostensibly to eliminate the need for inoculation with B. japonicum. We characterized the bradyrhizobia populations from 70 soils in 9 African countries according to nodulation phenotypes: B. spp. effectively nodulate cowpea, B. spp. (TGx) effectively nodulate TGx soybean and cowpea, B. japonicum nodulates N. American and TGx soybean and cowpea. Populations of B spp.(TGx) and B. japonicum were ≤102 g-1 in 85% and 91% of the soils indicating inoculation should increase soybean yields. B spp.(TGx) and B. japonicum were not detected in 26% and 67% of the soil samples. Population size was not related to soil physiochemical properties but was related to prior management. B. spp.(TGx) and B. japonicum populations≥103 g -1 soil were more frequent at research stations than farmers’ fields and where soybean had previously been grown. Of 258 isolates we made using TGx soybean as a trap host, only 27% were highly effective on TGx soybean. Most of the effective isolates were also effective on N. American soybean. RFLP analysis of 16S rDNA from B. spp.(TGx), B. japonicum, and B. spp. strains showed that most B. spp.(TGx) strains were phylogenetically related to B. spp. Like B. elkanii strains, B. spp.(TGx) strains do not have the nodY gene. Additional genetic analyses, evaluation of cross inoculation with tropical legumes and IAR analysis indicated the B. spp.

Quantitative trait loci mapping of western corn rootworm (coleoptera: Chrysomelidae) host plant resistance in two populations of doubled haploid lines in maize (Zea mays L.)

Abstract Over the last 70 yr, more than 12,000 maize accessions have been screened for their level of resistance to western corn rootworm, Diabrotica virgifera virgifera (LeConte; Coleoptera: Chrysomelidae), larval feeding. Less than 1% of this germplasm was selected for initiating recurrent selection or other breeding programs. Selected genotypes were mostly characterized by large root systems and superior root regrowth after root damage caused by western corn rootworm larvae. However, no hybrids claiming native (i.e., host plant) resistance to western corn rootworm larval feeding are currently commercially available. We investigated the genetic basis of western corn rootworm resistance in maize materials with improved levels of resistance using linkage disequilibrium mapping approaches. Two populations of topcrossed doubled haploid maize lines (DHLs) derived from crosses between resistant and susceptible maize lines were evaluated for their level of resistance in three to four different environments. For each DHL topcross an average root damage score was estimated and used for quantitative trait loci (QTL) analysis. We found genomic regions contributing to western corn rootworm resistance on all maize chromosomes, except for chromosome 4. Models fitting all QTL simultaneously explained about 30 to 50% of the genotypic variance for root damage scores in both mapping populations. Our findings confirm the complex genetic structure of host plant resistance against western corn rootworm larval feeding in maize. Interestingly, three of these QTL regions also carry genes involved in ascorbate biosynthesis, a key compound we hypothesize is involved in the expression of western corn rootworm resistance.

Parasitism of Northern Corn Rootworms (Chrysomelidae: Diabrotica barberi) by Celatoria diabroticae (Tachinidae) in South Dakota: New Geographic Record

Corn rootworms (Coleoptera: Chrysomelidae) are devastating pests of maize (Poaceae: Zea mays L.). Recently, there has been increased interest in biological control of corn rootworms, in part due to their invasion of Europe (Kuhlmann and van der Burgt, 1998; Kuhlmann et al., 2005). One group of parasitoids under investigation for their potential as natural enemies of Diabrotica species are flies in the genus Celatoria (Diptera: Tachinidae) (Kuhlmann and van der Burgt, 1998; Gamez-Virues and Eben, 2005; Kuhlmann et al., 2005; Dequech et al., 2006). Female parasitoids insert eggs containing fully developed first instar larvae directly into adult beetles (Zhang et al., 2003). In North and South America, four Celatoria species attack beetles within Diabrotica and related chrysomelid genera, including: C. compressa Wulp, C. bosqi Blanchard, C. setosa (Coquillett), and C. diabroticae (Shimer) (reviewed in Cabrera Walsh, 2004). Celatoria compressa is found in Mexico, C. bosqi is known from South America, and C. setosa and C. diabroticae are the two species found in North America (O’Hara and Wood, 2004; reviewed in Cabrera Walsh, 2004). Celatoria diabroticae has a widespread distribution across the United States (reviewed in Arnaud, 1978; O’Hara and Wood, 2004) and parasitizes D. virgifera virgifera LeConte, D. undecimpunctata howardi Barber, and D. undecimpunctata undecimpunctata Mannerheim (Arnaud, 1978; reviewed in Cabrera Walsh, 2004). There is little information on the incidence of parasitism of D. barberi Smith & Lawrence by C. diabroticae, although it has been reported in Kansas and Illinois [reported as D. longicornis (Say); reviewed in Chiang, 1973; Fischer, 1983]. To our knowledge, this is the first report of C. diabroticae parasitizing D. barberi in South Dakota.