Frank M. Davis

Impact of Bt Cottons Expressing One or Two Insecticidal Proteins of Bacillus thuringiensis Berliner on Growth and Survival of Noctuid (Lepidoptera) Larvae

Abstract A series of laboratory assays were performed to compare the relative impact of commercial and experimental cultivars of cotton, Gossypium hirsutum (L.), expressing zero, one, or two insecticidal proteins of Bacillus thuringiensis Berliner, on several lepidopteran pests. Assays in which larvae were fed fresh plant tissue indicated that dual-toxin B. thuringiensis (Bt) cultivars, expressing both Cry1Ac and Cry2Ab endotoxins of B. thuringiensis, were more toxic to bollworms, Helicoverpa zea (Boddie), fall armyworms, Spodoptera frugiperda (J. E. Smith), and beet armyworms, Spodoptera exigua (Hübner), than single-toxin cultivars expressing Cry1Ac. Assays in which lyophilized plant tissue was incorporated into artificial diet also indicated improved activity of the dual-toxin Bt cultivar compared with single-toxin plants. Both bollworm and tobacco budworm, Heliothis virescens (F.), growth was reduced by Bt cotton, particularly the dual-toxin cultivar. Although assays with lyophilized tissues were done using largely sublethal doses, bollworm survival was reduced by the dual-toxin cultivar. It appears that this newly developed Bt cotton expressing two toxins will be more effective and have a wider range of activity on these lepidopteran pests.

2-Hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one (N-O-ME-DIMBOA), a possible toxic factor in corn to the southwestern corn borer.

The southwestern corn borer (SWCB),Diatraea grandiosella Dyar, is a major pest of corn,Zea mays L., in the southern United States. The damage to corn is caused primarily by larval feeding on leaf, ear, and stem tissues. In this study, 2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one (N-O-Me-DIMBOA) was identified by MS and NMR as present in corn whorl surface waxes. This compound has evidently not been isolated previously, but its glucoside has been reported in corn, wheat, andCoix lachryma. It is present in the waxes in a higher concentration than DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) and 6-MBOA (6-methoxybenzoxazolinone). It was toxic to the SWCB in a stress diet, but it was less toxic to this insect than 6-MBOA when incorporated in the standard rearing diet. Nevertheless, it may have some role in the resistance of corn to the SWCB because the total surface wax content is higher in resistant lines than in susceptible lines.

Influence of Whorl Region from Resistant and Susceptible Corn Genotypes on Fall Armyworm (Lepidoptera: Noctuidae) Growth and Development

Abstract The effect of diets prepared from whorl tissue of resistant and susceptible corn genotypes, Zea mays L., on the larval growth, development, and physiology of fall armyworm, Spodoptera frugiperda (J. E. Smith), was analyzed. Larvae reared on an optimized artificial diet had a higher growth rate and developed faster than those reared on lyophilized whorl tissue from resistant and susceptible genotypes. Larvae reared on the resistant material were smaller and had a longer developmental period. Larvae reared on yellow-green and green whorl sections from resistant plants were significantly smaller than those reared on the same sections of susceptible plants. There was no significant difference in weight when larvae were reared on the yellow whorl regions from either resistant or susceptible lines. Physiological indices were determined for larvae fed resistant and susceptible lyophilized and fresh whorl material. Larvae fed resistant lyophilized material had significantly lower growth rate (GW) and efficiency of conversion of ingested food to body substance (ECI) than those reared on artificial diet or susceptible material. However, there were no significant differences in consumption index (CI), approximate digestibility (AD) and efficiency of conversion of digested food to body substance (ECD) between larvae reared on lyophilized tissue from resistant and susceptible genotypes. Larvae reared on fresh yellow-green whorl sections from resistant plants had significantly lower GW, ECI, and ECD than those reared on susceptible material. In contrast, no significant differences in any of the estimated food consumption and utilization indices were observed between larvae reared on fresh yellow whorl sections from resistant or susceptible plants. These results suggest that some components of whorls from resistant plants, especially the yellow-green region, inhibit food utilization in fall armyworm larvae.

DIFFERENTIAL GROWTH OF FALL ARMYWORM LARVAE (LEPIDOPTERA: NOCTUIDAE) REARED ON THREE PHENOTYPIC REGIONS OF WHORL LEAVES FROM A RESISTANT AND A SUSCEPTIBLE MAIZE HYBRID

Two laboratory bioassays were conducted to determine the effect of feeding selected whorl leaf regions of a resistant and a susceptible maize, Zea mays L., hybrid on fall armyworm, Spodoptera frugiperda (J. E. Smith) growth. In one bioassay, larvae were fed fresh excised whorl leaf tissue and in the other, they were fed reconstituted diets containing ground lyophilized leaf tissue from three phenotypic leaf regions of both hybrids. Results of the two bioassays were similar. Differences in larval weights were found for those larvae fed tissue from leaf regions within and across hybrids. The largest differences among treatments were found within the resistant hybrid, thus showing that regions of the same whorl leaf differ in suitability of food source for larval growth.

Enhancing Inherent Fall Armyworm (Lepidoptera: Noctuidae) Resistance of Corn With Bacillus thuringiensis Genes

Fall armyworm, Spodoptera frugiperda (J. E. Smith), is a serious pest of corn, Zea mays L., in the southern United States. Larvae feed extensively on leaves and other parts of the plant. Germplasm exhibiting a moderate level of resistance to leaf feeding damage has been identified and released. This germplasm has been used in breeding programs for developing corn hybrids with resistance to fall armyworm and other Lepidoptera. In recent years, much effort has also been devoted to developing corn hybrids with genes from Bacillus thuringiensis (Bt) that encode insecticidal proteins. Some of these hybrids have exhibited moderate resistance to fall armyworm damage. In this investigation hybrids with both native genetic resistance and genes from Bt encoding insecticidal proteins were evaluated for resistance to fall armyworm in field tests and laboratory bioassays. Hybrids with both types of resistance sustained less fall armyworm damage than hybrids that had only native genetic resistance or genes from Bt encoding insecticidal proteins alone. Larvae that fed on hybrids with both types of resistance were significantly smaller after feeding on plants in the field or on lyophilized whorl leaf tissue in a laboratory bioassay for 10 d than larvae fed on susceptible hybrids or hybrids with only one type of resistance. Both traditional host plant resistance and transformation of corn with genes from Bt provide hybrids with moderate levels of resistance, but when used together, they are complementary. Deployment of hybrids with both types of resistance should reduce losses to fall armyworm and also reduce the rate of buildups of resistance to Bt in fall armyworm populations.

Combining ability in maize for fall armyworm and southwestern corn borer resistance based on a laboratory bioassay for larval growth.

The fall armyworm, Spodoptera frugiperda (J. E. Smith), and southwestern corn borer, Diatraea grandiosella Dyar, are major insect pests of maize, Zea mays L., in the southern USA. Both insects feed extensively on leaves of plants in the whorl stage of growth. A diallel cross of seven inbred lines with different levels of susceptibility to leaf feeding damage in the field was evaluated in a laboratory bioassay for fall armyworm and southwestern corn borer larval growth. Diets were prepared from lyophilized leaf tissue of field-grown plants of the inbred lines and their 21 F1 hybrids. One inbred line, Tx601, exhibited heavy leaf damage in field tests but showed moderate resistance in the laboratory bioassay. Both general and specific combining ability were highly significant sources of variation in the inheritance of fall armyworm and south-western corn borer larval growth in the laboratory bioassay. Tx601 showed excellent general combining ability for reduced larval growth of both species.

RESISTANCE OF A MAIZE POPULATION, FAWCC(C5), TO FALL ARMYWORM LARVAE (LEPIDOPTERA : NOCTUIDAE)

Field tests at Mississippi State, MS, and Tifton, GA, were conducted to evaluate the effect of resistance of a maize, Zea mays (L.), germplasm population, ‘GT-FAWCC(C5)’, to feeding by larvae of the fall armyworm, Spodoptera frugiperda (J. E. Smith). Plants of selected maize entries were infested at the 8 and 12 leaf stage with two applications of 15 larvae per plant. Resistance traits measured were leaf damage at 7 and 14 days after infestation and number and weight of surviving larvae per plant at 7 and 10 days after infestation. Leaf damage ratings at both 7 and 14 days after infestation and the number and weight of surviving larvae per plant on GT-FAWCC(5) at 7 and 10 days after infestation on GT-FAWCC(C5) equalled the number and weight of surviving larvae on ‘MpSWCB-4’, the resistant check. Both the resistant check and GT-FAWCC(C5) were significantly more resistant to whorl damage than the susceptible check, ‘Ab24E × SC229’, for all resistance traits. It is evident that antibiosis (low weight) and nonpreference (fewer larvae per plant and fewer larvae preferring leaf samples) mechanisms of resistance are present in the GT-FAWCC(C5) population as well as for MpSWCB-4.

Southwestern corn borer damage to maize: Influence of plant age at time of larval infestation

Abstract A maize ( Zea mays L.) hybrid was infested with 30 southwestern corn borer [ Diatraea grandiosella (Dyar)] larvae per plant at 4, 5, 6, 7 or 8 weeks after planting in a three-year study conducted at Mississippi State, Mississippi. Visual ratings of leaf feeding damage were highest when plants were infested 5 weeks after planting. Plant height increased linearly with the age of plants at infestation. Height of infested plants was significantly less than uninfested plants only when infestations were made 4 weeks after planting. The relationship between age of plants at infestation and yield was curvilinear. Yields were significantly reduced at all infestation times; however, the least yield reduction occurred when plants were infested 6 weeks after planting. To evaluate resistance to southwestern corn borer damage, infestation at 4 or 5 weeks after planting appeared to be most satisfactory. Yield reductions following infestation at 7 and 8 weeks after planting indicated that, when infestations are late, leaf feeding damage ratings alone may not be satisfactory indicators of plant damage.

Insect Colony, Planting Date, and Plant Growth Stage Effects on Screening Maize for Leaf-Feeding Resistance to Fall armyworm (Lepidoptera: Noctuidae)

Field experiments were conducted at Mississippi State, MS and Tifton, GA to determine effects of laboratory insect colony, planting date, and plant growth stage on screening maize, Zea mays L., for leaf-feeding resistance to the fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith). The experiments were conducted using a randomized complete block design with treatments in a factorial arrangement with 6 replications. Treatments consisted of 2 insect colonies, an early and a late planting period, 2 plant growth stages, and 4 single cross maize hybrids (2 susceptible and 2 resistant to leaf-feeding by FAW) at each location. Each plant in an experiment was infested with 30 neonate FAW larvae when the plants of the second planting within each planting period reached the V 4 (Tifton) or V 8 (Mississippi State) stage. Each plant was visually scored for leaf damage 7 and 14 days after infestation. Statistical analyses revealed interactions among factors resulting in inferences having to be made using nonmarginal means. Significant differences in rating scores within each factor (insect colony, planting date, and plant growth stage) were found for some comparisons. However, none of these factors appreciably altered our ability to distinguish between resistant and susceptible genotypes which is the objective of screening.