Pyramids of QTLs enhance host-plant resistance and Bt-mediated resistance to leaf-chewing insects in soybean.

KEY MESSAGE: QTL-M and QTL-E enhance soybean resistance to insects. Pyramiding these QTLs with cry1Ac increases protection against Bt-tolerant pests, presenting an opportunity to effectively deploy Bt with host-plant resistance genes. Plant resistance to leaf-chewing insects minimizes the need for insecticide applications, reducing crop production costs and pesticide concerns. In soybean [Glycine max (L.) Merr.], resistance to a broad range of leaf-chewing insects is found in PI 229358 and PI 227687. PI 229358's resistance is conferred by three quantitative trait loci (QTLs): M, G, and H. PI 227687's resistance is conferred by QTL-E. The letters indicate the soybean Linkage groups (LGs) on which the QTLs are located. This study aimed to determine if pyramiding PI 229358 and PI 227687 QTLs would enhance soybean resistance to leaf-chewing insects, and if pyramiding these QTLs with Bt (cry1Ac) enhances resistance against Bt-tolerant pests. The near-isogenic lines (NILs): Benning(ME), Benning(MGHE), and Benning(ME+cry1Ac) were developed. Benning(ME) and Benning(MGHE) were evaluated in detached-leaf and greenhouse assays with soybean looper [SBL, Chrysodeixis includens (Walker)], corn earworm [CEW, Helicoverpa zea (Boddie)], fall armyworm [FAW, Spodoptera frugiperda (J.E. Smith)], and velvetbean caterpillar [VBC, Anticarsia gemmatalis (Hübner)]; and in field-cage assays with SBL. Benning(ME+cry1Ac) was tested in detached-leaf assays against SBL, VBC, and Southern armyworm [SAW, Spodoptera eridania (Cramer)]. In the detached-leaf assay, Benning(ME) showed the strongest antibiosis against CEW, FAW, and VBC. In field-cage conditions, Benning(ME) and Benning(MGHE) suffered 61 % less defoliation than Benning. Benning(ME+cry1Ac) was more resistant than Benning(ME) and Benning (cry1Ac) against SBL and SAW. Agriculturally relevant levels of resistance in soybean can be achieved with just two loci, QTL-M and QTL-E. ME+cry1Ac could present an opportunity to protect the durability of Bt genes in elite soybean cultivars. These results should assist the development of effective pest management strategies, and sustainable deployment of Bt genes in soybean.

Corn stand and yield loss from seedling injury by southern corn rootworm (Coleoptera: Chrysomelidae).

Southern corn rootworm, Diabrotica undecimpunctata howardi Barber, can cause severe stand loss in reduced tillage corn after a vetch cover crop. Trials conducted over 3 yr found that clothianidin and thiamethoxam neonicotinoid seed treatments and conventional granular insecticides applied in-furrow or as a T-band were very effective against southern corn rootworm. Treatments were categorized as no control (untreated), partial control, and full control based on the extent of seedling injury. In 2 yr, damaged plants were individually marked and their survival, tiller (i.e., secondary stem) production, and ear and grain weight measured. About one-half of plants injured by larvae produced a tiller, with most plants producing tillers within 30 d after planting. All dead-hearted plants not producing a tiller died and were not evident by mid-season. In no control plots with severe damage, healthy plants produced more ears per plant and secondarily more grain weight per plant than healthy plants in full control plots. Plants with early tillers in no control produced approximately 38% of the grain weight of healthy plants compared with only approximately 6% in full control plots, whereas plants with late tillers in no control plots produced < 10% of the grain weight of healthy plants and produced no grain in full control plots. Therefore, in stands with severe seedling damage, remaining healthy plants and damaged plants producing a tiller within 30 d of planting responded to lower plant population by producing more ears and grain per plant. However, in full control plots with low levels of damage, damaged plants producing a tiller often persisted until harvest but produced very little grain thereby acting as weeds with the stand.

Evaluation of pest vulnerability of 'Benning' soybean value added and insect resistant near isogenic lines.

Crop enhancement with value added traits may affect vulnerability to insects, and evaluating the susceptibility levels of the various value added traits in elite germplasm would aid in developing integrated pest management strategies. During 2007-2008, five 'Benning' soybean (Glycine max (L.) Merr) lines with different value added nutritional traits and four insect resistant quantitative trait loci (QTL) lines were evaluated in an effort to determine their pest vulnerability under artificial and natural insect pest populations. The lines showed variable susceptibility to lepidopterous insect pests classified as defoliators and stem feeders in replicated greenhouse and field tests. The study was carried out in Athens and Midville, GA. The green cloverworm (Hypena scabra (F.)) was the most common lepidopteran defoliator occurring in the fields. Other caterpillar pests found included the soybean looper (Pseudoplusia includens (Walker)), the bollworm (Helicoverpa zea (Boddie)), and the velvetbean caterpillar (Anticarsia gemmatalis (Hübner)). Data indicated that there was no significantly increased pest susceptibility among the value added cultivars with improved nutritional qualities, with the insect resistant quantitative trait loci lines Benning M and Benning MGH consistently being less susceptible to lepidopterous (Noctuidae) leaf injury.

Plant-incorporated Bacillus thuringiensis resistance for control of fall armyworm and corn earworm (Lepidoptera: Noctuidae) in corn.

Fall armyworm, Spodoptera frugiperda (J.E. Smith), and corn earworm, Helicoverpa zea (Boddie), perennially cause leaf and ear damage to corn, Zea mays L., in the southeastern United States. Transgenic Bacillus thuringiensis (Bt) hybrids with the Bt11, MON810, or 176 events expressing the Cry1Ab insecticidal endotoxin from were evaluated for control fall armyworm and corn earworm at seven locations in Georgia during 1999 and 2000. Corn was planted at the recommended time for each location and 1 and 2 mo later in the southern locations. All Bt events consistently reduced whorl infestation and damage, although event 176 did not prevent whorl damage in the later plantings in the southern locations in both years. All events also reduced seedling damage by the lesser cornstalk borer, Elasmopalpus lignosellus (Zeller), in one trial and stalk infestations and tunnel length by southwestern corn borers, Diatraea grandiosella Dyar, in another trial. Hybrids containing Bt11 and MON810 events reduced ear infestations in all trials, although reductions were small in later plantings. Nevertheless, both events reduced grain damage from earworms and armyworms by an average +/- SE of 52.5 +/- 5.1% in all trials. The hybrid containing event 176 did not reduce ear infestations and damage. Total grain aflatoxin concentrations were not significantly affected by Bt resistance in any trial (N = 17). Yield responses were variable with the prevention of yield loss being proportional to the severity of insect damage. Although plantings made after the recommended time did not consistently benefit from Bt resistance, Bt11 and MON810 events were effective in reducing damage to field corn when large infestations occurred. The Bt11 and MON810 events mitigated the risk of severe lepidopteran damage to corn, thereby making later plantings of corn feasible in double-cropping systems.

A QTL that enhances and broadens Bt insect resistance in soybean.

Effective strategies are needed to manage insect resistance to Bacillus thuringiensis (Bt) proteins expressed in transgenic crops. To evaluate a multiple resistance gene pyramiding strategy, eight soybean (Glycine max) lines possessing factorial combinations of two quantitative trait loci (QTLs) from plant introduction (PI) 229358 and a synthetic Bt cry1Ac gene were developed using marker-assisted selection with simple sequence repeat markers. Field studies were conducted in 2000 and 2001 to evaluate resistance to corn earworm (Helicoverpa zea) and soybean looper (Pseudoplusia includens), and detached leaf bioassays were used to test antibiosis resistance to Bt-resistant and Bt-susceptible strains of tobacco budworm (TBW; Heliothis virescens). Based on defoliation in the field and larval weight gain on detached leaves, lines carrying a combination of cry1Ac and the PI 229358 allele at a QTL on linkage group M were significantly more resistant to the lepidopteran pests, including the Bt-resistant TBW strain, than were the other lines. This is the first report of a complementary additive effect between a Bt transgene and a plant insect resistance QTL with an uncharacterized mode of action that was introgressed using marker-assisted selection.

Effect of pubescence tip on soybean resistance to lepidopteran insects.

DNA marker analysis has mapped a quantitative trait locus for soybean, Glycine max (L.) Merr., resistance to the corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), on the USDA soybean genetic linkage map near the classical gene Pb, which conditions pubescence tip. This study was initiated to determine the effect of pubescence tip on resistance to H. zea larvae and to examine the effect on beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), and soybean looper, Pseudoplusia includens (Walker) (Lepidoptera: Noctuidae), larvae. The effect of blunt (pb) and sharp (Pb) pubescence tip was tested in antixenosis and antibiosis bioassays on H. zea, S. exigua, and P. includens larvae with near-isolines and insect-resistant and -susceptible genotypes differing in pubescence tip morphology. Sharp pubescence tip significantly reduced defoliation (antixenosis) from H. zea, S. exigua, and P. includens and weight gain (antibiosis) of H. zea. The weight gain of P. includens was unaffected, and S. exigua weight gain was significant for one pair of near-isolines differing in pubescence tip but not the other. The results indicate that sharp pubescence tip would be beneficial to introgress into elite soybean germplasm due to its association with resistance to H. zea, S. exigua, and P. includens.