Assessing Soybean Genotypes for Resistance to Helicoverpa armigera (Lepidoptera: Noctuidae).

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is an important pest capable of colonizing several species of cultivated plants, including soybean [Glycine max (L.) Merrill]. The primary management strategy for this insect is chemical control and adoption of Bt plants. However, alternative environmentally friendly strategies should be investigated and adopted as available for sustainable management of this insect pest. One potential underutilized method for insect control is plant resistance. The objective of this study was to categorize resistance (antixenosis and antibiosis) of 22 soybean genotypes on H. armigera in a laboratory setting. A preliminary experiment was conducted using soybean leaves and pods to determine the most promising genotypes for resistance. For each selected genotype, we collected biological parameters, leaf intake, and performed confinement tests. The genotypes PI 227687, PI 274453, PI 274454, PI 229358, PI 171451, 'IAC 17', and 'IAC 19' expressed resistance against H. armigera. Each genotype lowered larval survivorship. The genotypes PI 229358, PI 227687, PI 274453, and PI 274454 expressed resistance by prolonging larval development and reducing fifth-instar larvae weight. 'IAC 19' reduced leaf consumption and lowered the weight of fifth-instar larvae. Antixenosis was found in genotypes PI 227687, PI 274454, and 'IAC 19'. These genotypes might be used in breeding programs focusing on soybean resistance to lepidopterans.

Characterization of Antixenosis in Soybean Genotypes to Bemisia tabaci (Hemiptera: Aleyrodidae) Biotype B.

Bemisia tabaci biotype B (Gennadius) is one of the most important soybean pest worldwide. Herein, 15 soybean genotypes were evaluated, to characterize the occurrence of antixenosis to B. tabaci biotype B. Initially, a multiple-choice test with all genotypes was carried out, evaluating the settling and oviposition preference at 3 d after infestation, and the colonization by nymphs after 48 d of infestation. Subsequently, a no-choice test, using 14 genotypes, was conducted with infested plants individually, and the number of eggs was counted after 72 h. Then, 10 genotypes were selected (indicative of resistance and susceptibility), which were evaluated for whitefly settling 24, 48, and 72 h after infestation and for oviposition 72 h after infestation. The trichomes of the leaflets were characterized for density, size, and inclination to establish possible correlations with the settling and oviposition in the genotypes. In the first multiple-choice test, involving 15 genotypes, 'IAC-17,' 'IAC-19,' and UX-2569-159 expressed antixenosis against B. tabaci. 'Jackson,' 'P98Y11,' and PI-229358 exhibited the same behavior in the no-choice test. In the multiple-choice test, 'Jackson,' 'P98Y11,' and 'TMG1176 RR' were the least attractive and least used for oviposition. The antixenosis shown by 'Jackson,' 'P98Y11,' and PI-229358 may be related to the characteristics of the trichomes (lower density and inclined). Based on the experiments carried out, 'IAC-17,' 'IAC-19,' 'Jackson,' 'P98Y11,' PI-229358, TMG1176 RR, and UX-2569-159 are considered promising for resistance to B. tabaci biotype B and may be exploited in soybean breeding programs for resistance to insects.

Intraguild Competition of Three Noctuid Maize Pests.

The western bean cutworm Striacosta albicosta (Smith), the fall armyworm Spodoptera frugiperda (J. E. Smith), and the corn earworm Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) are among the major lepidopteran pests of maize in the United States, belonging to the same guild and injuring the reproductive tissues of this crop. Here, intraguild competition of these lepidopterans on non-Bt maize was evaluated through survival analysis of each species under laboratory and field conditions. Competition scenarios were carried out in arenas containing maize silk or ear tissue, using larvae on different stadium of development. Fitness cost competition studies were conducted to examine the influence of intraguild competition and cannibalism and predation rates on larval development. The survival of S. albicosta competing with the other species was significantly lower than in intraspecific competition, even when the larvae were more developed than the competitor. For S. frugiperda, survival remained high in the different competition scenarios, except when competing in a smaller stadium with H. zea Larvae of H. zea had a high rate of cannibalism, higher survival when competing against S. albicosta than S. frugiperda, and reduced survival when the H. zea larvae were at the same development stadium or smaller than the competitors. Based on fitness cost results, the absence of a competitor for the feeding source may confer an advantage to the larval development of S. frugiperda and H. zea Our data suggest that S. frugiperda has a competitive advantage against the other species, while S. albicosta has the disadvantage in the intraguild competition on non-Bt maize.