Jeffrey P. Wilson

Impact of Brown Stink Bug (Heteroptera: Pentatomidae) Feeding on Corn Grain Yield Components and Quality

ABSTRACT Brown stink bug, Euschistus servus (Say) (Heteroptera: Pentatomidae), damage on developing corn, Zea mays L., ears was examined in 2005 and 2006 by using eight parameters related to its yield and kernel quality. Stink bug infestations were initiated when the corn plants were at tasseling (VT), mid-silking (R1), and blister (R2) stages by using zero, three, and six in 2005 or zero, one, two, and four bugs per ear in 2006, and maintained for 9 d. The percentage of discolored kernels was affected by stink bug number in both years, but not always affected by plant growth stage. The growth stage effect on the percentage of discolored kernels was significant in 2006, but not in 2005. The percentage of aborted kernels was affected by both stink bug number and plant growth stage in 2005 but not in 2006. Kernel weight was significantly reduced when three E. servus adults were confined on a corn ear at stage VT or R1 for 9 d in 2005, whereas one or two adults per ear resulted in no kernel weight loss, but four E. servus adults did cause significant kernel weight loss at stage VT in 2006. Stink bug feeding injury at stage R2 did not affect kernel damage, ear weight or grain weight in either year. The infestation duration (9 or 18 d) was positively correlated to the percentage of discolored kernels but did not affect kernel or ear weight. Based on the regression equations between the kernel weight and stink bug number, the gain threshold or economic injury level should be 0.5 bugs per ear for 9 d at stage VT and less for stage R1. This information will be useful in developing management guidelines for stink bugs in field corn during ear formation and early grain filling stages.

Spatial patterns of aflatoxin levels in relation to ear-feeding insect damage in pre-harvest corn.

Key impediments to increased corn yield and quality in the southeastern US coastal plain region are damage by ear-feeding insects and aflatoxin contamination caused by infection of Aspergillus flavus. Key ear-feeding insects are corn earworm, Helicoverpa zea, fall armyworm, Spodoptera frugiperda, maize weevil, Sitophilus zeamais, and brown stink bug, Euschistus servus. In 2006 and 2007, aflatoxin contamination and insect damage were sampled before harvest in three 0.4-hectare corn fields using a grid sampling method. The feeding damage by each of ear/kernel-feeding insects (i.e., corn earworm/fall armyworm damage on the silk/cob, and discoloration of corn kernels by stink bugs), and maize weevil population were assessed at each grid point with five ears. The spatial distribution pattern of aflatoxin contamination was also assessed using the corn samples collected at each sampling point. Aflatoxin level was correlated to the number of maize weevils and stink bug-discolored kernels, but not closely correlated to either husk coverage or corn earworm damage. Contour maps of the maize weevil populations, stink bug-damaged kernels, and aflatoxin levels exhibited an aggregated distribution pattern with a strong edge effect on all three parameters. The separation of silk- and cob-feeding insects from kernel-feeding insects, as well as chewing (i.e., the corn earworm and maize weevil) and piercing-sucking insects (i.e., the stink bugs) and their damage in relation to aflatoxin accumulation is economically important. Both theoretic and applied ramifications of this study were discussed by proposing a hypothesis on the underlying mechanisms of the aggregated distribution patterns and strong edge effect of insect damage and aflatoxin contamination, and by discussing possible management tactics for aflatoxin reduction by proper management of kernel-feeding insects. Future directions on basic and applied research related to aflatoxin contamination are also discussed.

Preharvest aflatoxin contamination of corn and other grain crops grown on the U.S. Southeastern Coastal Plain

Preharvest aflatoxin contamination of grain grown on the U.S. Southeastern Coastal Plain is provoked and aggravated by both biotic and abiotic stress factors that influence infection by the Asperigillus group. Asperigillus flavus, Link ex Fr., is one of the principal toxigenic fungi of summer grains grown in the region, and the hot, humid weather patterns along with suboptimal summer rainfall favor the development of this organism. An array of arthropod species also contributes to the dispersal of this fungus as they attack and feed on the developing grain. Research on summer grains grown on the Coastal Plain has the expressed goal of reducing, and perhaps eliminating aflatoxin contamination in adapted germplasm using classical crop improvement methods to deploy host plant resistance. This research is complimented and enhanced by molecular techniques that have proven invaluable in the identification and development of superior germplasm. It also emphasizes the need to fully understand the biological interactions between fungus, arthropods, crops, and the environmental conditions that govern the aflatoxin contamination. Alternative cropping systems that avoid contamination are also integrated into this summary of this research progress.

Differential Responses of Forage Pearl Millet Genotypes to Chinch Bug (Heteroptera: Blissidae) Feeding

ABSTRACT Chinch bug, Blissus leucopterus leucopterus (Say) (Heteroptera: Blissidae), is one of the most important insect pests on forage pearl millet, Pennisetum glaucum L. R. Br., production in the southeastern United States. Twenty-nine forage pearl millet genotypes were assessed for chinch bug resistance by using stunt and necrosis ratings in combination with quantitative measurements of chlorophyll content and leaf photosynthetic rate. Plant stunt and leaf sheath necrosis ratings, and chlorophyll content in flag leaves differed among the 29 genotypes. Photosynthetic rate differed both among the noninfested control and among the chinch bug-infested plants. The chinch bug-infested plants had lower photosynthetic rate than the noninfested control plants. Inbreds with resistance superior to that of Tift 23DB were identified for hybrid development. When the 29 pearl millet genotypes were assessed by the six parameters by using cluster analysis, genotypes 07F-1226, 07F-1229, 07F-1231, 07F-1235, 07F-1238, 07F-1239, and 07F-1240 were the most resistant, whereas the genotypes 07F-1220, 07F-1221, 07F-1225, 07F-1227, 07F-1232, 07F-1246, and Tift 23DB were the most susceptible to chinch bug feeding. The rest of the genotypes expressed intermediate responses to the six parameters. To differentiate the physiological impact of chinch bug feeding on light and dark reactions of plant photosynthesis, photosynthesis capacity was assessed using light and CO2 (A/Ci) response curves on noninfested and chinch bug-infested plants of genotypes 07F-1246, 07F-1223, and 07F-1245, which expressed low, intermediate, and high chlorophyll content, respectively. Based on the A/Ci curves, photosynthesis capacity of injured leaves was suppressed in 07F-1223 and 07F-1246, whereas the chinch bug-injured 07F-1245 leaves showed an increase of photosynthetic rate compared with the noninfested plants. In contrast, light response curves were suppressed in the chinch bug-injured plants compared with the noninfested plants of all three genotypes, irrespective of their variations in insect injury ratings. This research demonstrated that visual stunt and necrosis rating methods in combination with chlorophyll and photosynthesis measurements could be used in screening forage pearl millet for chinch bug resistance and deciphering the underlying resistance mechanisms.

Evaluation of Pearl Millet Grain Hybrids for Resistance to Meloidogyne spp. and Leaf Blight Caused by Pyricularia grisea

Pearl millet, Pennisetum glaucum, has potential as a grain crop in the southeastern United States. Our objectives were to (i) determine the resistance and/or tolerance of pearl millet hybrids to Meloidogyne incognita race 3 and M. arenaria race 1; (ii) compare reproduction of Meloidogyne spp. on pearl millet and corn; and (iii) determine the disease severity of leaf blight caused primarily by Pyricularia grisea. In a field naturally infested with M. incognita, experimental pearl millet hybrids with inbreds 114 and 117 as the pollinators had fewer numbers of second-stage juveniles and more severe leaf blight than did HGM-100, a nematode-susceptible hybrid; hybrids with inbred 115 as the pollinator were similar to HGM-100 in both nematode numbers and foliar disease severity. Grain yields in pearl millet were greater in plots treated with 1,3-dichloropropene than in control plots and were negatively correlated with leaf blight severity. In a greenhouse experiment, both M. incognita and M. arenaria produced fewer eggs on pearl millet hybrids with pollinators 114, 117, 101, 102, and 103 than on hybrid HGM-100. Reproduction of M. incognita was less on the resistant pearl millet hybrids than on corn. Because both M. incognita and P. grisea can reduce grain yield of pearl millet, hybrids developed for the southeastern United States should be resistant to both pathogens.

Evaluation of Corn Germplasm Lines for Multiple Ear-Colonizing Insect and Disease Resistance

ABSTRACT Ear-colonizing insects and diseases that reduce yield and impose health threats by mycotoxin contaminations in the grain, are critical impediments for corn (Zea mays L.) production in the southern United States. Ten germplasm lines from the Germplasm Enhancement of Maize (GEM) Program in Ames, IA, and Raleigh, NC, and 10 lines (derived from GEM germplasm) from the Texas Agricultural Experiment Station in Lubbock, TX, were examined in 2007 and 2008 with local resistant and susceptible controls. Four types of insect damage and smut disease (Ustilago maydis) infection, as well as gene × environment (G × E) interaction, was assessed on corn ears under field conditions. Insect damage on corn ears was further separated as cob and kernel damage. Cob penetration rating was used to assess corn earworm [Helicoverpa zea (Boddie)] and fall armyworm [Spodoptera frugiperda (J.E. Smith)] feeding on corn cobs, whereas kernel damage was assessed using three parameters: 1) percentage of kernels discolored by stink bugs (i.e., brown stink bug [Euschistus servus (Say)], southern green stink bug [Nezara viridula (L.)], and green stink bug [Chinavia (Acrosternum) hilare (Say)]; 2) percentage of maize weevil (Sitophilus zeamais Motschulsky)—damaged kernels; and 3) percentage of kernels damaged by sap beetle (Carpophilus spp.), “chocolate milkworm” (Moodna spp.), and pink scavenger caterpillar [Pyroderces (Anatrachyntis) rileyi (Walsingham)]. The smut infection rates on ears, tassels, and nodes also were assessed. Ear protection traits (i.e., husk tightness and extension) in relation to insect damage and smut infection also were examined. Significant differences in insect damage, smut infection, and husk protection traits were detected among the germplasm lines. Three of the 20 germplasm lines were identified as being multiple insect and smut resistant. Of the three lines, entries 5 and 7 were derived from DKXL370, which was developed using corn germplasm from Brazil, whereas entry 14 was derived from CUBA117.

Evaluation of spatial and temporal patterns of insect damage and aflatoxin level in the pre-harvest corn fields to improve management tactics

Spatial and temporal patterns of insect damage in relation to aflatoxin contamination in a corn field with plants of uniform genetic background are not well understood. After previous examination of spatial patterns of insect damage and aflatoxin in pre‐harvest corn fields, we further examined both spatial and temporal patterns of cob‐ and kernel‐feeding insect damage, and aflatoxin level with two samplings at pre‐harvest in 2008 and 2009. The feeding damage by each of the ear/kernel‐feeding insects (i.e., corn earworm/fall armyworm damage on the silk/cob, and discoloration of corn kernels by stink bugs) and maize weevil population were assessed at each grid point with five ears. Sampling data showed a field edge effect in both insect damage and aflatoxin contamination in both years. Maize weevils tended toward an aggregated distribution more frequently than either corn earworm or stink bug damage in both years. The frequency of detecting aggregated distribution for aflatoxin level was less than any of the insect damage assessments. Stink bug damage and maize weevil number were more closely associated with aflatoxin level than was corn earworm damage. In addition, the indices of spatial–temporal association (χ) demonstrated that the number of maize weevils was associated between the first (4 weeks pre‐harvest) and second (1 week pre‐harvest) samplings in both years on all fields. In contrast, corn earworm damage between the first and second samplings from the field on the Belflower Farm, and aflatoxin level and corn earworm damage from the field on the Lang Farm were dissociated in 2009.

Foliar Resistance to Fall Armyworm in Corn Germplasm Lines that Confer Resistance to Root- and Ear-Feeding Insects*

ABSTRACT A holistic approach to developing new corn germplasm that confers multiple insect resistance in various plant tissues at different growth stages was examined. Eight corn germplasm lines were examined for their foliar resistance to fall armyworm [Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae)] and natural enemy attraction at V6–V8 (or 6–8 leaf) stages in 2008 and 2009. Four corn germplasm lines with known levels of resistance to root- and ear-feeding insects [‘CRW3(S1)C6’, ‘B37*H84’, ‘SIM6’ and ‘EPM6’], and four germplasm entries with different levels of S. frugiperda resistance (‘Mp708’, ‘Ab24E’, ‘FAW7061’ and ‘FAW7111’) were evaluated in the study. All plants were manually infested with 15–20 neonate S. frugiperda larvae per plant, and injury was rated 7 and 14 d after infestation. Based on cluster analysis of S. frugiperda injury rating and predator survey data, ‘Mp708’ and ‘FAW7061’ were the most resistant, whereas ‘Ab24E’ and ‘EPM6’ were the most susceptible to fall armyworm feeding. The western corn rootworm-resistant ‘CRW3(S1)C6’ showed resistance to S. frugiperda feeding. Surveys for the diversity and abundance of predators of S. frugiperda in each experimental plot were also conducted 7 d after infestation. ‘CRW3(S1)C6’ and ‘Ab24E’ had the highest and lowest predator abundance, respectively. However, there was no direct correlation between S. frugiperda injury ratings and predator abundance. The current study demonstrated the feasibility of developing foliage-, root-, and ear-feeding insect-resistant germplasm covering multiple corn growth stages. In addition, the possibility of utilizing plant volatiles to attract predators, and reduce pest populations and crop damage is discussed.