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.

Evaluation of corn germplasm lines for multiple ear-colonizing insect and disease resistance.

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 X environment (G X 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 serous (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.

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.

Impact of brown stink bug (Heteroptera: Pentatomidae) feeding on corn grain yield components and quality.

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. sercus 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.

Differential responses of forage pearl millet genotypes to chinch bug (Heteroptera: Blissidae) feeding.

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.

Changes in fungi and mycotoxins in pearl millet under controlled storage conditions.

Pearl millet is increasingly being grown as a premium-value grain for the recreational wildlife and poultry industries in the southern US. We conducted three experiments to assess grain mold development in storage conditions typically encountered in the region of production. Variables included production year, temperature, relative humidity, atmosphere, and grain moisture content. In the first experiment, grain was stored for 9 weeks at 20 or 25 degrees C and maintained at 86% or 91% relative humidity (r.h.). In the second experiment, grain was stored for 9 weeks at 20 or 25 degrees C in either air (aerobic) or N2 (anaerobic), and maintained at 100% r.h. In the third experiment, high-moisture grain was stored for 3 weeks at 20 or 25 degrees C and maintained at 100% r.h. Grain was sampled at weekly intervals and plated to determine changes in fungal frequency. Fungi isolated included Fusarium chlamydosporum (19% of grain), Curvularia spp. (14%), F. semitectum (16%), Alternaria spp. (9%), Aspergillus flavus (8%), "Helminthosporium"-type spp. (6%), and F. moniliforme sensu lato (3%). Year of grain production significantly affected isolation frequency of fungi. Isolation frequencies from low-moisture grain were rarely affected by temperature, relative humidity, or atmosphere treatments, but was affected by storage duration for some fungi. Changes in isolation of toxigenic fungi occurred in high-moisture grain. Isolation frequency of F. chlamydosporum increased in grain stored at 86% and 91% r.h. Incidence of A. flavus increased in high-moisture grain treatments, particularly at 25 degrees C. Incidence of deoxynivalenol was not affected by storage treatment. Low concentrations of nivalenol were detected in most grain incubated at 100% r.h. Zearalenone was detected only when grain moisture content was 20-22%. Aflatoxin contamination averaged 174 ng g(-1) over all treatments, and increased up to 798 ng g(-1) in high-moisture grain at stored at 25 degrees C.

United States Department of Agriculture-Agricultural Research Service research on improving host-plant resistance to pests.

Host-plant resistance is an efficient, economical and environmentally benign approach used to manage many pests and diseases of agricultural crops. After nearly a century of research, the resources and tools have become more refined, but the basic tasks in breeding for resistance have not changed. Resistance must be identified, incorporated into elite germplasm, and deployed in a form useful to growers. In some instances, biotechnology has expedited this process through incorporating a foreign gene(s) for resistance into elite germplasm. The USDA Agricultural Research Service (ARS) has made significant contributions in the development of germplasm with resistance to insects, nematodes and plant diseases. Because resistant plant varieties are an essential component of sustainable production systems, ARS is committed to developing techniques and germplasm to help meet this goal.