Southwestern Corn Borer Larvae Research Articles

Wheat Germ Hemicellulose Is an Absolute Requirement for Growth and Development of the Southwestern Corn Borer1–,

The southwestern corn borer, Diatraea grandiosella Dyar, is a major pest of corn, Zea mays L., in the southern United States. To search for plant allelochemicals, chemically defined insect diets are commonly used, but southwestern corn borer larvae did not grow and develop unless a subnutritional amount of wheat germ was incorporated in the amino acid diet. Fractionation led to identification of hemicellulose as the active component which was then characterized. The hemicellulose permitted insect growth on a protein-free amino acid diet. Microfloral involvement was ruled out because larvae grew and developed free of intestinal microbes. Therefore, this insect has an absolute requirement for hemicellulose that has evidently not been demonstrated with other invertebrates or vertebrates. This finding is of general interest because hitherto, the contributions of hemicellulose to nutrition have generally been considered to be associated chiefly with microfloral conversion to assimilable carbohydrates. Investigations should be conducted to determine whether hemicelluloses are also important or essential for growth of vertebrates including mammals by mechanisms that may not have been considered.

Injury and Interplant Compensation for Southwestern Corn Borer (Lepidoptera: Crambidae) Infestations in Field Corn

Growers that plant Bacillus thuringiensis (Bt) Berliner corn (Zea mays L.) hybrids are required to plant non-Bt corn for resistance management. Refuge in a bag (RIB) is an emerging approach for resistance management where, for some hybrids having multiple Bt traits for a target species, the refuge is planted as a blend of Bt and non-Bt corn. Studies were conducted to evaluate how southwestern corn borer (Diatraea grandiosella Dyar), when infested at different densities and growth stages, affected the yield of infested, non-Bt plants and neighboring Bt plants. Infesting non-Bt corn plants with southwestern corn borer larvae caused significant injury. Both the number of larvae infested on plants and the timing of these infestations affected the number of kernels per ear, total kernel weight, and the weight of individual kernels. Infestation timing was more important than the number of larvae inoculated onto plants, with pretassel infestations causing more yield loss. There was little compensation by Bt plants that were adjacent to infested plants. Thus, the risk of yield loss from stalk tunneling larvae in a refuge in a bag scenario should be directly proportional to the percentage of non-Bt plants and the level of yield loss observed in these non-Bt plants. Because current refuge in a bag systems have five or 10% non-Bt corn plants within the seed unit, the likelihood of substantial yield losses from infestations of corn boring larvae is remote given our results, especially for infestations that occur after silking has begun.

Fall Armyworm (Lepidoptera: Noctuidae) and Southwestern Corn Borer (Lepidoptera: Crambidae) Leaf Feeding Damage and Its Effect on Larval Growth on Diets Prepared from Lyophilized Corn Leaves

Fall armyworm, Spodoptera frugiperda (J.E. Smith), and Southwestern corn borer, Diatraea grandiosella Dyar, are economically important pests of maize (Zea mays L.) in the southeastern United States. These insects attack plants in both the vegetative and reproductive stage so f growth. Plant resistance is widely considered a desirable means for reducing losses to both insects, and corn germplasm lines with resistance to leaf feeding damage have been developed and released. Fall armyworm and southwestern corn borer larvae feeding on resistant genotypes grow more slowly than those feeding on susceptible genotypes. The objectives of the investigation were to evaluate 20 single cross maize hybrids for leaf feeding damage by southwest- ern corn borer and fall armyworm, to compare larval growth on laboratory diets prepared from lyophilized leaf tissue of single cross hybrids with different levels of resistance to leaf feeding, and to determine whether larval growth differed between diets prepared from leaf tissue collected from plants previously infested with southwestern corn borer or fall armyworm and non- infested plants. Both fall armyworm and southwestern corn borer larvae weighed significantly less when fed on laboratory diets prepared from lyophilized leaf tissue of resistant single cross hybrids than susceptible hybrids. When tissue from either resistant or susceptible plants that had been infested with either insect in the field was used in bioassays, larval weights were further reduced. It appears that both constitutive and induced defensive mechanisms may be operating.

Plant Resistance and Its Effect on the Peritrophic Membrane of Southwestern Corn Borer (Lepidoptera: Crambidae) Larvae

The southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), is a serious pest of corn, Zea mays L., in the southern United States. Corn germplasm lines with conventional genetic leaf-feeding resistance to this pest, the fall armyworm, Spodoptera frugiperda (J. E. Smith), and other lepidopterans have been released to the public by USDA–ARS scientists located in Mississippi. Recent studies suggest the insect resistant lines disrupt the integrity of the peritrophic membrane of the fall armyworm. The objectives of this study were to investigate any morphological differences in the structure of the peritrophic membrane of southwestern corn borer larvae feeding on resistant and susceptible corn hybrids and to quantify the damage. Larvae were reared under field and laboratory conditions on three corn hybrids (two resistant and one susceptible). Scanning electron microscopy was used to examine the peritrophic membrane for abnormalities such as holes or tears and to count the holes or tears in the membrane. Differences in the degree of damage to peritrophic membrane of larvae fed on resistant and susceptible plants were not detected. Up to five distinct layers of the membrane were observed in each larva. Variation in the amounts of damage to the peritrophic membrane observed from larvae feeding on all plant material was high. Plant resistance adversely affects growth and development of southwestern corn borer larvae, and further investigations are needed to explain the role of plant resistance and its relation to peritrophic membrane in southwestern corn borer larvae.

Plant Resistance and Its Effect on the Peritrophic Membrane of Southwestern Corn Borer (Lepidoptera: Crambidae) Larvae

The southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), is a serious pest of corn, Zea mays L., in the southern United States. Corn germplasm lines with conventional genetic leaf-feeding resistance to this pest, the fall armyworm, Spodoptera frugiperda (J.E. Smith), and other lepidopterans have been released to the public by USDA-ARS scientists located in Mississippi. Recent studies suggest the insect resistant lines disrupt the integrity of the peritrophic membrane of the fall armyworm. The objectives of this study were to investigate any morphological differences in the structure of the peritrophic membrane of southwestern corn borer larvae feeding on resistant and susceptible corn hybrids and to quantify the damage. Larvae were reared under field and laboratory conditions on three corn hybrids (two resistant and one susceptible). Scanning electron microscopy was used to examine the peritrophic membrane for abnormalities such as holes or tears and to count the holes or tears in the membrane. Differences in the degree of damage to peritrophic membrane of larvae fed on resistant and susceptible plants were not detected. Up to five distinct layers of the membrane were observed in each larva. Variation in the amounts of damage to the peritrophic membrane observed from larvae feeding on all plant material was high. Plant resistance adversely affects growth and development of southwestern corn borer larvae, and further investigations are needed to explain the role of plant resistance and its relation to peritrophic membrane in southwestern corn borer larvae.

Impact of Plant Resistance on Southwestern Corn Borer (Lepidoptera: Crambidae) Biology and Plant Damage

Southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), is a major insect pest of corn, Zea mays L., in the southern United States. Germplasm lines with resistance to southwestern corn borer have been developed and released by the USDA-ARS. Two single-cross hybrids produced by crossing germplasm lines with resistance to southwestern corn borer and a susceptible single-cross hybrid were infested with southwestern corn borer larvae in a 2-yr field test conducted in Mississippi. The susceptible hybrid sustained significantly more leaf damage and stalk tunneling than either resistant hybrid. The number of tunnels and the length of tunneling were significantly lower on the resistant hybrids. In 2003, up to 15 times more tunneling was observed on the susceptible hybrid. Larvae feeding on the resistant hybrids were delayed in their movement from the whorl to the stalk and larval survival was 50% lower on the resistant hybrids than on the susceptible hybrid. Larvae recovered from the susceptible hybrid 7-14 d after infestation weighed twice as much as those recovered from the resistant hybrids. Similar differences in larval weight were observed in the laboratory when larvae were reared on diets prepared from lyophilized tissue from the three hybrids. These results provide a foundation for other investigations designed to identify and determine the roles of specific genes and gene families associated with southwestern corn borer resistance in corn.

Impact of Plant Resistance on Southwestern Corn Borer (Lepidoptera: Crambidae) Biology and Plant Damage

Southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), is a major insect pest of corn, Zea mays L., in the southern United States. Germplasm lines with resistance to southwestern corn borer have been developed and released by the USDA–ARS. Two single-cross hybrids produced by crossing germplasm lines with resistance to southwestern corn borer and a susceptible single-cross hybrid were infested with southwestern corn borer larvae in a 2-yr field test conducted in Mississippi. The susceptible hybrid sustained significantly more leaf damage and stalk tunneling than either resistant hybrid. The number of tunnels and the length of tunneling were significantly lower on the resistant hybrids. In 2003, up to 15 times more tunneling was observed on the susceptible hybrid. Larvae feeding on the resistant hybrids were delayed in their movement from the whorl to the stalk and larval survival was 50% lower on the resistant hybrids than on the susceptible hybrid. Larvae recovered from the susceptible hybrid 7–14 d after infestation weighed twice as much as those recovered from the resistant hybrids. Similar differences in larval weight were observed in the laboratory when larvae were reared on diets prepared from lyophilized tissue from the three hybrids. These results provide a foundation for other investigations designed to identify and determine the roles of specific genes and gene families associated with southwestern corn borer resistance in corn.

Effects of Nitrogen Fertility on Bt Endotoxin Levels in Maize

Three Bt maize hybrids (two with Bt event MON-810, one with Bt event DBT-418) and one nonBt maize hybrid were grown at Stoneville, MS, USA in 2002 and 2003 with N-fertility rates of 0, 112, 224, and 336 kg N/ha, respectively. Tissue samples of the 7th leaf at growth stage V5 (5 fully extended leaves) and primary ear leaf sheaths and outer ear husks at growth stage R3 (milk) were collected and analyzed for endotoxin concentrations. At V5 CrylAb Bt concentrations of the MON-810 hybrids increased whereas there were no N-fertility effects on Cry1Ac in the DBT-418 hybrid. Bioassays from tissue samples at growth stage V5 of all N-fertility treatments for both Bt events were nearly 100% fatal to southwestern corn borer larvae (Diatraea grandiosella_Dyar). Endotoxin concentrations at growth stage R3 increased in both the primary ear leaf sheaths {[Bt] = 0.0006(kg N/ha) + 0.167 r2 = 0.8817 for both MON-810 and DBT-418} and outer ear husks {[Bt] = 0.0006(kg N/ha) + 0.148) r2 = 0.9985 and [Bt] = 0.0007 (kg N/ha) + 0.16) r2 = 0.8702 for MON-810 and DBT-418, respectively} as N fertilization increased in 2003. Bioassays determined that southwestern corn borers were susceptible to even the lowest [Bt] in tissues of MON-810 at R3 but not DBT-418. Diets made from the nonBt hybrid at all stages and N-fertility rates had a nearly 80.0% larval survival rate. Because other Noctuidae pests [i.e., corn earworm (Heliothis zea (Boddie)) and fall armyworm (Spodoptera frugiperda (J. E. Smith))] of corn are more tolerant to Bt toxins, growing Bt maize under adequate levels of N-fertility may be even more important to insure maximum protection from these pests.

Relative Concentration of Cry1A in Maize Leaves and Cotton Bolls with Diverse Chlorophyll Content and Corresponding Larval Development of Fall Armyworm (Lepidoptera: Noctuidae) and Southwestern Corn Borer (Lepidoptera: Crambidae) on Maize Whorl Leaf Profiles

To manage insect resistance to transgenic crops that express insecticidal proteins from Bacillus thuringiensis (Bt) Berliner, the U.S. Environmental Protection Agency recommends a refuge-based insect resistance management strategy where a percentage of non-Bt (refuge) crop is grown in proximity to a Bt-expressing crop. An important requirement for this strategy is that the toxin exists at a high effective dose for control of the target pest(s), so that heterozygous individuals in the population do not reach adulthood. Factors that cause reduced levels of toxin in the plant are a threat to this strategy. We quantified Cry1Ab from different areas of the maize, Zea mays L., leaf. In general, the distal tip of the V7 maize leaf had a higher concentration of Cry1Ab compared with the middle section of the V7 leaf, and the middle section of the developing V9 leaf had the lowest concentration of Cry1Ab. When these sections of maize tissue were fed to fall armyworm, Spodoptera frugiperda (J.E. Smith), and southwestern corn borer, Diatraea grandiosella Dyar, there was not a reduction in development or an increase in mortality with tissue that had higher concentrations of toxin. Another study tested the relative concentration of Cry1Ab between the white-yellow, yellow-green, and green portions of the developing ninth leaf within the maize whorl. There were differences in Cry1Ab concentration among these leaf areas. The green tissue had the highest concentration of toxin followed by the yellow-green and white-yellow tissues. Correlations between concentration of Cry1Ab and 5-d fall armyworm larval weights among the three leaf color profiles were all significant and negative, i.e., decreased concentration of Cry1Ab in the leaf tissue resulted in increased 5-d larval weights. There was 100% mortality to the southwestern corn borer larvae fed Cry1Ab maize leaf tissue. Differences in the amount of Cry1Ab in the developing V9 leaf profiles did not alter the absolute susceptibility of the southwestern corn borer to the toxin. In cotton, Gossypium hirsutum L., the amount of Cry1Ac was significantly lower in boll tips where flowers had remained attached compared with normal boll tips. Boll tips where the flowers remained attached are often the site where corn earworms, Helicoverpa zea (Boddie), penetrate Bt cotton bolls. This study demonstrated that, in two diverse plant species, tissue that has low chlorophyll content does not fully express Cry1A. Photosynthesis regulating factors related to mRNA transcription and translation should be studied for their effect on Cry1A production and insect control.

Southwestern corn borer (Lepidoptera: Crambidae) damage and aflatoxin accumulation in maize.

Aflatoxin, a potent carcinogen, is produced by the fungus Aspergillus flavus Link: Fr. Drought, high temperatures, and insect damage contribute to increased levels of aflatoxin contamination in corn, Zea mays L. Plant resistance is widely considered a desirable method of reducing aflatoxin contamination. Germplasm lines with aflatoxin resistance have been developed. This investigation was undertaken to determine whether crosses among these lines exhibited resistance to southwestern corn borer, Diatraea grandiosella Dyar, and to assess the effects of southwestern corn borer feeding on aflatoxin accumulation. Differences in ear damage among southwestern corn borer infested hybrids were significant. Estimates of general combining ability effects indicated that the lines Mp80:04, Mp420, and Mp488 contributed to reduced ear damage, and SC213 and T165 contributed to greater damage when used in hybrids. Mean aflatoxin levels were 254 ng/g for hybrids infested with southwestern corn borer larvae and 164 ng/g for noninfested hybrids in 2000 when environmental conditions were conducive to aflatoxin production. In contrast, the overall mean aflatoxin level for southwestern corn borer infested hybrids was only 5 ng/g in 1999 when environmental conditions did not favor aflatoxin accumulation. Crosses that included lines selected for aflatoxin resistance as parents (Mp80:04 and Mp313E) exhibited lower levels of aflatoxin contamination both with and without southwestern corn borer infestation in 2000. Only the experimental line Mp80:04 contributed significantly to both reduced southwestern corn borer damage and reduced aflatoxin contamination.

Pathogens associated with southwestern corn borers and southern corn stalk borers (Lepidoptera: Crambidae).

A study was undertaken to isolate entomopathogens of southwestern corn borer, Diatraea grandiosella Dyer, and southern corn stalk borer, Diatraea crambidoides (Grote). Field-collected diapausing larvae of southwestern corn borer (three sites in Mississippi) and southern corn stalk borer (one site in North Carolina), and a laboratory strain of D. grandiosella in the diapause state were maintained in a simulated winter followed by a simulated spring environnent. Few larvae (< or = 6%) collected from any of the field sites died in the winter environment, and most insect mortality (11-25%) occurred after transfer of the larvae to the simulated spring environment. Mortality during the simulated spring period differed among the collection sites, and the highest mortality was recorded for southwestern corn borers from Washington County (25%), followed by Marshall (16%) and Oktibbeha (11%) Counties. A high level of mortality was also observed in southern corn stalk borers during the simulated spring period (27%). No viruses were observed, but a number of bacteria, microsporidia, and fungi were isolated from both southwestern corn borer and southern corn stalk borer larvae and pupae. In most instances, numerous bacterial taxa were isolated from cadavers, but on some occasions a single taxon predominated. The most prevalent bacterial taxon from larval and pupal cadavers was Enterococcus faecalis (Andrewes & Horder) Schleifer & Kilppel-Balz, but Bacillus spp., Pseudomonas aeruginosa (Schroeter) Migula, and Serratia marcescens Bizio were frequently isolated as well. Few fungi (1-7%) were recovered from southwestern corn borer and southern corn stalk borer larvae and pupae. The most common entomopathogenic taxon isolated was Beauveria bassiana (Balsamo) Vuillemin from southern corn stalk borer larvae. Microsporidia were not isolated from southern corn stalk borers. However, Nosema spp. were isolated from southwestern corn borer cadavers from Washington (15%), Marshall (1%), and Oktibbeha (3%) Counties in Mississippi. In addition, we observed parasitism of southern corn stalk borer larvae by Macrocentrus cingulum Reinhard (Hymenoptera: Braconidae). No parasitism of southwestern corn borers was observed. Isolates of Bacillus, Beauveria, Entercoccus, Nosema, Pseudomonas and Serratia were all pathogenic to southwestern corn borer larvae under controlled environmental conditions, and with the exception of B. bassiana, these are novel pathogens of Diatraea corn borers.

Resistance to Southwestern Corn Borer in Corn after Anthesis

Southwestern corn borer, Diatraea grandiosella Dyar, is an important pest of corn, Zea mays L., in the southern USA and Mexico. Germplasm with resistance to attack by southwestern corn borer in the vegetative stages has been released; however, resistance to attack after anthesis has not yet been identified. The objectives of this investigation were to evaluate a diallel cross among a group of inbred lines, some of which appeared to exhibit resistance in laboratory and preliminary field tests, and to evaluate the effectiveness of visual ratings of leaf sheath and husk damage in identifying resistant hybrids. The nine‐parent diallel cross was grown for 2 yr and plants were infested with southwestern corn borer larvae 7 d after anthesis. Leaf sheaths and husks were visually rated and larvae recovered, counted, and weighed 14 d after infestation. Husks were also collected from an additional planting of the diallel cross within 3 d after silk emergence, lyophilized, and used in preparation of laboratory diets. Larvae were fed on the diets for 21 d and then weighed. Leaf sheath and husk ratings were not significant among hybrids; however, differences in number of larvae, mean larval weight, and total weight of larvae collected from a plant in the field test and mean larval weight in the laboratory among hybrids were significant. General combining ability (GCA) was a significant source of variation among hybrids for all traits. Estimates of general combining ability effects indicated that the parental inbreds, Mp305 and SC213, contributed to resistance of their hybrids.

Growth Responses of Southwestern Corn Borer (Lepidoptera: Crambidae) and Fall Armyworm (Lepidoptera: Noctuidae) Larvae Fed Combinations of Whorl Leaf Tissue from a Resistant and a Susceptible Maize Hybrid

Maize, Zea mays L., germplasm with resistance to leaf feeding by southwestern corn borer, Diatmea grandiosella Dyar, and fall armyworm, Spodoptera frugiperda (J. E. Smith), neonates has been developed and released. Laboratory studies were conducted over 2 yr to determine if this resistance extends to older larvae initially fed susceptible leaf tissue and if larvae initially fed resistant leaf tissue recover when switched to similar susceptible tissue. Our results indicate a resistance and a recovery response by larvae of both species, depending on leaf tissue feeding regime. Southwestern corn borer larvae fed susceptible tissue for 3 or 7 d before switching to resistant tissue gained significantly less weight after the switch than those fed on only susceptible tissue. In contrast, southwestern corn borer larvae fed resistant tissue for 3 or 7 d before switching to susceptible tissue gained significantly more weight after the switch than those fed resistant tissue only. Similar results were observed for the fall armyworm. An interesting response occurred with fall armyworm larvae fed resistant tissue after being initially fed susceptible tissue for 7 d. Although the larvae gained less weight after the tissue switch than those reared only on susceptible tissue, the number of days to pupation did not differ. However, the resulting pupae weighed only 138.5 mg compared with 182.1 mg for those reared on only susceptible tissue. Our findings provide evidence that resistance to later instars does occur in maize and useful information when considering using a mixture of resistant and susceptible seed as an integrated pest management strategy.

Southwestern Corn Borer Growth on Laboratory Diets Containing Lyophilized Corn Husks

Southwestern corn borer (Diatraea grandiosella Dyar) is a serious pest of corn (Zea mays L.) in the southern USA. Lack of reliable techniques for evaluating resistance after anthesis is a major problem. The objective was to develop a laboratory bioassay for evaluating southwestern corn borer larval growth on diets comprised primarily of lyophilized husks of corn inbred lines. Diets were prepared by adding 15 g of lyophilized busk tissue collected within 3 d after silk emergence to a mixture of 250 mL distilled water, 2400 mg agar, 12.5 mg gentamicin sulfate, 132 mg sorbic acid, and 528 mg ascorbic acid. Larvae were weighed after feeding on the test diet for 21 d. Larvae fed diets containing husks of different inbred lines varied significantly. Because southwestern corn borer larvae that infest corn plants at or after anthesis feed initially on husks, this bioassay is a potentially useful technique for identifying resistant germplasm.

Wheat Germ Hemicellulose Is an Absolute Requirement for Growth and Development of the Southwestern Corn Borer1–4

The southwestern corn borer, Diatraea grandiosella Dyar, is a major pest of corn, Zea mays L., in the southern United States. To search for plant allelochemicals, chemically defined insect diets are commonly used, but southwestern corn borer larvae did not grow and develop unless a subnutritional amount of wheat germ was incorporated in the amino acid diet. Fractionation led to identification of hemicellulose as the active component which was then characterized. The hemicellulose permitted insect growth on a protein-free amino acid diet. Microfloral involvement was ruled out because larvae grew and developed free of intestinal microbes. Therefore, this insect has an absolute requirement for hemicellulose that has evidently not been demonstrated with other invertebrates or vertebrates. This finding is of general interest because hitherto, the contributions of hemicellulose to nutrition have generally been considered to be associated chiefly with microfloral conversion to assimilable carbohydrates. Investigations should be conducted to determine whether hemicelluloses are also important or essential for growth of vertebrates including mammals by mechanisms that may not have been considered.

Southwestern Corn Borer (Lepidoptera: Pyralidae) and Fall Armyworm (Lepidoptera: Noctuidae): Comparative Developmental Biology and Food Consumption and Utilization

Developmental biology and food consumption and utilization of male and female southwestern corn borers, Diatraea grandiosella Dyar, and fall armyworms, Spodoptera frugiperda (J. E. Smith), were compared when reared on a meridic diet. Variations in some developmental parameters were sex-related. Female southwestern corn borer larvae (≥8 d old) and pupae were significantly heavier, had significantly wider head capsules in fourth and fifth instars, and had a significantly longer developmental period than males, but their pupal developmental periods did not differ. In fall armyworm, the only developmental parameter that differed between sexes was the pupal developmental period (Significantly shorter in females). Food consumption and utilization by selected larval age groups were estimated with indices for the amount of food ingested, diet assimilated, approximate digestibility (AD), efficiency of conversion of ingested food (ECI), and efficiency of conversion of assimilated food (ECD). Five-, 7-, and 8-d-old southwestern corn borer males and females had equivalent weight gains, and food consumption and utilization. However, 10-, 13-, and 15-d-old females gained significantly more weight, ingested and digested significantly more food, and had significantly higher ECI and ECD values than comparably aged males. The AD values of these three age groups did not differ significantly between sexes. In contrast, no significant differences in weight gained or in any of the estimated food consumption and utilization indices at all age groups studied were observed between male and female fall armyworm larvae. Thus, variation of developmental biologies of these lepidopterous species are not only speciesrelated but also sex-related. Use of developmental parameters of insects as indicators to measure or compare experimental treatment effects or as criteria for selecting insects to be used in an experiment must be done with caution.

Arrestant responses of southwestern corn borer larvae to free amino acids: Structure-activity relationships

The leaf-feeding resistance of corn or maizeZea mays L. to the southwestern corn borer, SWCB,Diatraea grandiosella Dyar has been attributed at least in part to decreased protein, increased crude fiber, and increased hemicellulose in the whorls of resistant genotypes. In this study, individual amino acids and sugars were evaluated as arrestants, with the objective of identifying those that gave weak or negative responses. Several structure-activity relationships were identified. Larvae responded to three-carbonn alkyl alpha amino acids more than to two-, four-, five-, and six-carbon compounds. Amino acids with terminal isopropyl functions gave decreased responses relative to theirn-alkyl counterparts. Dicarboxylic acids and their amides gave the lowest responses of all classes of amino acids. The normally occurring basic amino acids were all good arrestants. The guanido [HN:C(NH2)NH-] function was somewhat important to an arrestant response, as was the number of methylenes between the alpha and omega amino functions of diaminon-alkyl amino acids. Hydroxy amino acids were generally good arrestants unless the hydroxyl was located on a ring system. The two sugars present in expressed corn whorl juice, glucose and fructose, gave poor responses. However, two other sugars, mannose and arabinose, whose C-2 hydroxyls are conformationally in the axial position, were strongly arrestant. Formulated amino acid mixtures based on their content in whorl juice were as strong arrestants as whorl juice. However, the relative contributions of amino acids and sugars that are weak arrestants to the resistance of corn to SWCB larvae is uncertain because amino acid analyses did not reveal significantly higher contents of these amino acids in the whorl juices of resistant lines.

Mortality of Second-Generation Immatures of the Southwestern Corn Borer (Lepidoptera: Pyralidae)

The naturally occurring mortality of eggs and larvae of second-generation southwestern corn borer, Diatraea grandiosella (Dyar), in Kansas irrigated field corn, Zea mays L., was investigated. During 1983, corn plants in the study plots averaged 35.4 southwestern corn borer eggs per plant. Egg mortality was 33.2%; egg through third larval stage mortality was 86.2%; and 96.8% mortality occurred from egg through large larval stages. During 1984, plants in the study plots averaged 6.1 eggs per plant. Egg stage mortality was 32.6%; egg through third larval stage mortality was 87.2%; and 87.2% mortality occurred from egg through large larval stages. Trichogramma pretiosum (Riley) was the most significant factor contributing to egg mortality, accounting for 21.9% of total mortality in 1983 and 28.2% of total mortality in 1984.

Response of resistant and susceptible maize to different infestation levels of southwestern corn borer(Diatraea grandiosella Dyar)in Mexico

Abstract Four maize lines, Mp78:518 (resistant), P47S3 (intermediate to susceptible), Tz3 and Ki3 (susceptible), and three experimental hybrids, P47S3 × Mp78:518 (resistant), Tz3 × Ki3 and Ki3 × T×601 (susceptible), were Infested with 15, 30, 45 and 60 southwestern corn borer (SWCB) larvae per plant at the 6–7 fully extended leaf stage. There was a small linear increase in leaf feeding ratings on resistant genotypes (Mp78:518 and P47S3 × Mpρ78:518) as the number of larvae increased. On susceptible genotypes leaf feeding increased sharply at 30 larvae when compared to 15 larvae. Additional larvae resulted in little change as the ratings at 30 larvae on susceptible genotypes were in the upper limit of the scale. Leaf feeding was significantly less on resistant than on susceptible genotypes. Neither plant height nor yield of resistant genotypes was reduced significantly (? = 0.05) at any infestation level. The results indicate that infestation with 15 larvae per plant gives low damage ratings and does not ad...

Ovipositional Response of Southwestern Corn Borer (Lepidoptera: Pyralidae) and Fall Armyworm (Lepidoptera: Noctuidae) to Selected Maize Hybrids

Field cage experiments were done to determine if maize (Zea mays L.) hybrids with leaf-feeding resistance to larvae of southwestern corn borer, Diatraea grandiosella Dyar, and fall armyworm, Spodoptera frugiperda (J. E. Smith), also are less preferred for oviposition under choice conditions. Two resistant maize hybrids (Mp496 × Mp701 and Mp704 × Mp706) and two susceptible hybrids (SC229 × Tx601 and Ab24E × Va35) were tested. Southwestern corn borers laid significantly fewer eggs on the resistant than on the susceptible hybrids. Fall armyworms laid significantly fewer eggs on the resistant hybrids than on SC229 × Tx601 but not on Ab24E × Va35. Thus, females of both species exhibited preference in selection of hosts for oviposition with the resistant hybrids being less preferred.