Search

Field trials using Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) and Euxesta stigmatias Loew (Diptera: Ulidiidae) were conducted to evaluate resistance and potential damage interactions between these two primary corn, Zea mays L., pests against Lepidoptera-resistant corn varieties derived from both endogenous and exogenous sources. The endogenous source of resistance was maysin, a C-glycosyl flavone produced in high concentrations in varieties 'Zapalote Chico 2451' and 'Zapalote Chico sh2'. The exogenous resistance source was the Bacillus thuringiensis (Bt)11 gene that expresses Cry1A(b) insecticidal protein found in 'Attribute GSS-0966'. Damage by the two pests was compared among these resistant varieties and the susceptible 'Primetime'. Single-species tests determined that the Zapalote Chico varieties and GSS-0966 effectively reduced S. frugiperda larval damage compared with Primetime. E. stigmatias larval damage was less in the Zapalote Chico varieties than the other varieties in single-species tests. E. stigmatias damage was greater on S. frugiperda-infested versus S. frugiperda-excluded ears. Ears with S. frugiperda damage to husk, silk and kernels had greater E. stigmatias damage than ears with less S. frugiperda damage. Reversed phase high-performance liquid chromatography analysis of nonpollinated corn silk collected from field plots determined that isoorientin, maysin, and apimaysin plus 3'-methoxymaysin concentrations followed the order Zapalote Chico sh2 > Zapalote Chico 2451 > Attribute GSS-0966 = Primetime. Chlorogenic acid concentrations were greatest in Zapalote Chico 2451. The two high maysin Zapalote Chico varieties did as well against fall armyworm as the Bt-enhanced GSS-0966, and they outperformed GSS-0966 against E. stigmatias.

In the United States, insecticide is used extensively in the production of sweet corn due to consumer demand for zero damage to ears and to a sweet corn genetic base with little or no resistance to ear-feeding insects. Growers in the southern United States depend on scheduled pesticide applications to control ear-feeding insects. In a study of quantitative genetic control over silk maysin, AM-maysin (apimaysin and methoxymaysin), and chlorogenic acid contents in an F2 population derived from GE37 (dent corn, P1A1) and 565 (sh2 sweet corn, p1a1), we demonstrate that the P1 allele from field corn, which was selected against in the development of sweet corn, has a strong epistatic interaction with the a1 allele in sh2 sweet corn. We detected that the p1 gene has significant effects (P < 0.0001) not only on silk maysin concentrations but also on AM-maysin, and chlorogenic acid concentrations. The a1 gene also has significant (P < 0.0005) effects on these silk antibiotic chemicals. Successful selection from the fourth and fifth selfed backcrosses for high-maysin individuals of sweet corn homozygous for the recessive a1 allele (tightly linked to sh2) and the dominant P1 allele has been demonstrated. These selected lines have much higher (2 to 3 times) concentrations of silk maysin and other chemicals (AM-maysin and chlorogenic acid) than the donor parent GE37 and could enhance sweet corn resistance to corn earworm and reduce the number of applications of insecticide required to produce sweet corn.

Maysin, a C-glycosylflavone in maize silk, has insecticidal activity against corn earworm, Helicoverpa zea (Boddie), larvae. Sweet corn, Zea mays L., is a vulnerable crop to ear-feeding insects and requires pesticide protection from ear damage. This study was conducted to identify maize chromosome regions associated with silk maysin concentration and eventually to transfer and develop high silk maysin sweet corn lines with marker-assisted selection (MAS). Using an F2 population derived from SC102 (high maysin dent corn) and B31857 (low maysin sh2 sweet corn), we detected two major quantitative trait loci (QTL). It was estimated that 25.6% of the silk maysin variance was associated with segregation in the genomic region of npi286 (flanking to p1) on chromosome 1S. We also demonstrated that a1 on chromosome 3L had major contribution to silk maysin (accounted for 15.7% of the variance). Locus a1 has a recessive gene action for high maysin with the presence of functional p1 allele. Markers umc66a (near c2) and umc105a on chromosome 9S also were detected in this analysis with minor contribution. A multiple-locus model, which included npi286, a1, csu3 (Bin 1.05), umc245 (Bin 7.05), agrr21 (Bin 8.09), umc105a, and the epistatic interactions npi286 x a1, a1 x agrr21, csu3 x umc245, and umc105a x umc245, accounted for 76.3% of the total silk maysin variance. Tester crosses showed that at the a1 locus, SC102 has functional A1 alleles and B31857 has homozygous recessive a1 alleles. Individuals of (SC102 x B31857) x B31857 were examined with MAS and plants with p1 allele from SC102 and homozygous a1 alleles from B31857 had consistent high silk maysin. Marker-assisted selection seems to be a suitable method to transfer silk maysin to sweet corn lines to reduce pesticide application.

J. Econ. Entomol. 92(3): 746-753 (1999) Sweet corn is one of the highest ranking vegetable crops in the United States in terms of both market value and total acreage: it also ranks among the top vegetable crops in terms of pesticide usage. Ear damage caused by insects is the most significant concern to producers. An estimated 25-40 insecticide applications may be made in a single season to control ear-feeding insects. Plant resistance is an alternative strategy to alleviate environmental concern when pesticides are used. In this study we investigated the feasibility of transferring antibiosis to corn earworm. Helicoverpa zea (Boddie), from dent corn with high silk maysin to sweet corn. Relationships among silk maysin concentration, antibiosis to corn earworm, and silk browning in their F 2 segregating populations were determined. Lyophilized high maysin silks bioassays produced low larval weights, thus providing evidence that damage to sweet corn by corn earworm could be reduced by increasing silk maysin concentration. Simple correlation coefficients indicated a highly significant negative association between silk maysin concentrations and 8-d corn earworm larval weights (r -0.80, P< 0.0001). Silk browning was also closely associated with silk maysin concentrations and antibiosis. Diets containing mid-dark silks reduced larval weights by 68% compared with larval weights (588 mg) on the bean diet control. Maysin analyses and dry-silk bioassays also revealed that if maysin is present in F 2 plants from sweet corn kernels, it is always in a higher concentration than in plants from dent kernels, These results suggest that sweet corn parents in these populations may have a recessive gene, which may be closely linked to sh2 locus enhancing/regulating maysin synthesis when crossed to a maysin producing dent parent.

A better understanding of the genetic basis of antibiosis to the corn earworm, Helicoverpa zea (Boddie), in maize, Zea nays L., silks will help breeders develop hybrids with improved resistance to this pest. We identified maize chromosome regions associated with antibiosis to the corn earworm, and compared the results with those previously obtained for concentration of silk maysin, a C-glycosyl flavone that inhibits larval growth. We evaluated antibiosis with a dried-silk bioassay of 76 (GT114 X GT119)F 2:3 lines, derived by self-pollinating F2 plants from the high and low tails of the distribution for silk maysin concentration. Variation in 8-d larval weights resulting from silks of the F 2:3 lines was compared with variation in restriction fragment length polymorphism genotypes at 39 flavonoid pathway loci or linked markers of corresponding F 2 plants. The multiple-locus model that best explained variation of larval weights included a major effect at the p1 region of chromosome 1 and smaller effects in the umc166b region (chromosome 1) and the sh1 region (chromosome 9). The model for maysin concentration of the corresponding F2 plants was nearly identical, with the exception that the sh1 locus was replaced by the nearby u1tlciosa locus. Our results provide evidence that in this population silk maysin concentration and corn earworm antibiosis are under similar genetic control and that the p1 locus plays the major role in determining variation in both traits. Because we evaluated the tails of the distribution and probed only at flavonoid pathway loci, all chromosome regions associated with corn earworm resistance in this population may not have been detected.

Silks of 50 field corn, Zea mays L., inbreds were evaluated in 4 separate bioassays (fresh silks, 2- and 4-g oven-dried silks/100 inl diet, and silk methanol extract deposited on celufil) for their effects on the growth of corn earworm, Helicoverna zea (Boddie) larvae. The silks also were assayed for maysin, isomaysin, and apimaysin plus 3'-methoxymaysin content. Significant differences in larval growth were found among silks from inbreds within each bioassay. The dried silk bioassay was easier to mix and dispense and permitted the use of silks of germplasm with varying maturity. A highly significant negative relationship was found between weight of larvae and maysin content from fresh silk, and maysin and isomaysin content from oven-dried silk. The correlation between larval weight and apimaysin plus 3'-methoxymaysin was not significant. The correlation between larval weight and total flavones from oven dried silks seas enhanced (-0.667 for maysin alone to -0.979 for total flavones) compared with that between larval weight and maysin alone.

Plant resistance to insects, specifically antibiotic resistance, offers biologically, economically, and environmentally sound alternatives to pesticides for controlling the corn earworm, Helicoverpa zea (Boddie), and fall armyworm, Spodoptera frugiperda (J. E. Smith), in corn, Zea mays L. Our study was done to determine the relationship between maysin concentration in corn silks and growth of corn earwonn and fall armyworm larvae. Silks, or methanol extracts from the silks of a large number of corn entries mixed in meridic diets, were evaluated by feeding them to neonate larvae and measuring subsequent larval growth at 8 or 9 d. Silks from several corn entries showed high antibiosis and a high maysin content. A range of highly significant negative relationships between weight of larvae and maysin concentration was observed.

Aflatoxin contamination in preharvest maize, Zea mays L., grain is a serious problem in the United States, especially in the southeastern production areas. Development of genotypes with resistance to ear-infesting insects, resistance to Aspergillus spp., and resistance to production of aflatoxin are maize-breeding objectives. Two cultivars, Mo20W × Teosinte and ‘lbadan B’, have been identified as having resistance to earinfesting insects when compared with the widely grown check hybrid B73 × Mo17. Although it was not determined if there was resistance to growth of Aspergillus flavus (Link ex Fries) or aflatoxin production per se, it was confirmed that these two cultivars produced less aflatoxin in preharvest grain than B73 × Mo17. Also, Mo20W × Teosinte tended to have less aflatoxin than ‘lbadan B’.

The corn earworm, Helicoverpa zea (Boddie), is the most important pest of corn, Zea mays L., in the southeastern United States. This study was conducted to determine whether tolerance or antibiosis mechanisms (or both) of resistance function in 18 populations of corn that are being developed for resistance to ear and leaf damage in the field to the corn earworm and fall armyworm, Spodoptera frugiperda (J. E. Smith). The populations and resistant and susceptible checks were evaluated in the field for corn earworm injury and husk tightness and in the laboratory for feeding responses of neonate corn earworm larvae on fresh silks or oven-dried silks mixed in a meridic diet. Several populations approached the resistance level of the resistant check, ‘Zapalote Chico’, to ear damage in the field, but none equaled ‘Zapalote Chico’ in husk tightness. In 1987, the growth reduction of corn earworm larvae that were fed silk-diets of five populations was similar to the growth reduction of larvae that were fed a silk diet of ‘Zapalote Chico’. In 1988, only one population showed a growth reduction similar to that of ‘Zapalote Chico’, However, none of the populations approached the level of resistance exhibited by ‘Zapalote Chico’ when other antibiotic measurements such as development time and weight of pupae were considered. Based on data from both the field and laboratory, the resistance of recently released populations, ‘DDSA(C5)’ and ‘DDSB(C5)’, is primarily tolerance. Tolerance to ear damage in the field is also present in the 18 populations evaluated, although a few populations also possess a minor level of antibiosis.

Six recently released dent inbreds of corn, Zea mays L., with resistance to ear damage by corn earworm, Helicoverpa zea (Boddie), larvae were evaluated in the field and laboratory to determine mechanisms ofresistance. ‘GT117’ and ‘GT118’ were tolerant to corn earworm based on ear damage in the field and negligible biological effects on larvae that were fed fresh silks, silk diets, or both, in the laboratory. ‘GT114’ was susceptible to corn earworm in the field but highly antibiotic when larvae were fed silks in the laboratory. Development time to pupation for larvae that were fed silk diets of ‘GT114’ was more than twice that required for larvae that were fed check diets. Weight of pupae for larvae fed silk diets of ‘GT114’ was significantly less than for larvae that fed on any of the other entries, including the resistant check ‘Zapalote Chico’. ‘CT115’ possessed resistance to ear damage in the field as well as antibiosis in the silks in laboratory bioassays. ‘GT114’ will require additional breeding for husk characters to make the antibiotic silks effective against ear damage by larvae of the corn earworm in the field.

Two maize (Zea mays L.) breeding populations with very high concentrations of maysin, a silk-expressed flavone glycoside, were tested for their ability to resist ear damage by the corn earworm, Helicoverpa zea Boddie, under field conditions. Tests were conducted in 2000 and 2001 at multiple locations in Georgia. The high maysin populations, EPM6 and SIM6, as well as resistant and susceptible checks, were scored for silk-maysin content, H. zea damage, and husk characters. In 2000, there was a negative correlation between husk tightness and earworm damage at three of five locations, while there was no significant correlation between damage and maysin content at any location. In 2001, EPM6 and SIM6 had approximately ten times the maysin content of the low-maysin control genotypes; nevertheless, earworm damage to EPM6 and SIM6 was either greater than or not significantly different from the low-maysin genotypes at all locations. The resistant control genotype, Zapalote Chico, had significantly less earworm damage than EPM6 and SIM6 for both years at all locations. The results of this study highlight the importance of identifying and quantifying husk and ear traits that are essential to H. zea resistance in maize.

Fresh and lyophilized silks of a susceptible corn, Zea mays (L.), ‘Stowell’s Evergreen’ (SEG), and a resistant line, ‘Zapalote Chico 2451# (PC3)’ (ZC), were incorporated as base-plant materials into standard and substandard corn/soy/milk solids(CSM) and pinto bean (PB) diets and used to measure corn earworm (CEW), Heliothis zea (Boddie), larval growth and development. Eighty grams of fresh ZC silks had to be mixed into 300 ml of the standard CSM diet to reduce CEW larval weights as much as on those reared on fresh ZC silks. When added to the CSM diet, ZC silks held in the laboratory or left on the ear in the field for 5 days did not have the resistance that was shown by silks blended immediately or refrigerated or frozen for 5 days. Significant weight differences were detected between larvae fed on lyophilized SEG and those fed on ZC silk powder mixed in 300 ml of diet. All concentration levels (2, 4, 8, and 16 g per diet) of ZC silk powder added to PB diets produced significantly smaller larvae than did the SEG silk powder added to those diets. Results from the CSM diets were more variable, but ZC resistance was shown at higher concentration levels. Weights of larvae fed for 10 days and longer indicated that the resistance factor was more effective in the PB diets than in the CSM diets.

A 2-year study was conducted in the laboratory on mechanisms of resistance of corn silks to feeding and behavior of the fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith). Antibiotic factors present in silks of ‘Zapalote Chico’ result in production of small larvae and pupae and a longer life cycle of FAW compared with those fed silks of ‘Stowell’s Evergreen’ sweet corn. Results of five separate choice tests indicated a high degree of non preference by larvae of FAW for ‘Zapalote Chico’ silks compared with silks of ‘Stowell’s Evergreen’.

Significant differences were found among silks of corn, Zea mays L., for both relative maysin concentration and corn earworm, Heliothis zea (Boddie), larval weight. A decrease in maysin concentration was found in only one test year as age of silks Increased. However, in all test years (1979–1983), significant decreases in corn earworm growth were found as the age of silks increased. Since no significant relationship was found between maysin concentration of corn silks and corn earworm growth, maysin is probably one of several factors that condition resistance to corn earworms in ‘Zapalote Chico’ silks.

Introduction of Aspergillus flavus Link ex Fr. group spores onto external areas of developing corn, Zea mays L., plants did not subsequently yield a high incidence of the fungus in larvae of the European corn borer (ECB), Ostrinia nubilalis (Hubner), collected from internal portions of the plants; addition of spores of an aflatoxin-producing strain of A. flavus did not inhibit the naturally occurring, toxin-positive population of A. flavus propagules from infecting and contaminating developing kernels with aflatoxin before harvest; only a limited ECB transfer of A. parasiticus spores to the developing kernels occurred; ca. 12% of the visually characterized A. fiavus -group isolates were identified as aflatoxin producers; neither origin of the ECB larvae collected from plants (stalks or ears) nor site of fungal inoculum (silks or leaf axils) appeared related to the occurrence of A. flavus group isolates in the insects; and in Georgia, higher levels of aflatoxin in mature kernels were observed in hand-infested ECB samples than in control corn.

Two field applications of 10 or more Heliothis zea (Boddie) eggs at intervals of 0-10 days or a single application of at least 30 eggs produced damage to corn comparable to that produced by any method of larval infestation tested. Repeated applications were more important than number of eggs per application in producing results comparable to current larval infestation methods. Under low natural infestation, the percentage of infested cars was substantially increased by a single application of eggs, but was still less than the artificial larval infestation. Differences among hybirds in their response to levels of infestation were attributed largely to a greater percentage of escapes (ears that escaped damage) for certain hybrids, because the interaction between hybrids and infestation levels disappeared when infestation percentages among hybrids were uniform. The effect of timing or number of applications was quite constant among hybrids when the number of escapes was not a factor. Practical aspects of substituting egg-infestation for the conventional method of larval infestation are discussed.

Flights of Contarinia sorghicola (Coquillett) at Tifton, Georgia, were monitored with colored sticky traps on the perimeter of (1969) and coming out of (1970) a plot of sorghum. Yellow traps and white traps caught significantly more sorghum midges than orange traps or green traps. As distance from the infestation source increased, the number of sorghum midges trapped in the surrounding sorghum plots decreased significantly. Part of the population of sorghum midges did not move ill the direction of the surrounding sorghum plots, an indication of some drift during their flights after emergence.

Five commercial corn hybrids were evaluated for ear corn yield and damage by Sitophilus oryzae (L.). Hybrid plots were artificially infested by 3 methods: doghouses, birdhouses, and tepees. Paired rows within each plot were used for slit- and unslit-husk treatments. Differences in yield and damage among infestation method means were not significant at the 5% level. When husks were slit, hybrid differences indicated that factors other than husk tightness contributed to insect resistance. On the basis of cost, effort, and analyses, considering standard errors, means, and correlations, the birdhouse method of infesting and rating by degree damage per infested car was chosen as the best combination for producing artificial infestations and evaluating damage.

Corn earworm, Heliothis zea (Boddie), injury varied considerably from one planting date to another in any given location and season. Although differences among planting dates at Tifton, Georgia, were not completely overcome, erroneous indications of resistance in susceptible hybrids were averted by artificial infestation. Simple correlations of injury rating with solar radiation during the silking period and days to 50% silk were significant at the 1 and 5% levels, respectively. Standard partial regression coefficients indicated that radiation was many times more efficient than maturity in predicting injury. When mean squares from the analysis of injury ratings were adjusted for radiation, only planting-date variation was removed. Adjustment for maturity affected only the source of variation due to hybrids. The removal of environmental effects from injury readings through adjustment for solar radiation should help in the identification of genotypes which have earworm resistance.

Correlations obtained between the means for 36 inbred Lines over 2 years indicated that only husk characters and feeding stimulation (among 14 characters measured) prodded reliable enough information to use in identifying plant genotypes resistant to injury by the corn car-worm. Heliothis zea (Boddie). Lines with husk length in excess of that required to cover the car tended to have more injury, but the mechanical protection of tight husks reduced damage considerably. Identification of genotypes with resistance unrelated to husk characters appears possible through laboratory evaluations of the feeding stimulation caused by extracts of kernels. These results suggest that resistance is conditioned by both physical and chemical factors. Relationships between other characters and corn earworm injury fluctuated widely from year to year.

The magnitude of variance ratios obtained from analyses, when 3 corn earworm injury measurement scales (Centimeter, Survey, and Visual) were used on 5 sweet corn single crosses, indicated that under artificial infestation the Centimeter Scale based on depth of penetration gave the most precise measurements. Deviations from normal were found for the frequency distribution of inbred means obtained by a previously used measurement method (Injury Index). Tests for normality of the distribution of individual plants measured on the Centimeter Scale also indicated anormality. The anormalities occurred in both segregating and nonsegregating generations and appeared to be more closely associated with certain inbreds. Anormality of the Centimeter Scale was largely attributed to grouping in a single injury class all ears which had feeding on unfilled tips. A Revised Scale, which eliminates this discrepancy and generates frequency distributions in nonsegregating generations that are essentially normal, is proposed.

When several levels of aflatoxin B1 or G1 were fed to 2nd instars of the corn earworm, Heliothis zea (Boddie), the fall armyworm, Spodoptera frugiperda (J. E. Smith), and the European cornborer, Ostrinia nubilalis (Hubner), weight was reduced from 0 to 96% compared with untreated larvae. A dose of 2.5 µg of aflatoxin B1 or G1 in 10 ml of diet was significantly more toxic than other doses to the 2nd instar of all test insect species and also was toxic to 5th-instar corn earworms. Aflatoxin had no detectable effect on the viability of eggs produced by treated insects. Overall, corn earworms seemed more susceptible to aflatoxin than the other insect species; and aflatoxin B1 seemed more toxic than aflatoxin G1. Natural populations of corn earworms in cornfields may be significantly reduced by larval feeding on contaminated grain.

In tests comparing the effectiveness of 3 methods of infesting whorl-stage corn with Spodoptera frugiperda (J. E. Smith), the modified “Bazooka” proved to be the best method based on resultant visual damage ratings. The “Bazooka” is a device made primarily of Plexiglas, and larvae of fall armyworms can be mechanically dispensed into whorls of corn plants. It was easier and more efficient than using the camel-hair brush or pinning egg masses into the whorl. Infestation of every plant per plot produced higher leaf-damage ratings than infestation of either every 2nd or every 3rd plant/plot. Differences in leafdamage ratings were detected at infestation rates of 10, 20 and 40 larvae/plant. Rates of 20 or 40 larvae/plant should be used to insure adequate infestations at the 10–12 leaf stage.

Approximately 100 inbred corn, Zea Mays L., lines in replicated tests of four or more hybrid and reciprocal cross combinations were evaluated for grain weight loss due to feeding by Sitophilus zeamais Motschulsky. Results showed highly significant differences among inbreds within three different groups of lines, indicating substantial genetic variation for resistance to maize weevil feeding. Several of the single crosses performed at significantly different levels of resistance than their reciprocals. However, many of the best performing single crosses did not differ greatly from their reciprocals, thus indicating that resistance was being conferred to the cross from both inbreds. High-performance crosses, involving the inbreds of 91549 38A, L578, and L690, should be useful for introgressing resistance into commercial lines and for the development of composites that can be used as breeding populations in a recurrent selection program for improving resistance to feeding by the maize weevil.

Maysin (rhamnosyl-6-C-(4-ketofucosyl)-5,7,3′, 4′ tetrahydroxyflavone (I), isolated from corn silks of the exotic strain Zapalote Chico’ has been shown to severely retard the growth of Heliothis zea (Boddie) when incorporated into the insect's diet. Data indicate that the presence of this flavone glycoside may be related to the natural resistance of corn against corn earworm.

Evidence is presented that both nonpreference and antibiosis resistance to feeding are manifested in the silks of ‘Zapalote Chico’ (ZC) corn to Heliothis zea (Boddie). Fewer larvae survived on ZC silks than on silks of 471-U6 × 81-1 or ‘Stowell’s Evergreen’ (SEG). Larvae reared on ZC silks weighed significantly less than those reared on silks from 471-U6 × 81-1 or SEG. In two choice tests, after 4 days, significantly more larvae which had been placed on silks of ZC were found on silks of SEG (19 vs. 8] % and 7.5 vs. 92.5%). When larvae were placed on SEG silks for 4 days, significantly more larvae were found on SEG silks than on ZC silks (89 vs. 11 % and 92.5 vs. 7.5%). When CEW larvae were not placed on silks but were given a choice of SEG and ZC silks, significantly more larvae chose SEG silks (94 vs. 6%).

Four dent corn hybrids (3 in 1974 and one in 1975) were tested in split plot experiments that included arificial vs. natural infestation of ears with Heliothis zea (Boddie), inoculation of ears with Aspergillus flavus Link ex Fries vs. no inoculation, and ears sprayed with stirofos vs. nonsprayed ears in 1974; included inoculation vs. noninoculation and stirofos treated bags vs. nontreated bags in 1975. Aflatoxin B1 production and corn earworm injury were recorded. No evidence was found in either year for a significant relationship between corn earworm injury and the presence of aflatoxin B1. The insecticide regimen which was designed to totally control insects in both years did not eliminate insect damage nor did it effectively preclude A. flavus infection of the ears and subsequent aflatoxin production.

Resistant sweet corn hybrid 471-U6 ✕ 81–1 that was not treated with insecticide had 48% more ears free of damage from an artificial infestation (heavy population pressure) of Heliothis zea (Boddie), than a susceptible hybrid treated with 7 applications of Gardona® (2-chloro-l-(2, 4, 5-trichlorophenyl) vinyl dimethyl phosphate). When exposed to a natural field infestation of corn earworms, the susceptible hybrid treated with 7 applications of Gardona had only 8% more damage-free ears than the untreated resistant hybrid. When the resistant hybrid was treated with Gardona and exposed to a natural infestation of insects and to the artificial infestation, it had 15 and 4% more damage-free ears, respectively, than when untreated. The results indicated that resistant hybrids may require less insecticide than susceptible hybrids to achieve an equivalent high level of earworm damage control.

Methods of artificially infesting corn with eggs of Heliothis zea (Boddie) were evaluated by using 4 applicators, various rates of eggs per silk, 2 infestation dates (100% silk and 7 days later), and 2 times (AM or PM) on 4 different sweet-corn hybrids. Infestation with 30 eggs/silk yielded larvae adequate to obtain damage equal to 3 larvae/silk. A pressure applicator (handlotion dispenser) was judged superior to other applicators (chromatographic sprayer, hypodermic syringe, and squeeze bottle) because of its overall performance with respect to accuracy, ease in handling, and capacity to transfer the eggs suspended in agar with minimum egg damage.

Corn lines previously classified as resistant on the basis of car damage possess tolerance as another mechanism of resistance to Heliothis zea (Boddie). Resistant hybrids supported levels of infestation that seriously damaged susceptible hybrids.

Several methods of measuring resistance of corn kernels to Sitophilus zeamais Motschulsky were evaluated for their efficiency and adaptability. Among the 6 methods studied (grain weight loss, total progeny, percent of damaged kernels, percent of parent mortality, percent of progeny mortality, and weight per weevil), total progeny rated comparable to or better than any of the other methods for each criterion used for evaluation. Progeny mortality data are of questionable usefulness when weevil populations exceed 3 or 4 weevils per gram of grain. Grain weight loss, the best economic indicator, rated well for all criteria as a good resistance measurement, except that genetic differences among genotypes were difficult to detect for this method.

The husks of selected hybrids of corn, Zea. mays (L.), provided significant protection against the ravages of several insect species and contributed to increased yields. The corn earworm, Heliothis zea (Boddie), and the pink scavenger caterpillar, Sathrobrota rileyi (Walsingham), were more damaging than Sitophilus zeamais Motschulsky. Correlations among damage ratings indicated that pink scavenger caterpillar and corn earworm damage were more closely related than damage associated with either earworm or pink scavenger caterpillar in combination with S. zeamais . The corn earworm, pink scavenger caterpillar, and S. zeamais ranked in that order of economic importance in these tests. Both husks and/or kernels of some hybrids were resistant to insect damage.

Bagging ears of a dent corn hybrid in the field was effective in reducing damage by naturally occurring populations of Lepidoptera, but was not effective in reducing the production of aflatoxin on the ear. Insects produced conditions in the infested ear that were conducive to aflatoxin production and insect species differed in their relationship to increased aflatoxin production. Ears infested with European corn borer, Ostrinia nubilalis (Hubner), had the highest concentrations of aflatoxin B1.

Ears of corn, Zea mays L., hybrids infested with maize weevils, Sitophilus zeamais Motschulsky, previously exposed to spores of Aspergillus flavus (Link) had almost twice as much fungal infection as ears infested with unexposed weevils. Hybrids differed in amount of insect damage and fungal infection. Apparently, the maize weevil can contribute significantly to increased A. flavus infection on corn ears by transporting spores and damaging corn kernels.