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Ostrinia nubilalis (Hubner) (Lepidoptera: Crambidae) is one of the most damaging insect pests of corn. Studies were conducted to determine whether live larval counts obtained from corn whorls were predictive of the amount of larval tunneling that would result in the stalk of the plant 40 d later at the end of larval development. Whorls from plants treated with Dipel 10G (6,400 IU per whorl) and untreated controls, both infested with O. nubilalis neonates, were evaluated for the number of live larvae in 50 whorls 5, 7, 9, and 12 d after Bacillus thuringiensis (Berliner) (Bacillales: Bacillaceae) application. Forty days after larval infestation, 25 plants from each plot were split from tassel to base, and the length of larval tunneling was recorded. There was a strong relationship between numbers of live larvae in the plant whorl and the length of larval tunneling that resulted. While linear at each location, there was significant variation in the relationship among locations, indicating that comparisons could not be made between years or locations within a year. Blocks within a location on a given year did not vary significantly and reproducible results were obtained each year within a given location as well as on any of the whorl pulling dates evaluated. Because of its ease of use, predictability, and rapid return of results, we propose this technique as an additional method to shot-hole feeding and stalk splitting to evaluate the effectiveness of O. nubilalis management strategies.

To quantify yield losses of corn, Zea mays (L.), caused by larvae of the European corn borer, Ostrinia nubilalis (Hubner) (Lepidoptera: Pyralidae), a model describing the within-plant distribution of larvae is proposed and discussed. This paper is restricted to the interorgan and vertical distribution of larvae for pretasseling infestation. Field data collected at Versailles were used to study larval distribution and to estimate and test the model. The model proposed is based on four hypotheses deduced from field data: larval distribution in the tassel is influenced mainly by corn development; distribution in the ear of fourth and fifth instars depends on infestation date; distribution of first instars in the ear is conditional upon ear appearance; and vertical distribution of larvae in vegetative organs depends on corn development. For the 11 infestation conditions, the proposed mathematical model used to quantify the relations between larval distribution and main factors (corn development, infestation conditions) allows the percentages of larvae in ear, tassel, and internodes to be described with a precision of 10-15%.

Corn earworm, Helicoverpa zea (Boddie), male catch in pheromone traps was monitored in western Oregon processing sweet corn, Zea mays L., fields during 1986-1990. Peaks in catches of males and degree-day calculations indicate only two generations per year. Using nominal values for the crop, treatment costs, and estimated loss of $16.63/ha per 10% infestation, an economic injury level was estimated at 27.4% infested ears. Correlation and regression were used to identify variables for predicting percentage ears infested on a per-field basis in 55 untreated fields in 1986-1989. A linear regression of date of 5% silk and moth catch from first tassel to 5% silk explained 63% of variation in infestations. Additionally, a variable accounting for a change in pheromone trap and lure type and two indicator variables accounting for relatively earlier beginning and higher increases of infestations for 1988 improved the model significantly (adjusted R2 = 0.73). Because July moth counts were higher in 1988 than in other years, the model was formulated so the indicator variables are used when moth captures are > 1.5 per trap per night during the month of July. A validation using 30 fields monitored in 1990 showed good correlation with model predictions (R2 = 0.69, P 5%, the model overpredicfed damage by a mean of $5.63/ha in seven fields and underpredicted damage by a mean of $5.86/ha in two fields. Overall, the model tended to overpredict damage in 1990, which had very light infestations.

The ability of Orius insidiosus (Say) to search for, find, and destroy eggs of European corn borer, Ostrinia nubilalis (Hubner), and corn earworm, Helicoverpa zea (Boddie), was investigated in the field. Mobility of O. insidiosus on individual corn plants, attack rates on corn borer and corn earworm eggs, and searching capacity on corn plants were studied. Egg position on the corn plant and predator density affected egg mortality. Mortality rates rose with increased predator numbers. O. insidiosus searched in the corn silks first, followed by searching on the corn leaves but not in the tassel.

Oviposition by European corn borer, Ostrinia nubilalis Hübn, was examined in relation to sweet corn development from 1994 to 1996, and related to harvest infestation levels. Stepwise multiple regression and linear regression showed that 79-87% of the variability of larvae per ear or proportion of ears infested at harvest was explained by the number of egg masses laid from about anthesis to brown silk stages. The analyses indicated three periods of oviposition with differing implications to harvest infestation level: (1) eggs laid from 784-337 degree-days (DD) before harvest (before green tassel) had very low correlation to harvest infestation; (2) eggs laid from 336-169 DD before harvest (green tassel to green silk) were highly correlated with harvest infestation; and (3) eggs laid during the last 168 DD of sweet corn development (green silk to harvest) had low to moderate correlation with harvest infestation. The 336-169 DD period corresponded to the anthesis to brown silk growth stages, which was approximately 14-21 d long, and would be the likely period for optimum chemical control. The results of this study are compared with existing recommendations from the midwestern and northeastern U.S., and potential explanations for the patterns observed are discussed.

Plant damage and fresh-market yield lossesfrom feeding by fall armyworms, Spodoptera frugiperda (J. E. Smith), at five stages of plant development and five levels of larval infestation were determined for sweet corn ( Zea mays L.). Fall armyworms feed on leaves and on tissue developing within the whorls from the early-whorl to midwhorl stages of plant development. Tassel damage occurs primarily during the midwhorl to late-whorl stages of development and ceases after tassels begin to emerge and become exposed. Once tassels emerge, fall armyworms attack the developing ears, bore into the young ears, and cause fresh-market yield loss. Plants infested during the early-tassel stage of development have significantly fewer fresh-marketable ears at harvest compared with all other stages. Leaf-feeding damage, the percentage of damaged tassels, and the percentage of damaged ear shoots increase as the number of fall armyworm larvae per plant increases.

Location of eggs and larvae of Asian corn borer, Ostrinia furnacalis Guenee, on mid-whorl, late-whorl, tassel, silk, and mature stages of sweet corn was examined by dissecting plants. As the plant aged, larva feeding locations shifted from the whorl to the tassel and then to all parts of the plant depending on instar. First and second instars initially fed in the whorl, then the tassel, and finally in the ear. In the tassel they were concentrated within the florets, feeding on anthers. In the ear, first and second instars were on the silk and between shuck leaves. Older instars also fed in the whorl, then in the tassel, ear, and stalk. In the tassel, third instars were in the florets but older instars bored in the stem or branches or webbed groups of florets or branches together. In the ear they bored in the shank and cob or fed on silk or kernels. The stalk was the most common feeding site for last instars. Screening for host-plant resistance should concentrate on resistance in tassels and the ear.

Mechanically extracted maggots of Archytas marmoratus (Townsend) were evaluated against larva populations of Spodoptera frugiperda (J. E. Smith) and Heliothis zea (Boddie) in whorl and early tassel-stage sweet corn, Zea mays L., grown within 1.8-m3 screened cages. A. marmoratus maggots applied in an aqueous carrier at 5.4, 10.9, and 21.7 per plant against late-instar S. frugiperda reduced subsequent adult emergence by 40.2, 56.6, and 66.5%, respectively. A. marmoratus maggots applied at 5.5, 12.5, and 25.0 per plant against late-instar H. zea reduced subsequent adult emergence by 56.0, 56.9, and 81.9%, respectively. Maggots applied on the soil surface at rates to 25 per 100 cm2 against larvae of S. frugiperda in search of pupation sites had no significant effect on the subsequent emergence of adult moths. Likewise, volumes of aqueous carrier at rates of 0.05–1.25 ml per plant had no effect on the performance of A. marmoratus maggots against H. zea larvae in corn.

During a 2-year period, two popcorn hybrids (Iopop 12 and Indiana AES experimental hybrid 8365) were infested with European corn borer (ECB), Ostrinia nubilalis (Hubner), egg masses at five plant-growth stages (mid-whorl, late whorl, tassel emergence, anthesis, and 14 days after anthesis). Six levels of infestations were used (none, natural, 0.5, 1, 2, and 4 egg masses per plant). Considerable yield loss occurred in Iopop 12 from as few as 0.5 egg masses per plant at all plant-growth stages. Yield losses in hybrid 8365 were significant but less than in Iopop 12. Yield potential of Iopop 12 was greater than that of hybrid 8365, but potential yield losses and damage from ECB infestation were also greater. ECB infestation had no effect on popping expansion of either hybrid.

Relationships among stresses caused by nitrogen deficiency, high plant population levels, and western corn rootworm (WCR) ( Diabrotica virgifera virgifera LeConte) injury and their effects on phenological development and grain yield of corn ( Zea mays L.) were determined in a 2-yr field study. WCR infestation did not significantly affect silk development in 1984, although yields were 88.8 and 80.6% of those of the control at densities of 600 and 1,200 eggs per 30.5-cm row, respectively. Effects from root injury appeared to be compounded by moisture-stressed conditions in 1985, and asynchrony between tassel and silk development resulted in increased plant barrenness. Grain yields in 1985 were 80.2 and 55.6% of those of undamaged plants at the 600- and 1,200-egg infestation levels. Evidence from plant density by rootworm interaction on silking interval, barrenness, and yields suggests that the corn plant can tolerate a certain amount of root damage when plant densities are low. A significant nitrogen by rootworm interaction on grain yields provided evidence that root injury interferes with nitrogen uptake. The disruption of phenology of the injured corn plants appeared to be the result of decreased plant turgor resulting from WCR feeding.

A 4-Year study on the effect of infestation by the European corn borer ( Pyrausta nubilalis (Hba.)) on the growth of corn plants shows the following points: 1) Borer infestation adversely affects the elongation of the plants, results in smaller leaves, shorter internodes, and a delay in both tackling and silking. 2) Reduction in size of leaves increases progressively toward the tip of the plant. 3) The reduction in the length of internodes, however, is greater in internodes near the ear and tassel than in other internodes. 4) The decrease in the length of internodes began when only the leaf blades were injured by the borers, and without the physical destruction or obstruction of the vascular bundles in the stalk. 5) These facts indicate a need for investigations on (a) feeding activates of the young larvae and plant reaction to such feeding; (b) whether a phytotoxic secretion may be involved; and (c) the possibilities of interrelationships between micro-organisms and corn borer infestation.

Quantitative methods were developed for evaluating corn for corn leaf aphid, Rhopalosiphum maidis (Fitch), resistance. Aphids were confined in micro cages on the whorl leaves of 7-weeks-old corn inbreds grown in the greenhouse. Percentage of survival and rate of reproduction of the confined aphids were found to be reliable criteria for determining resistance among corn genotypes. Percentage of survival and rate of reproduction in the greenhouse studies and the intensity of tassel infestation found under field conditions of 1965 were all highly correlated. All inbreds grown in the greenhouse appeared to become more susceptible during maturation, with Illinois sweet corn inbred 465a showing the least natural infestation at maturity. In preference tests using detached leaves from 6-weeks-old plants, the aphids preferred susceptible Illinois sweet corn inbred 325a and the F1 hybrid over the resistant Illinois inbred 465a. In other studies, aphids were transferred from resistant inbred 465a to susceptible inbred 325 to determine if toxic materials that permanently interfere with reproduction were present in the resistant line; no toxic effects were observed.

Experiments to study time of insecticide application in relation to control of the European corn borer ( Pyrausta nubilalis (Hbn.)) were carried out at Ankeny, Iowa, in 1951-56. Two applications of DDT gave significantly better borer control than one application in all experiments. Each year the best control was obtained following applications when egg laying was almost completed. The minimum points on regression curves represented the estimated optimum treatment dates. Criteria used for predicting time of insecticide application, as they existed on the dates of optimum treatment, were compared with the recommended or suggested criteria. In 5 out of 6 years the number of borer-degree days on the date of optimum treatment was below the suggested 1100, averaging about 900. A tassel bud-plant height ratio of 35 appeared to be more suitable than the recommended range of 30 to 50. Except for 1 year plants in plots with the best borer control were consistently taller than the recommended 35 inches. Plant height and unhatched egg masses were found to be unreliable. Data on moth emergence were consistent with recommendations; however, this criterion is unsuitable for use by the average grower. The recommendation to treat when 75% of the plants show evidence of recent feeding in the whorls was the most rapid and simple to use.

Shoot and tassel bags used in hand pollination of field corn inbreds and single crosses were treated with insecticides to test their effectiveness in preventing damage by larvae of the corn earworm (Heliothis zea (Boddie)) and fall armyworm (Laphygma frugiperda (J. E. Smith)). Materials tested in descending order of effectiveness were: aldrin, heptachlor, dieldrin, chlordane, DDT, and toxaphene. Shoot bags coated on the inside with 1% concentrations of aldrin or heptachlor, or with their open ends dipped in, 5% concentrations prevented ear damage when left on the ears following hand pollination. The use of treated bags on garden sweet corn appeared to have little practical value.

Effectiveness of a nuclear polyhedrosis virus and Bacillus thuringiensis Berliner for control of the corn earworm, Heliothis zea (Boddie), was investigated on successive sweet corn plantings in southern California in 1965, 1966 and 1967. Effectiveness of the treatments was determined by using the classification ‘marketable ears’ (less than 25% of the car from the tip injured by larval feeding) as delined in the Agricultural Code of California in the sections pertaining to marketing standards for green corn. In 1965, 4 applications of a 2.5 larval unit (LU) virus dust formulation at 3-day intervals, using the individual-ear, brush-dust method, resulted in an average of 35.8% increase in marketable ears and 42% ears free of larval injury. The virus dust was more effective than a Blacillus dust or a combination of the, two when applied to the silk. In 1966. 4 applications of a virus spray to the silk at 3-day intervals at the rate of 228 LU per acre resulted in a significant increase (27%) in marketable ears and a lower number of live larvae present at harvest. Virus sprays applied to the tassels were ineffective in reducing the larval population thereon, and a tassel + silk treatment was not significantly better than a silk treatment alone for control of the larvae affecting the cars. The 228 LU virus Tate per acre was consistently better than 114 LU per acre. Similar applications of a virus spray at 194 LU per acre in 1967 resulted in a significant increase (24%) in marketable cars and a lower number of live larvae at harvest. Effectiveness of the virus treatment was not increased significantly by the addition of mineral oil, emulsifier, or wetting agent. From 36 to 70.6% of the larvae, collected from cars in the control plots and reared individually in the laboratory, were killed by virus transmitted by ovipositing adults, moving from treated to untreated silk. During the 3-year study, 77 to 87% marketable ears were obtained with virus sprays and 91 to 100% with virus dust applications.