Insect Injury Research Articles

176 Substrate on Apple Fruit Increased Sooty Blotch Incidence

The potential mechanism of susceptibility of apple (Malus ×domestica Borkh.) fruit to sooty blotch due to carbohydrate leachates was examined. Fruit received applications of 0.5% apple juice to the fruit cuticle in order to mimic leachates. Nine cultivars of apple were selected from a variety block of potentially disease resistant breeding stock. Intact fruit on selected limbs received 0.5% apple juice applications on eight evenly spaced occasions during development. Trees received a minimal fungicide program only. Leachate samples from the fruit cuticles were collected monthly by immersion of whole, detached fruit in dH2O for 60 min. At harvest, fruit were evaluated for skin roughness, fruit rots, sooty blotch, sooty mold, flyspeck, and insect injury. Despite drought conditions, sooty blotch and sooty mold showed strong increases in disease intensity ratings, while flyspeck and skin texture damage showed moderate increases in intensity due to the dilute juice applications. Fruit rots and insect damage were unaffected by the juice applications.

COMPARISON OF SOIL INSECTICIDE APPLICATION METHODS IN A CONSERVATION TILLAGE SYSTEM FOR CONTROLLING CORN WIREWORMS ON FIELD CORN, 1999

COMPARISON OF SOIL INSECTICIDE APPLICATION METHODS IN A CONSERVATION TILLAGE SYSTEM FOR CONTROLLING CORN WIREWORMS ON FIELD CORN, 1999: This study was conducted at the Mountain Branch Experiment Station, Blairsville, GA, to evaluate the efficacy of various methods of applying soil insecticides for the control of corn wireworms. Each treatment was applied to the two middle rows of 25-ft long, 4-row plots. A RCB design with four replications was used. One untreated check was included in each replication. Seed was planted on 19 May using a four row Cole no-till planter. Granular insecticides were applied at planting using three methods: a 7-inch band application, a 7-inch T-band application, and precision placement of insecticide onto the planted hill using a jab planter for seed placement and a bazooka for insecticide application. All insecticide rates are specified as lb (AI)/acre based on 38-inch rows. On 4 Jun, 20 plants were selected at random in untreated rows and examined for insect injury and the presence of larvae. All plants had wireworm injury and 7 plants had late instar corn wireworm larvae present in the soil nearby. On 8 Jun, 26 Jun, and 19 Jul, wireworm plant damage ratings were assessed for wireworm damage by surveying all of the plants in the two middle rows of each plot for feeding holes, tillered plants, and severely stunted plants. The total number of damaged and undamaged plants was recorded. The middle two rows of each plot were mechanically harvested on 26 Oct using a Hage small plot one row combine. Whole kernel corn was weighed in lb in the field to determine yield.

Insect Resistance in Sweetpotato Plant Introductions

One hundred U.S. sweetpotato [Ipomoea batatus (L.) Lam.] plant introductions (PIs) and four control cultivars were screened for insect injury in 1993. Of the least injured by insects, 56 and 31 were tested again in 1994 and 1995, respectively. Among control cultivars, the most highly resistant was `Regal' (moderately resistant), followed by `Beauregard' (susceptible), `Centennial' (susceptible), and `Jewel' (susceptible). Stem and root injury by the sweetpotato weevil (SPW) [Cylas formicarius elegantulus (Summers)] and root injury by the wireworm (Conoderus sp.)–Diabrotica sp. (cucumber beetle)– Systena sp. (flea beetle) (WDS) complex were measured. SPW stem injury was less severe (P ≤ 0.05) in 1994 and 1995 in PIs 508523, 531116, and 564107 than in control cultivars. PIs 508523 and 531116 also suffered less SPW root injury than did `Regal'. In the six PIs with least SPW root injury, PIs 538354, 564149, 508523, 538286, 531116, and 564103, 70% to 85% of the roots were not injured compared with 36% in `Regal' and 6% in `Jewel'. SPW root injury scores (0 = no injury; 5 = severe injury) in those PIs averaged 0.5 vs. 2.3 for `Regal'. Only in PI 538286 was WDS injury to roots less than in `Regal' over 2 years. However, eight additional accessions suffered less WDS injury than `Regal' in 1995 and four of those were among the six with least SPW injury. The lower levels of combined insect injury found in these four PIs (compared to `Regal') show that PIs have potential use for increasing insect resistance in sweetpotato improvement programs.

Feeding Injury, Oviposition, and Nymphal Survivorship of the Potato Leafhopper on Red Maple and Freeman Maple Clones

Feeding injury, oviposition, and nymphal survivorship of the potato leafhopper, Empoasca fabae (Harris), were measured among 5 red maple, Acer rubrum L., clones and 2 Freeman maple, A. × freemanii E. Murray, cultivars. Symptoms of feeding injury among the 5 red maple clones varied in severity among trees in the field. Red maple clone 56026 was the most susceptible, and red maple clones 57775 and 55410 were the most tolerant. The Freeman cultivars were most tolerant to feeding, with ‘Jeffersred’ showing no sign of insect injury. Oviposition was significantly higher on the susceptible clone 56026, but significantly lower on Jeffersred than on the other clones. Fewer nymphs became adults when reared on ‘Indian Summer’ than when reared on the other clones. Although the data showed that red maple is more susceptible to feeding injury by the potato leafhopper than Freeman maple, no clear-cut relationship exists between susceptibility to feeding injury and host acceptability for oviposition or host suitability for nymphal development. The differences among the maple clones tested in this study suggest that red maple and Freeman maple may have different mechanisms of resistance against the potato leafhopper.

Insect injury to linseed in south-east England

Pests were monitored on linseed crops in Hertfordshire, Cambridgeshire and Bedfordshire from 1987 to 1990. Adult flea beetles ( Aphthona euphorbias and Longitarsus parvulus) caused severe injury to seedlings; their numbers have markedly increased since 1990 and they are now important pests of linseed in the UK. Two generations of Thrips angusticeps bred on linseed, causing damage to growing points and flower buds and approximately 14% yield loss to a crop in Hampshire. Mirids, mostly Calocoris norvegiens and Lygus rugulipennis, caused injury similar to that caused by thrips. Mirids were more abundant close to field boundaries with woody plants, whereas thrips were fewer at these boundaries and rather evenly distributed within the rest of the field. Few other insects and no aphid colonies were found. Experiments with insecticide treatments provided evidence of compensation for insect injury by additional flower production.

A characterisation of rice pests and quantification of yield losses in the rice-wheat system of India

The relationship between rice cropping practices, biotic constraints, and rice yield levels in the eastern part of Uttar Pradesh, India was studied. The investigation was conducted at the centre of a geographic area representative of the rice-wheat system of South Asia, and covers a large diversity of production situations undergoing intensification. A survey procedure was used to collect data for 3 consecutive years (1993–1995) in 251 individual farmers' fields. Two analytical approaches were used, the emphasis shifting from yield determining variables, that are mostly qualitative in nature, to quantitative, and predominantly yield-reducing, variables. The first approach was intended to characterize relationships among variables using cluster and correspondence analyses, while the second approach was aimed at generating yield loss estimates using combinations of principal components and step-wise multiple regressions. Seven patterns of cropping practices were distinguished, reflecting a wide variation in production situations, especially in terms of use of fertilizers and manure, and degree of water control. Six types of disease profiles, four insect injury profiles, and four weed infestation patterns were identified. Correspondence analysis, based on patterns of cropping practices and injury profiles, yielded a path of increasing attainable yield associated with varying levels of intensification and combinations of injuries. The use of principal component analysis with multiple regression generated estimates of yield reductions due to rice diseases, insects and weeds. The analysis highlighted the effects of changes in patterns of cropping practices on injuries caused by weeds, brown spot, sheath blight and dead hearts. The significance of these findings with respect to current and future IPM needs is discussed.

Benefit of Chemical and Cultural Control in Managing Pumpkin Viruses and Vectors

Pumpkins rank third in acreage among Ohio's fresh market vegetables. Recently, increasing incidence of insect-vectored viruses have threatened the economic potential of this crop. Studies were initiated in 1992 to obtain information on abundance of insect pests, to evaluate the effect of insecticides, reflective mulch and row covers on pest density and yield of marketable fruit. In 1994 and 1995, statewide virus surveys were conducted to determine virus types infecting the pumpkin crop. In 1992 the greatest insect injury of concern was cucumber beetle feeding on fruit rinds, Insecticides lowered pest density, but there was no effect on marketable yield. In 1993, yield of virus-infected fruit was lower were reflective mulch was used than where rowcovers or foliar insecticides were used. Marketable yield and virus incidence in 1994 were not affected by reflective mulch, rowcovers or stylet oil due to the late arrival of the virus. In 1995, aphid infestations at three locations were significantly lower on plants on reflective mulch than on plants on bare ground. Results of the 1994 virus survey showed that watermelon mosaic virus (WMV) was the most common virus in Ohio pumpkins. The watermelon mosaic developed late in the season and fruit deformity was not severe enough to affect marketable yield. There was some incidence of cucumber mosaic and squash mosaic virus but zucchini yellow mosaic virus was not detected in 1994. Results of ELISA testing of samples collected in August/September 1995 at 27 farms were 18 farms positive for watermelon mosaic virus, 5 sites positive for squash mosaic, 4 sites positive for papaya ringspot and 1 site positive for zucchini yellow mosaic.

Grape, Season-Long Broad-Spectrum Control, 1994

Abstract Insecticides were applied to mature 14 year old grape vines at the Trevor Nichols Research Complex near Fennville, MI at a rate of 50 gpa using a FMC 1029 airblast sprayer. Treatments were arranged in a completely randomized design of single 50-foot-long rows of vines replicated 4 times. Insecticides were applied, as indicated in the table, on the following dates: 4 May (BS), 25 May (1C), 9 Jun (Bloom), 22 Jun (3C), 3 Jul (4C), 17 Jul (5C), 29 Jul (6C), 12 Aug (7C), 25 Aug (8C) and 6 Sep (9C). Second generation grape berry moth application was timed for one wk after sustained catch in pheromone traps. Phytotoxicity damage was rated on the untreated control and the Danitol plots on 22 Jul. Twenty-five leaf samples were rated as clean or damaged. A leaf was rated as damaged if it had spots or blemishes that could not be attributed to disease or insect injury. All damaged leaves had ≤25% of their surface injured. Damage evaluations for grape berry moth, and Japanese beetle were performed on 27 Sep. Damage was assessed by sampling 25 fruit clusters from each replicate. Each cluster was rated for the presence or absence of injury from each of the three insect pests.

Second-level integrated pest management in commercial apple orchards

AbstractAs historical background helpful to understanding current concepts and practices of apple pest management, we review the origin and rise of key pests of apple in North America and the evolution of approaches to their management, culminating with the concept of integrated pest management (IPM). We propose four levels of integration of orchard pest management practices. First-level IPM integrates chemically based and biologically based management tactics for a single class of pests, such as arthropods, diseases, weeds or vertebrates. Second-level IPM, the focus of our effort here, integrates multiple management tactics across all classes of pests. We describe components of second-level IPM for Massachusetts apple orchards, which are threatened each year by an exceptionally broad range of injurious pests. We illustrate the tentative advantages and shortcomings of second-level IPM using 1993 data from six commercial orchard test blocks. Our predominant approach was to use chemically based tactics for controlling arthropods, diseases and weeds early in the growing season, and afterwards to rely exclusively (for insects) or largely (for other pests) on biologically based tactics, such as cultural, behavioral, and biological controls. Compared with nearby first-level IPM blocks, insecticide use in 1993 was reduced substantially (about 30%), with only slightly more insect injury to fruit and little difference in populations of foliar insect pests. The results for mite pests and diseases were less encouraging although summer pruning significantly reduced disease injury caused by flyspeck. We discuss how second-level IPM poses special biological or operational challenges to apple pest management practitioners. The concept has merit, but refinements are necessary before it can be recommended broadly to commercial apple growers in Massachusetts as an economical and reliable alternative to first-level IPM.

WINTER COVER CROPS AND NITROGEN FERTILIZER EFFECTS ON TOMATO AND BEAN PRODUCTION: A THREE YEAR SUMMARY

Winter cover crops (wheat or rye and crimson clover) in combination with three levels of nitrogen fertilizer (0, 60, 120 kg/ha) were evaluated as to their influence on bean and tomato production (fruit yield, disease and insect injury on fruit) over a three year period (1991-1993). A split plot design was used with the cover crop as the main treatment and nitrogen rate as the split treatment. Results indicate that total marketable and cull yields for bean increased significantly in 1992 but decreased again in 1993. Tomato yields were significantly greater in 1991 than in 1992 and 1993 for both early and total marketable yields while early cull yield increased each year and total cull yield was highest in 1993. Cover crop had an effect on non-marketable tomato fruit. There was a higher incidence of cracked and insect damaged tomato fruit in association with clover or fallow treatment. Marketable yields responded in a quadratic manner while the number of cull fruit increased linearly for both bean and tomato as nitrogen rate increased. The incidence of diseased bean pods increased linearly as nitrogen rate increased. Catfaced tomato fruit responded in a quadratic manner and cracked tomato fruit increased linearly as nitrogen increased. Results from the three year evaluation do not indicate an influence of cover crop on marketable yields of bean and tomato.

118 ACTUAL AND SIMULATED MITE-FEEDING EFFECTS ON TART CHERRY PHOTOSYNTHESIS IN THE FIELD

European red mite (Panonychus ulmi) populations were monitored in a tart cherry (Prunus cerasus `Montmorency') orchard and the effects on photosynthesis determined. Mites levels were controlled in some trees by miticide applications to establish different cumulative mite*days in the trees. Photosynthetic inhibition caused by insect injury was also simulated by spraying other trees with 78 ppm Terbacil at one of four different times during the season, The mite*days accumulated in 1993 ranged from 937 to 2205, however, there were no differences in single leaf or whole tree CO2 assimilation, chlorophyll a fluorescence, or chlorophyll levels among the different levels of mite damage. Likewise, there were no differences in these same parameters among the Terbacil-treated trees except that photosynthesis was reduced on treated trees for 10-14 days, after which photosynthesis recovered to the level of the controls. There were no differences in yield or fruit quality among any treatments, and cold hardiness and return fruiting characteristics will be measured.

Interactions Among the Soybean Looper (Lepidoptera: Noctuidae), Threecornered Alfalfa Hopper (Homoptera Membracidae), Stem Canker, and Red Crown Rot in Soybean

Field experiments in 1988–1990 determined the (1) effects of defoliation by Pseudoplusia includens (Walker) (soybean looper) or basal stem girdling by Spissistilus festinus (Say) (threecornered alfalfa hopper) on soybean stem canker severity and (2) effects of the soybean looper defoliation on red crown rot incidence. Compared to ungirdled plants, stem canker severity was increased while stem length, stem diameter, and yield were reduced for girdled plants. Stem canker severity, number of perithecia, yield, stem length, and stem diameter were reduced on defoliated soybean, compared to undefoliated soybean. Red crown rot incidence, perithecial production, and yields were reduced on defoliated soybean compared to undefoliated soybean. This suggests different types of insect injury may either increase or decrease the severity of soybean diseases.

Cotton Yield Response to Cotton Fleahopper (Hemiptera: Miridae) Infestations on the Lower Gulf Coast of Texas

Insect density-crop yield response functions were developed for the cotton fleahopper, Pseudatomoscelis seriatus (Reuter), attacking cotton, Gossypium hirsutum L., on the Lower Gulf Coast of Texas. Yield response to insect injury (yield with injury divided by yield without injury or yield without insecticide divided by yield with insecticide) was regressed on insect density. Densities of insects were manipulated by controlling plant exposure to cotton fleahopper using field cages or treating indigenous cotton fleahopper infestations with insecticides. Density-yield regression equations were developed for seven cultivars of cotton during the phenological period of plant development when cotton is susceptible to cotton fleahopper injury; i.e., from fifth true-leaf through first week after initiation of anthesis (flowering). Significant differences were observed among cultivars in response of yield to cotton fleahopper densities. These differences were influenced by trichome density and other factors. ‘TAMCOT SP 37H’, one of the most hirsute cottons, was very tolerant of high cotton fleahopper densities. ‘Stoneville 213’ was intermediate in trichome density and in response to cotton fleahopper densities, whereas, ‘TAM COT CAB-CS’, ‘TAM COT CAMD-E’, ‘TAMCOT SP21’, ‘TAMCOT SP2IS’, and ‘TAMCOT SP37’ were very susceptible to cotton fleahopper densities. ‘TAMCOT SP37’ was the most hirsute cultivar, ‘TAMCOT CAMD-E’ was intermediate in trichome density, and ‘TAMCOT CAB-CS’, ‘TAMCOT SP21’, and ‘TAMCOT SP 21S’ were glabrate. Methods are presented to calculate distinct economic injury levels for varying combinations of management costs and market values using the density-yield regression equations for each cultivar.

Comparison of Insect Injury-Cotton Yield Response Functions and Economic Injury Levels for Helicoverpa zea and Heliothis virescens (Lepidoptera: Noctuidae) in the Lower Gulf Coast of Texas

Insect injury-crop yield response functions (i.e., regression equations) and economic injury levels (ElLs) were developed and compared for the bollworm, Helicoverpa zea (Boddie), and the tobacco budworm, Heliothis virescens (F.), attacking cotton, Gossypium hirsutum (L.). Plant injury was sampled 7 d after plants were infested with neonates. Cotton cultivars used were ‘Deltapine 50’ and ‘TOMCAT CD3H’ at growth stages of one-third-grown flower bud (square), second week after initiation of anthesis (flowering), and 30 d after initiation of anthesis. Injury-yield regression equations were significantly different between insect species and cotton cultivars and among growth stages and combinations of cultivar, insect species, and growth stage. ElLs ranged from 4 to 9% and 3 to 7% injured reproductive organs (i.e., flower buds and capsules [bolls]) at one-third- grown flower bud and second week after initiation of anthesis, respectively. Yield was not influenced by insect injury when cotton was infested 30 d after initiation of anthesis. Differences in ElLs between insect species ranged from 0 to 1 and 3 to 5% injured reproductive organs on ‘Deltapine 50’ and ‘TOMCAT CD3H’, respectively. ElLs as determined from analysis of variance (ANOVA) ranged from 12 to 32% injured reproductive organs. Regression analysis proved to be superior to ANOVA in calculating ElLs that vary with market values and management costs.

Economic Injury Levels and Development and Application of Response Surfaces Relating Insect Injury, Normalized Yield, and Plant Physiological Age

Normalized yield (i.e., yield with injury divided by yield without injury) of cotton, Gossypium hirsutum L., was observed for various levels of plant injury by Helicoverpa zea (Boddie) or Heliothis virescens (F.) at different plant physiological ages (i.e., stages of plant growth). These data were then used to develop functions describing response surfaces composed of insect injury, cotton yield, and plant physiological age. Normalized yield can be predicted from response surface functions when injury and plant physiological age are known. Plant physiological age was either degree-days, or an index of plant development determined using a mechanistic, physiological cotton plant simulation model. The generalized function (i.e., regression equation) describing response surfaces was In Y = b1 ln X + b2 X + b3 X 2 + b4 X 3 + b5 T + b6 T 2 + b7 T 3 + b8 XT, where Y = normalized yield, X = injured reproductive organs, and T = plant physiological age (i.e., as degree-days or plant physiological age index). Regressions were highly significant and fit the data well based on coefficients of multiple correlation, mean square errors, Mallows' Cps , predicted residual error sum of squares, and slopes significantly different from zero for parameters of independent variables. The yield response of cotton to injury changed continuously over physiological age. One response surface function quantified an infinite set of two-dimensional insect injury–plant yield response curves. Different sections of the generalized insect injury–plant yield response curve were observed at different plant physiological ages. Cotton yield was increasingly sensitive to injury from one-third-grown flower bud to first week after anthesis, was decreasingly sensitive to injury from first week after anthesis until 30 d after anthesis, and was unaffected by injury of neonates at 30 d after anthesis. Response surface regression functions were significantly different among cultivar–insect species combinations within degree-days or the plant physiological age index, indicating that cotton responded differently to bollworm and budworm injury and cotton cultivars responded differently to injury. For a given set of economic costs and returns, the economic injury level (ElL) is not static nor a straight line, but is dynamic, varying with plant physiological age and forming a complex continuous curve. Overcompensation increased EILs, steep descents in the response surface resulted in low EILs, and tolerance or inherent impunity rapidly increased EILs.

ROWCOVERS AFFECT INSECT INJURY TO STRAWBERRIES

Rowcovers applied to strawberries have documented value for increased earliness and yield. The effect of rowcovers on insect damage to strawberries was investigated in this study. Nonwoven rowcovers were applied over strawberries in the fall with and without malathion to determine their effect on tarnished plant bug (Lygus lineolaris) and strawberry bud weevil (Anthomonus signatus) injury over two harvest seasons. Rowcovers increased the “umber and weight of marketable fruit. Tarnished plant bug injury was reduced by the use of rowcovers in 1990, regardless of insecticide application. I” 1991, rowcovers reduced tarnished plant bug injury only when a fall insecticide was applied. Rowcovers increased the number of flower buds killed by the strawberry bud weevil where no insecticide was used in 1990, but had no significant effect on the number of buds killed in 1991. The effect of rowcovers on insect injury to strawberries appears to depend upon the overwintering habits of the insects, and the prevailing weather patterns during a given season.

Apple, Seasonal Insecticide Evaluations, 1990

Abstract Experimental insecticide treatments were applied to 15-year-old ‘York’ and 13-year-old ‘Redspur Delicious’ trees. Originally the test was established in the ‘Redspur Delicious’ orchard, but was moved to the ‘York’ trees following bloom because of poor fruit set in the ‘Delicious’ planting. A randomized block design was used and each treatment was replicated 4 times. Trees were sprayed to runoff using a Bean hydraulic sprayer operating at 400 psi. Treatments were applied on 16 Mar, 2 Apr, 16, 31 May, 13, 26 Jun, 12 Jul, and 2 Aug. Mite populations were evaluated by randomly selecting 20 leaves per replicate (10 on 7 Jun), brushing onto a sticky glass plate and counting all motile mites and viable mite eggs with a binocular microscope. Predators of mites were evaluated by a 3 minute observation period per tree. RAA colonies per tree were counted on ‘Redspur Delicious’ trees during a 2-minute visual observation period per tree. Ground cover was closely mowed and all drop apples collected were examined for insect injury weekly from mid Aug to harvest. This drop fruit was graded for insect damage and the first 2 wk ‘drops’ were graded for fruit finish. Drop fruit finish was graded on a scale of 1-5 with 1 being the best.

Soybean Yield Responses and Intraspecific Competition from Simulated Seedcorn Maggot Injury

AbstractThe reproductive and competitive responses of plants to early season stresses are not well known. In field experiments from 1983 to 1986 we examined these effects for one system; simulated seedcorn maggot (SCM), Delia platura (Diptera: Anthomyiidae) injury to soybean, Glycine max (L.) Merr. Injury included different plant densities (simulating stand reductions) and different ratios of plumule‐injured to uninjured plants (simulating different levels of plumule destruction). Our objectives were to describe (i) how simulated SCM injury affected total yield, yield components, and intraspecific competition between injured and uninjured plants, and (ii) to determine the value of the replacement series experimental design for examining intraspecific competition from injury. Plant density did not influence plot yields but did affect yield components of injured and uninjured plants. The proportion of injured to uninjured plants influenced plot yields in 1983 and 1984 but not in 1985 or 1986. Uninjured soybean were much stronger competitors than injured soybean. Yield reductions of injured plants were attributable to competition from uninjured plants. Specifically, competition seemed to increase shading and reduce assimilate availability of injured plants. The replacement series design was a powerful technique for describing intraspecific competition arising from insect injury.

Control of Colorado Potato Beetle with Natural Product Insecticides, Grand Forks, 1990

Abstract Natural product insecticides used in this experiment were Bacillus thuringiensis var. san diego (MYX 1806), B. thuringiensis var. tenebrionis (Trident and BK-102) and azadirachtin (Margosan-O) from the neem tree, Azadirachta indica. Plots were planted 15 May at the Research Farm, Red River Valley Potato Growers Association, Grand Forks, N.D. Plots were 6 rows wide, 16.8 (55 ft) long, with 91 cm (36 inches) spacing between rows and 30 cm (12 inches) between plants. The two center rows of each plot were planted with certified 'Red Pontiac' seed pieces, the remaining four rows were planted with certified, B-size, 'Russet Burbank.' Plots were separated on all sides by 1.8 m (6 ft) of cultivated soil. A completely randomized plot design with 4 replications was used. Plots were scouted daily from CPB egg masses beginning 21 Jun. When egg masses were found these were flagged and checked daily for hatch. Timing of first spray application was based on egg hatch. Three basic spray schedules were used: first application at 30% hatch of flagged egg masses and subsequent application on either a 7 d or 10 d schedule, or first application at 30% hatch of all eggs (unflagged) laid on or before that sampling date and subsequent applications on a 7 d schedule. Plots were sprayed 1-3 times for each generation of CPB larvae: 29 Jun (30% hatch of flagged eggs—1st generation) 2 Jul (30% total hatch—1st generation), 5,9, 12, 16, 19 and 31 Jul (30% hatch of flagged eggs—2nd generation), 6 Aug (30% total hatch—2nd generation), 10, 13 and 14 Aug. Insecticidal sprays were applied using a tractor-mounted boom sprayer, with a single Teejet nozzle over each row. Sprays were applied in 150 liters water/ha (40 gal/ acre) at 275 kPa (40 psi). All data were collected from the two center rows of each plot. CPB larvae (small = 1st and 2nd instar, large = 3rd and 4th instar) were counted on 20 randomly selected whole plants per plot, PLH nymphs were counted on 35 mid-plant leaves per plot. Leaf loss to defoliation (by CPB) and hopperbum (injury caused by PLH) were estimated visually (by 2 individuals per sampling date) as percent foliage lost. CPB were counted on 4, 10 and 17 Jul, and 1, 8 and 17 Aug; defoliation was estimated on 10, 17 and 24 Jul, 1, 8 and 15 Aug; PLH were counted on 4, 10, 17 and 24 Jul, 1, 8 and 17 Aug; hopperbum was estimated on 1, 8 and 15 Aug. Insect data and plant injury ratings were expressed as mean insect numbers or plant injury × number of days elapsed (= Area under Curve). AUC for CPB was calculated separately for 1st and 2nd generations, and then summed. Yields were obtained 2 Oct by harvesting one center row from each plot. Two way analysis of variance was used to test for treatment differences in pest densities, plant injury and yield. The 1990 growing season at Grand Forks was characterized by severe drought from late Jun through harvest.

Soybean Growth Responses and Intraspecific Competition from Simulated Seedcorn Maggot Injury

AbstractThe consequences of early‐season insect injury on subsequent plant development often are obscure. Further, such injury may produce intraspecific competition between injured and uninjured plants. We examined these issues by studying the impact of simulated seedcorn maggot (SCM) [Delia platura (Meigen) (Diptera: Anthomyiidae)], injury to soybean [Glycine max (L.) Merr.], in field studies from 1983 to 1986 in Iowa. Simulated SCM injury includes plant density (simulating stand reductions) and normal to Y‐plant ratios (simulating various proportions of plumule injury). The Y‐plants are plants that have had the plumule destroyed and subsequently develop two main stems. Our objectives were to describe how simulated SCM injury affected: (i) development of soybean stands, individual normal plants, and individual Y‐plants, and (ii) intraspecific competition between normal and Y‐plants. Higher crop growth rates (CGRs) and leaf area indices (LAIs) were associated with higher plant densities. Generally, plumule‐injured plants were more affected by plant density than uninjured plants. Examinations of competition between uninjured and plumule‐injured plants indicated that uninjured plants were competitively superior to plumule‐injured plants. Competitive interactions were influenced by plant density, Phenological delay associated with plumule injury contributed to height differences between uninjured and injured plants, which led to substantial differences in leaf area, number of nodes, and other developmental parameters between injured and uninjured plants. Consequently, unequal competitive ability between uninjured and injured plants was identified as a major consequence of plumule injury.