Megha N. Parajulee

Effects of elevated CO2 and transgenic Bt cotton on plant chemistry, performance, and feeding of an insect herbivore, the cotton bollworm

Effects of elevated atmospheric CO2 (double‐ambient CO2) on the growth and metabolism of cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), fed on transgenic Bacillus thuringiensis (Berliner) (Bt) cotton [Cry1A(c)], grown in open‐top chambers, were studied. Two levels of CO2 (ambient and double‐ambient) and two cotton cultivars (non‐transgenic Simian‐3 and transgenic GK‐12) were deployed in a completely randomized design with four treatment combinations, and the cotton bollworm was reared on each treatment simultaneously. Plants of both cotton cultivars had lower nitrogen and higher total non‐structural carbohydrates (TNC), TNC:Nitrogen ratio, condensed tannin, and gossypol under elevated CO2. Elevated CO2 further resulted in a significant decrease in Bt toxin level in GK‐12. The changes in chemical components in the host plants due to increased CO2 significantly affected the growth parameters of H. armigera. Both transgenic Bt cotton and elevated CO2 resulted in a reduced body mass, lower fecundity, decreased relative growth rate (RGR), and decreased mean relative growth rate in the bollworms. Larval life‐span was significantly longer for H. armigera fed transgenic Bt cotton. Significantly reduced larval, pupal, and adult moth weights were observed in the bollworms fed elevated CO2‐grown transgenic Bt cotton compared with those of bollworms reared on non‐transgenic cotton, regardless of the CO2 level. The efficiency of conversion of ingested food and of digested food of the bollworm were significantly reduced when fed transgenic Bt cotton, but there was no significant CO2 or CO2× cotton cultivar interaction. Approximate digestibility of larvae reared on transgenic cotton grown in elevated CO2 was higher compared to that of larvae fed non‐transgenic cotton grown at ambient CO2. The damage inflicted by cotton bollworm on cotton, regardless of the presence or absence of insecticidal genes, is predicted to be more serious under elevated CO2 conditions because of individual compensatory feeding on host plants caused by nitrogen deficiency.

Impact of Elevated CO2 on Tri-Trophic Interaction of Gossypium hirsutum, Aphis gossypii, and Leis axyridis

Abstract Increased levels of atmospheric carbon dioxide (CO2) are likely to affect the trophic relationships that exist between plants, their herbivores, and the herbivores’ natural enemies. We examined interactions across three trophic levels, a transgenic Bacillus thuringiensis Berliner cotton, an aphid herbivore (Aphis gossypii Glover), and a coccinellid predator (Leis axyridis Pallas) as affected by elevated CO2 concentrations. Experimental CO2 levels included ambient and 2× and 3× ambient CO2 levels. Plant height, biomass, leaf area, and carbon:nitrogen (C:N) ratios were significantly higher in plants exposed to elevated CO2 levels compared with that in plants exposed to ambient CO2. More dry matter and fat content and less soluble protein were found in A. gossypii in elevated CO2 treatments. Cotton aphid fecundity significantly increased with increased CO2 concentrations. The aphid fecundity significantly increased through successive generations reared on plants grown under elevated CO2, suggesting that the changes in plant quality resulting from elevated CO2 altered aphid quality as prey for L. axyridis through the food chain. Significantly higher mean relative growth rates were observed in lady beetle larvae under elevated CO2 treatments. Also, the larval and pupal durations of the lady beetle were significantly shorter and aphid consumption rates increased when fed A. gossypii from elevated CO2 treatments. Our data showed that increasing CO2 concentrations could alter the preference of lady beetle to aphid prey and enhance the biological control of aphids by lady beetle. This study provides the first empirical evidence that changes in prey reared on host plants grown at different levels of CO2 can alter the feeding preference of their natural enemies.

Evaluation of the Functional Response of Selected Arthropod Predators on Bollworm Eggs in the Laboratory and Effect of Temperature on Their Predation Efficiency

Abstract A functional response study of the eight most common arthropod predators of cotton bollworm, Helicoverpa zea (Boddie), eggs was conducted in the laboratory. Predators were starved for 24 h, and single predators were exposed to different prey density treatments. Predation response was observed at 6, 12, and 24 h after feeding trials began. At the highest prey density (150 eggs per predator), Hippodamia convergens Guerin-Meneville and Collops quadrimaculatus (F.) adults and Chrysopa oculata Say larvae showed the highest consumption rates (116, 85, and 119 eggs/24 h, respectively), followed by H. convergens larvae (51 eggs/24 h), adult Geocoris punctipes (Say) (45 eggs/24 h), and adult Scymnus loewii Mulsant, Orius insidiosus (Say), Notoxus spp., and Nabis capsiformis Germar (1, 1, 10, and 12 eggs/24 h, respectively). Adult Notoxus spp., N. capsiformis, and O. insidiosus showed type 1 functional response, whereas adult C. quadrimaculatus, G. punctipes, H. convergens, and larval H. convergens and C. oculata showed type 2 response. All predators consumed the highest number of bollworm eggs at 35°C and the lowest numbers at 15°C; predation rate at 35°C was four times higher than that at 15°C. The prey density–dependent behavior of predators and effect of temperature on their predation behavior are discussed.

Long-term impacts of elevated carbon dioxide and transgenic Bt cotton on performance and feeding of three generations of cotton bollworm

The effects of elevated carbon dioxide (CO2) on growth, metabolism, and performance of three generations of cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), were studied. The insects were continuously fed transgenic Bacillus thuringiensis (Berliner) (Bt) cotton [Cry1A(c)] grown in open‐top chambers. Two levels of CO2 (ambient and 2× ambient) and two cotton cultivars (non‐transgenic Simian‐3 and transgenic Bt GK‐12) were used and bollworm larvae were reared on all four treatment combinations for three generations. CO2 level and cotton variety significantly affected the growth and food digestibility parameters of H. armigera, with the exception of larval consumption rate for cotton variety. Overall, elevated CO2 and transgenic Bt cotton both increased larval lifespan, food consumption rate, relative consumption rate, and approximate digestibility, while decreasing pupal weight, survival rate, fecundity, frass output, relative and mean relative growth rates (RGR/MRGR), and the efficiency of conversion of ingested and digested food (ECI/ECD). Moreover, there were significant CO2*variety interactions on pupal weight and ECD, and CO2*generation interactions on pupal weight, frass output, and MRGR. Furthermore, transgenic Bt cotton significantly decreased the population‐trend index compared to non‐transgenic cotton for the three successive bollworm generations, especially at elevated CO2. Damage inflicted by the cotton bollworm on cotton, irrespective of the presence of insecticidal genes, is predicted to be higher under elevated CO2 conditions because of individual compensatory feeding on host plants. Conversely, population abundance is presumed to be lower under elevated CO2 compared to that under ambient CO2, particularly in combination with transgenic technologies.

Behavioral Response of Helicoverpa armigera (Lepidoptera: Noctuidae) to Cotton with and Without Expression of the CrylAc δ-Endotoxin Protein of Bacillus thuringiensis Berliner

Behavioral responses of larvae and adults of cotton bollworm, Helicoverpa armigera to cotton with (GK-12) and without (SI-3) expression of the CrylAc δ-endotoxin protein of Bacillus thuringiensis (Bt) Berliner were observed during 2001 and 2002. Our results showed that 8.3% individuals fed with flowers and bolls of GK-12 could develop from neonate to pupa; however, pupal weight decreased by 48.6% and duration of development was delayed by 7.6 days compared with those fed with flowers–bolls of SI-3. Deterrence index (DI) of larvae decreased in later instars, which indicated that the Bt toxin decreased with age. Feeding frequency of 4th-instar larvae on GK-12 leaves decreased by 38.8%, but movement frequency increased by 37.1%. Larvae moved at least one plant away by the age of 10 days in both pure and mixed plantings of SI-3 and GK-12 in the field. Adults preferred to lay eggs on SI-3. The total number of eggs deposited on SI-3 plants in 3 days were about 232 and 95% greater than that on GK-12 plants at bud–flower stage and flower–boll stage, respectively. Based on the behavior of larva and adults in response to the transgenic cotton, the potential effect of refuge strategy in resistance management of H. armigera is discussed.

Quantification of diapausing fourth generation and suicidal fifth generation cotton bollworm, Helicoverpa armigera, in cotton and corn in northern China

Cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), is a major pest of cotton and corn crops in northern China. A phenological differentiation between fourth generation cotton bollworms from cotton and those from corn fields was observed in northern China during 1999–2000. The proportion of pupation in late fall was marginally higher in cotton fields compared to that in corn fields; however, the proportions of fall emergence of moths from cotton fields were significantly higher than those from corn fields. The proportion of spring emergence of moths was also significantly higher for larvae collected from cotton (28.0%) than from corn (14.5%). The overwintering duration of females was significantly shorter than that of males in both crops. Moreover, the overwintering duration of bollworm populations from cotton was significantly longer than that from corn. The early spring population of H. armigera came from both cotton and corn fields, but the spring emergence of moths from larvae collected from cotton took about 5 days longer to reach 100% emergence compared to that from corn.

Influence of Constant Temperatures on Life History of Immature Lygus elisus (Hemiptera: Miridae)

Abstract Life history characteristics of Lygus elisus Van Duzee were studied at 10, 15, 20, 25, 30, and 35°C in the laboratory. The egg incubation period, instar-specific nymphal development, survivorship, and longevity of L. elisus were influenced by temperature. Eggs did not hatch at 10°C. For the five remaining selected temperature treatments, the incubation period was longest at 15°C and decreased nonlinearly with an increase in temperature. Temperature influenced the developmental period differently for different instars, with the second stadium being the shortest at 10, 20, 25, and 30°C, whereas first and third stadia were the shortest at 15 and 35°C, respectively. The final stadium was longest across all six temperatures. The relationships between temperature and total durations were described by the same equation for both males and females. Total nymphal duration was not significant with sexes. Sex ratio (proportion of males) of emerging adults of L. elisus did not deviate from 1:1. Both instar-specific and total nymphal survivorship varied significantly with temperature. Total nymphal survivorship was highest at 15°C and lowest at 10°C. Adult longevity ranged from 16 (35°C) to 122 d (15°C), with a curvilinear response to temperature. Females survived ≈10 d longer than males at 20°C, but survivorship of males and females was similar at other temperatures. These life history data will be useful in developing a computer model simulating L. elisus population dynamics in the field.

Toxicity of terpenes secreted by the predator Xylocoris flavipes (Reuter) to Tribolium castaneum (Herbst) and Oryzaephilus surinamensis (L.)

Abstract Four terpene alcohols, linalool, geraniol, α-terpineol, and nerol, which are compounds produced by Xylocoris flavipes (Reuter), were tested for toxicity against adults of Tribolium castaneum (Herbst) and Oryzaephilus surinamensis (L.) using a Petri dish assay. Dose-response studies were conducted for each compound singly and in a combination that mimicked the concentrations of these volatiles in exocrine secretions of X. flavipes . Linalool and α-terpineol were toxic to T. castaneum in a dose-dependent fashion, but geraniol and nerol were not toxic during the 24 h bioassay. The mixture of the four compounds was several times less toxic than linalool and α-terpineol for T. castaneum , even when exposed to large amounts. All four terpene alcohols and the mixture were toxic to O. surinamensis , with α-terpineol proving most toxic and linalool the least toxic. Toxic effects of linalool and α-terpineol against O. surinamensis occurred within very narrow ranges, suggesting the possibility of a threshold concentration. Variation in toxicity among similar compounds and between insect species for the same compounds should be examined in studies that assess terpenoids for toxicity against stored-product insects.

Influence of Constant Temperatures on Life History Parameters of the Cotton Aphid, Aphis gossypii, Infesting Cotton

Abstract Laboratory clip-cage studies were conducted to quantify the temperature-dependent development, survivorship, and reproduction and to generate life history characteristics and population growth parameters of the cotton aphid, Aphis gossypii Glover, on phenologically standardized greenhouse-grown cottons at 10, 15, 20, 25, 30, and 35°C. The developmental thresholds were estimated to be 6.3, 6.7, 5.9, 5.9, and 6.3°C for first to fourth instars and for total nymphal development, respectively. The maximum rate of development were estimated to occur at 32.2, 30.8, 30.4, 30.0, and 30.2°C for first to fourth instars and for total nymphal development, respectively. Increased temperature resulted in more rapid decline in survivorship, which was particularly sharp at 35°C, dropping from 94 to 17% in 5 d. Number of days elapsed until first deposition of progeny increased progressively and sharply at temperatures 10 (26 d) to 15 (15 d) to 20°C (8 d) and stabilized at 5 d for 25, 30, and 35°C. Average lifetime fecundity of females rose from a low of 9.76 progeny at 10°C to a peak of 58.9 progeny at 30°C and declined sharply to 17.3 at 35°C. Finite rate of population growth was highest at 25°C and lowest at 10°C. Although stage-specific developmental maxima occurred between 30 and 32°C, a nonlinear regression model estimated 28.6°C to be the optimum temperature for overall cotton aphid development, reproduction, and population increase.

Computer Model for Simulating Population Dynamics of the Predator Lyctocoris campestris (Heteroptera: Anthocoridae) in Stored Shelled Corn

Abstract A computer model for simulating population dynamics of Lyctocoris campestris (F.), a predator of stored-product insects, was developed using data from the literature and newly collected data for adult longevity and fecundity. Immature development time and survival, adult longevity, fecundity, and cannibalism were simulated. The model was validated at ambient environmental conditions during three seasons in Wisconsin. The model accurately simulated development of the predator feeding on larvae of the Indianmeal moth, Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae), in the validation studies. The model will be useful in optimizing mass-rearing and augmentative releases of this predator for biological control of stored-product insect pests.