Survivorship of Helicoverpa zea and Heliothis virescens on cotton plant structures expressing a Bacillus thuringiensis vegetative insecticidal protein

A series of tests quantified bollworm, Helicoverpa zea (Boddie), and tobacco budworm, Heliothis virescens (F.), larval survival on plant structures of a nontransgenic cotton (Gossypium hirsutum L.), 'Coker 312', and two transgenic cottons expressing Vip3A protein or both Vip3A + CrylAb proteins (VipCot). Vegetative and reproductive structures including terminal leaves, flower bud (square) bracts, whole debracted squares, flower petals, flower anthers, and intact capsules (bolls) were harvested from plants in field plots. Each structure was infested with 2-d-old larvae from one of the two heliothine species. Larvae were allowed to feed for 96 h on fresh tissue. Survivorship at 96 h after infestation was significantly lower on all structures of Vip3A and VipCot cotton lines compared with similar structures of Coker 312. VipCot plant structures generally resulted in lower larval survivorship compared with similar structures of the Vip3A cotton line. H. zea survivorship ranged from 4 to 28% and from 1 to 18% on Vip3A and VipCot plant structures, respectively. H. virescens survivorship ranged from 10 to 43% and from 2 to 12% on Vip3A and VipCot plant structures, respectively. H. virescens survivorship was higher on VIP3A plant structures compared with that for H. zea on similar structures. These differences between species were not observed on plants from the cotton line expressing VipCot proteins. The results for these plant structures demonstrate that the combination of proteins expressed in VipCot cotton lines are more effective than Vip3A cotton lines against this heliothine complex.     

Distribution of bollworm,Helicoverpa zea (Boddie), injured reproductive structures on genetically engineered Bacillus thuringiensis var. kurstaki Berliner cotton

Bollworm, Helicoverpa zea (Boddie), larvae are commonly observed feeding in genetically engineered Bollgard cotton. Although no information is currently available characterizing the levels of injury bollworms cause, aproximately 25% of the Bollgard acreage in the United States receives at least one insecticide application annually targeting bollworm populations. Studies were conducted to determine the levels of fruiting form injury that can occur from bollworm larvae feeding on white flowers of two types of genetically engineered cotton. The two types of genetically engineered cotton included the original Bollgard that produces one protein (Cry1Ac) from Bacillus thuringiensis variety kurstaki Berliner and Bollgard II that produces two proteins (Cry1Ac + Cry2Ab) from B. thuringiensis kurstaki. In one study, individual larvae (24 +/- 6 h old) were placed in first position white flowers of Deltapine 5415 (non-Bollgard) and Deltapine NuCOTN 33B (Bollgard). Larval infestations were made on 50 plants for each of 5 d during 2000 and 2001. Each plant was visually examined at 3 d and every 2 d thereafter, until larvae were no longer recovered. Larvae injured a total of 46.6 fruiting forms per 50 plants on non-Bollgard cotton, compared with only 18.9 fruiting forms per 50 plants on Bollgard cotton. Mean larval injury per insect was 4.3 fruiting forms on non-Bollgard cotton compared with 2.7 fruiting forms on Bollgard cotton. In a second study, individual larvae (24 +/- 6 h old) were placed in first position white flowers of Deltapine 50 (non-Bollgard), Deltapine 50B (Bollgard), and an experimental Bollgard II line. Larval infestations were made on 10 plants per day for each of six consecutive days during 2001. Larvae injured a total of 25.0 fruiting forms per 10 plants on non-Bollgard, 11.5 on Bollgard, and 6.4 on Bollgard II cottons. Mean larval injury per insect was 6.6 fruiting forms on non-Bollgard, 3.5 on Bollgard, and 0.8 on Bollgard II cottons. These data indicate that supplemental insecticide applications may be necessary to prevent yield losses on Bollgard cotton. In contrast, injury to Bollgard II cotton was minimal and may not require additional insecticide applications for bollworms.     

Behavior of bollworm (Lepidoptera: Noctuidae) larvae on genetically engineered cotton

Reports of bollworm, Helicoverpa zea (Boddie), larvae feeding in white flowers of Bollgard cotton have been relatively common since the commercialization of this technology in 1996. Field studies were conducted in Louisiana to determine if differences in bollworm larval behavior occuron non-Bollgard (cultivar 'Deltapine 5415') and Bacillus thuringiensis (Bt), Bollgard ('NuCOTN 33B') cottons. Larvae were placed on the terminal foliage of either single cotton plants or on all plants within 1-m row micro-plots. On preflowering cotton plants, significantly more bollworms moved from the site of infestation (terminal) on Bollgard plants compared with that on non-Bollgard plants. On individual flowering plants, the number of nodes larvae moved from the terminal and the number of infested bolls was greater on Bollgard cotton plants. Similar differences between Bollgard and non-Bollgard plants in the percentage of infested terminals and squares were observed at 48-h after infestation when 1-m rows were infested. These data will be used to refine scouting protocols for bollworm larvae on Bollgard cotton.     

Feeding behavior of bollworm and tobacco budworm (Lepidoptera: Noctuidae) larvae in mixed stands of nontransgenic and transgenic cotton expressing an insecticidal protein

Feeding behavior of third-instar bollworm, Helicoverpa zea (Boddie), and tobacco budworm, Heliothis virescens (F.), was observed in pure and mixed stands of nontransgenic and transgenic cotton (BTK), Gossypium hirsutum L., expressing an insecticidal protein CryIA(c) from a bacterium, Bacillus thuringiensis Berliner subsp. kurstaki. Five plant stands composed of BTK and non-BTK plants were evaluated; two pure stands and three mixed stands. Percentage ratios of BTK to non-BTK plants in the stands were 100:0, 75:25, 50:50, 25:75 and 0:100, respectively. In all stands with BTK plants, fewer bollworm and tobacco budworm larvae were found on BTK plants than non-BTK plants 24 h after infestation with third instars. At 48 h, significantly fewer tobacco budworm larvae, but not fewer bollworm larvae, were found on BTK plants. However, the number of larvae of either insect did not increase on non-BTK plants compared with the initial infestation density of three larvae per plant. The number of obacco budworm injured flower buds, and capsules was lower in all plant stands containing BTK plants compared with the pure stand of non-BTK at 48 h after infestation. Higher numbers of larvae on non-BTK plants were possibly the result of larval intoxication, reduced feeding, and increased plant abandonment and death on BTK plants rather than a classical feeding preference. Unexpectedly, the number of flower buds and capsules injured by bollworm and tobacco budworm when averaged per plant for all plants in a stand, differed little among the 75:25, 50:50 and 25:75 plant mixtures. These data suggest that larvae of both species frequently moved among plants, feeding indiscriminately on BTK and non-BTK plants.     

Effects of natural enemies on relative fitness of Heliothis virescens genotypes adapted and not adapted to resistant host plants

We investigated the potential of two natural enemies of Heliothis virescens (F.) (Noctuidae) to affect its rate of adaptation to tobacco containing Bacillus thuringiensis Berliner toxin. Larval fitness of two laboratory strains of H. virescens, one adapted to B. thuringiensis toxin and one not adapted, was compared on toxic and nontoxic plants, in the presence of the parasitoid Campoletis sonorensis (Cameron) (Ichneumonidae) or the entomopathogenic fungus Nomuraea rileyi (Farlow) Samson. By exposing larvae to plants and enemies for no more than 24 h, we focussed on the behavioral rather than physiological component of their interaction with toxic plants and natural enemies. Parasitism of H. virescens larvae by C. sonorensis during exposure periods of 1–4 h was lower on toxic plants than nontoxic plants and was lower for nonadapted larvae than for toxin-adapted larvae. Decreased larval feeding damage on toxic versus nontoxic plants, and by nonadapted versus adapted larvae, may explain differences in parasitism, because C. sonorensis locates host larvae using cues from damaged plants. Effects of plant toxicity and larval strain on H. virescens survival were numerically consistent with effects on parasitism, but they were not statistically significant. When mean larval survival is used to estimate fitness of the nonadapted genotype relative to the toxin-adapted genotype, we find that C. sonorensis is expected to delay adaptation to toxic plants. Percent infection by N. rileyi of H. virescens larvae exposed to fungus-treated plants for 24 h was greater when plants were toxic, and was greater for nonadapted larvae than toxin-adapted larvae. There were corresponding decreases in larval survival on toxic compared to nontoxic plants, and of nonadapted compared to adapted larvae. Interaction of effects of plant line and larval strain on survival was significant in the presence of fungus, but not in the absence of fungus, which indicates that the effect of toxic plants on the relative fitness of toxin-adapted and nonadapted larvae was mediated by fungus. As in the interaction with C. sonorensis, behavior of larvae on plants may explain differences in susceptibility to N. rileyi. Because nonadapted larvae moved more than toxin-adapted larvae on toxic plants, nonadapted larvae may have been more likely to encounter a lethal dose of conidia. In contrast with C. sonorensis, N. rileyi, which decreased the fitness of the nonadapted genotype relative to the adapted genotype, is expected to accelerate adaptation to toxic plants.     

Fitness cost of resistance to Bt cotton linked with increased gossypol content in pink bollworm larvae

Fitness costs of resistance to Bacillus thuringiensis (Bt) crops occur in the absence of Bt toxins, when individuals with resistance alleles are less fit than individuals without resistance alleles. As costs of Bt resistance are common, refuges of non-Bt host plants can delay resistance not only by providing susceptible individuals to mate with resistant individuals, but also by selecting against resistance. Because costs typically vary across host plants, refuges with host plants that magnify costs or make them less recessive could enhance resistance management. Limited understanding of the physiological mechanisms causing fitness costs, however, hampers attempts to increase costs. In several major cotton pests including pink bollworm (Pectinophora gossypiella), resistance to Cry1Ac cotton is associated with mutations altering cadherin proteins that bind this toxin in susceptible larvae. Here we report that the concentration of gossypol, a cotton defensive chemical, was higher in pink bollworm larvae with cadherin resistance alleles than in larvae lacking such alleles. Adding gossypol to the larval diet decreased larval weight and survival, and increased the fitness cost affecting larval growth, but not survival. Across cadherin genotypes, the cost affecting larval growth increased as the gossypol concentration of larvae increased. These results suggest that increased accumulation of plant defensive chemicals may contribute to fitness costs associated with resistance to Bt toxins.     

Identification of a novel elite genotype for in vitro culture and genetic transformation of cotton

Hypocotyls of cotton (Gossypium hirsutum L.) cultivars cv. YZ-1, Coker 312 and Coker 201 were inoculated on Murashige and Skoog callus induction medium. YZ-1 exhibited a very high regeneration potential, with 81.9 % of the explants inoculated differentiated into embryogenic callus within 8–10 weeks. During the process of callus maintenance (subculture for 1 to 3 years), the total embryos number in Coker 312 and Coker 201 calli dropped sharply, and the percentage of embryo germination decreased. On the contrary, the callus of YZ-1 consistently maintains a high frequency of plant regeneration after long-time subculture. Transgenic kanamycin-resistant calli of Coker 201 partially lost the ability of somatic embryogenesis and plant regeneration. The stress produced by the transformation procedure slightly affected somatic embryogenesis and plant regeneration of YZ-1, which showed minimum loss of plant regeneration ability.     

Effect of plant age, larval age, and fertilizer treatment on resistance of a cry1Ab-transformed aromatic rice to lepidopterous stem borers and foliage feeders

The resistance of vegetative, booting, and flowering stage plants of a variety of an aromatic rice, Oryza sativa L., transformed with a Bacillus thuringiensis Berliner cry1Ab gene under control of the maize phosphoenolpyruvate carboxylase (PEPC) promoter was evaluated against four lepidopterous rice pests--the stem borers Chilo suppressalis (Walker) (Lepidoptera: Crambidae) and Scirpophaga incertulas (Walker) (Lepidoptera: Pyralidae), and the foliage feeders Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae) and Naranga aenescens Moore (Lepidoptera: Noctuidae). Plants of the cry1Ab-transformed line (no. 827) were more resistant to young larvae of S. incertulas, C. suppressalis, and C. medinalis than control plants at the vegetative stage but not at the flowering stage. Survival of 10-d-old stem borer larvae did not differ on cry1Ab plants and control plants at either the vegetative or flowering stage, but the development of 10-d-old C. suppressalis larvae was retarded on the vegetative stage cry1Ab plants. Immunological analysis also showed an apparent decline in Cry1Ab titer in leaf blades and leaf sheaths at the reproductive stage. In experiments comparing three fertilizer treatments (NPK, PK, and none), there was a significant interaction between fertilizer treatment and variety on larval survival only in whole-plant assays at booting stage with C. suppressalis. On cry1Ab plants, larval survival did not differ significantly among the three fertilizer levels, whereas on control plants survival was highest with the NPK treatment. cry1Ab plants tested at the sixth and seventh generations after transformation were more resistant than control plants to N. aenescens and C. suppressalis, respectively, suggesting that gene silencing will not occur in line 827. The results of the experiments are discussed in terms of resistance management for B. thuringiensis toxins in rice.     

Population dynamics of Spodoptera litura (Lepidoptera: Noctuidae) on Bt cotton in the Yangtze River Valley of China

Genetically modified cotton that produces a crystalline protein from Bacillus thuringiensis subsp. kurstaki (Berliner) (Bt) has been widely deployed to manage lepidopteran insect pests in cotton growing areas worldwide. However, susceptibility of different insect species to Bt protein varies, which may affect lepidopteran pest populations in the field. Studies on effects of two transgenic cotton lines (BG1560 and GK19) carrying a Cry1A gene on common cutworm Spodoptera litura F. (Lepidoptera: Noctuidae), were conducted during 2002-2005 in the cotton planting region of the Yangtze River valley of China. Results showed that common cutworm larvae had low susceptibility to Bt cotton. There was no significant difference in larval population densities in conventional and Bt cotton fields. However, the larval populations of the insect on conventional plants treated with chemical insecticides for control of target pest of Bt cotton were significantly lower than that in Bt cotton fields. These results indicated that the common cutworm was the potential to become a major and alarming pest in Bt cotton fields, and therefore efforts to develop an effective alternative management strategy are needed.     

Oviposition preference,larval performance and adaptation of Trichoplusia ni on cabbage and cotton

Most female herbivores ensure to lay eggs where their offspring can develop successfully. The oviposition preferences of females affect strategies in pest management. In this study, the performance of two cohorts of Trichoplusia ni larvae on cabbage and cotton (after they had been transferred from their original host plants) were investigated. The preferences of female moth ovipositing and larval feeding on these two host plants were observed. The results indicated that plants significantly affected oviposition preference of the female adults and development and survival of larvae of T. ni. All females preferred to lay eggs on cabbage than cotton regardless from which host they originated. The detrimental effects of cotton on the development and survival of T. ni larvae originated from cabbage (CaTn) increased with the increase of the larval age when they were transferred. In addition, the host plant change did not significantly affect the development and survival of larvae of T. ni originating from cotton (CoTn). Larvae of CaTn preferred cabbage plants as compared to cotton plants, whereas larvae of CoTn did not show a significant choice. Although the adult females preferred laying eggs on cabbage, they did not show preferences between cotton and cabbage in a Y‐tube olfactometer test. The hypothesis of oviposition preference and performance of larvae was supported by the results of CaTn, whereas they not supported by those from CoTn. Based on these results, the strategy to manage this serious pest was discussed.     

Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) larval feeding behavior on transgenic maize (MON 863) and its isoline

Diabrotica species (Coleoptera: Chrysomelidae) larval behavior studies have posed a challenge to researchers because of the subterranean life cycle of this pest. To fully understand how the western corn rootworm, Diabrotica virgifera virgifera LeConte, injures the maize, Zea mays L., root system, its behavior must be studied. For example, larvae that can detect an area of the root that has a lower amount of toxin, whether from an insecticide or a transgenic maize plant, have an increased chance of survival. This study assessed D. v. virgifera larval feeding behavior on rootworm-susceptible maize and maize containing a biotechnology-derived trait (MON 863) with resistance to D. v. virgifera first instar feeding. Maize plants were grown in a medium that allowed for direct observation and measurements during feeding of larval stadia. Neonates were placed on maize seedlings, and data were taken at 3, 6, 9, and 12 d postinfestation on resistant and susceptible maize. On rootworm-susceptible maize, neonate larvae aggregated at the root tips and began actively feeding, and then they moved to older root tissue. Conversely, some larvae that ingested Cry 3Bb1 from the resistant maize exhibited no movement. Other larvae on the resistant maize moved continuously, sampling root hairs or root tissue but not actively feeding. The continuously moving larvae had visibly empty guts, suggesting possible nonpreference for the resistant root. This study contributes to our understanding of D. v. virgifera larval behavior and provides insight into questions surrounding the potential evolution of behavioral and biochemical resistance to Cry3Bb1.     

Metal uptake by iron-efficient and inefficient oats

Metal uptake by oats depending on plant responses to Fe-deficiency stress was investigated. Coker 227 oats classified as Fe-efficient and TAM 0–312 oats as Fe-inefficient cultivars (Hopkins et al., 1992) were grown either alone or in combination in three sandy soils using a pot experiment. These soils were from a field trial with sludge-borne metals applications leading to an increased metal content. Plant shoots were harvested one month after growth. Because soil pH increased from 5.4 to 6.8, shoot Fe level decreased in the Fe-inefficient TAM 0–312 oats compared to Coker 227 oats when plants were grown alone. In combination, TAM 0–312 oats had a negative impact on the availability of Fe in the Fe-efficient Coker 227 oats. Especially, Coker 227 and TAM 0–312 shoots showed chlorosis in mixed culture with high Zn and Mn content in the soil (soil B). However, Fe content in TAM 0–312 shoots in mixed culture did not increase compared to monoculture in all soils. In metal-contaminated soils, TAM 0–312 oats grown alone obtained less Zn and Cd than Coker 227 oats. Additionally at soil pH 6.8, shoot Ni and Mn levels were also lower in TAM 0–312 oats than in Coker 227 oats. Shoot Zn, Cd, and Ni levels decreased in Coker 227 oats from mixed cultures, and were not different compared to those in TAM 0–312 oats. Cu uptake was similar in all treatments except for the mixed culture in soil B. Coker 227 oats have been found to release a phytosiderophore whereas TAM 0–312 did not (Brown et al., 1991). Results indicated that phytosiderophores may lead to a higher Zn, Cd and Ni supply in the rhizosphere of Coker 227 oats and to higher metal contents in their shoots than in TAM 0–312 oats which did not activate such mechanisms.     

Western corn rootworm larval movement in SmartStax seed blend scenarios

Insect resistance management (IRM) can extend the lifetime of management options, but depends on extensive knowledge of the biology of the pest species involved for an optimal plan. Recently, the Environmental Protection Agency (EPA) registered seed blends refuge for two of the transgenic Bacillus thuringiensis (Bt) corn products targeting the western corn rootworm, Diabrotica virgifera virgifera LeConte. Larval movement between Bt and isoline plants can be detrimental to resistance management for high dose Bt products because the larger larvae can be more tolerant of the Bt toxins. We assessed movement of western corn rootworm larvae among four spatial arrangements of SmartStax corn (expressing both the Cry34/35Ab1 and Cry3Bb1 proteins) and isoline plants by infesting specific plants with wild type western corn rootworm eggs. Significantly fewer western corn rootworm larvae, on average, were recovered from infested SmartStax plants than infested isoline plants, and the SmartStax plants were significantly less damaged than corresponding isoline plants. However, when two infested isoline plants surrounded a SmartStax plant, a significant number of larvae moved onto the SmartStax plant late in the season. These larvae caused significant damage both years and produced significantly more beetles than any other plant configuration in the study (including isoline plants) in the first year of the study. This plant configuration would occur rarely in a 5% seed blend refuge and may produce beetles of a susceptible genotype because much of their initial larval development was on isoline plants. Results are discussed in terms of their potential effects on resistance management.     

Effect of water-deficit stress on cotton plants expressing the Bacillus thuringiensis toxin

Bacillus thuringiensis (Bt) crops require a high dosage of Bt toxin to delay development of insect resistance, in particular, when the refuge strategy is applied. This strategy is threatened by plant developmental and environmental factors that might reduce Bt toxin concentration and Bt efficacy in Bt crops. Growth of Bt (Cry1Ac) cotton under prolonged, moderate water deficit as a single stress factor was evaluated. Bt cotton plants were analysed for physiological performance, Bt toxin concentration and Bt efficacy. For performance analysis, leaf and total plant dry weight and leaf area were measured. Bt toxin concentration was determined by an immuno‐assay. Effects of Bt toxin on growth and mortality of African cotton bollworm, Helicoverpa armigera, larvae were measured in different plant organs. Leaves from young plants exposed for 30 days to moderate water deficit had both higher Bt toxin concentrations and were more effective against larvae than leaves, flowers or bolls from mature flowering plants exposed to 60 days of moderate water deficit. Although growth of Bt cotton plants under moderate water‐deficit conditions decreased Bt concentrations in leaves, flowers and bolls, this had no effect on efficacy against first‐instar cotton bollworm larvae. No significant evidence was found that moderate water deficit, as a single stress factor, decreases Bt efficacy in Bt cotton.     

Effects on oviposition behaviour and larval development of Spodoptera littoralis by herbivore-induced changes in cotton plants

Herbivore feeding induces chemical defence responses in plants. In this study we investigate how herbivore-induced changes in cotton plants, Gossypium hirsutum L., influence the oviposition behaviour and larval development of Spodoptera littoralis Boisd. (Lepidoptera: Noctuidae). In two-choice experiments female moths preferred to oviposit on small plants (3–4 leaves) that had been fed on by 3rd to 4th instar larvae (72%) over non-damaged control plants. However, when using larger plants (8 to 10 true leaves) the preference was reversed, with only 31% of the eggs deposited on the induced plants. The same trend was found with plants that had been given a similar level of damage by 6th instar larvae. However, the difference between the treatments was in both cases smaller with only 60% of the eggs deposited on the preferred plant treatment. If cotton plants subjected to artificial damage were compared with undamaged plants, none of the treatments were preferred for oviposition. No significant difference was found in larval weight, pupal weight, survival, or development time, between larvae reared their entire development on leaves from induced or from non-induced plants.     

Effects of Cry1Ab-expressing corn anthers on the movement of monarch butterfly larvae

Decreased larval feeding and weight of the monarch butterfly, Danaus plexippus L., have been detected after 4 d of exposure in the laboratory to a high density of Bacillus thuringiensis (Bt)-expressing anthers. One hypothesis is that larvae exposed to Bt anthers exhibit increased wandering, resulting in less feeding and lower weight gain. To test this hypothesis, 2-d-old monarch butterfly larvae exposed to milkweed leaf disks with no anthers, anthers that express Bt (Cry1Ab, event MON810), or other non-Bt anthers were observed using a video-tracking system. As had been shown in previous studies, larvae exposed to Bt anthers fed less and gained less weight than larvae exposed to non-Bt or no anthers, yet there was no evidence of feeding on anthers. Total distance moved, maximum displacement from release point, percentage of time spent moving or near anthers, or mean turn angle did not differ across treatments. However, larvae exposed to Bt anthers spent more time off milkweed leaf disks than those exposed to no anthers and were more likely to move off the leaf than larvae exposed to non-Bt anthers. Results suggest that larvae exposed to Bt anthers behave differently and that ingestion may not be the only way Bt can affect nontarget insects like the monarch butterfly.     

Cross-resistance responses of CrylAc-selected Heliothis virescens (Lepidoptera: Noctuidae) to the Bacillus thuringiensis protein vip3A

One susceptible and three Cry1Ac-resistant strains of tobacco budworm, Heliothis virescens (F.) (Lepidoptera: Noctuidae), were used in laboratory studies to determine the level of cross-resistance between the Bacillus thuringiensis (Berliner) toxins Cry1Ac and Vip3A by using concentration-mortality and leaf tissue experiments. Concentration-mortality data demonstrated that the three Cry1Ac-resistant H. virescens strains, YHD2, KCBhyb, and CxC, were at least 215- to 316-fold resistant to Cry1Ac compared with the susceptible strain, YDK. Results from Vip3A concentration-mortality tests indicated that mortality was similar among all four H. virescens strains. Relative larval growth on Cry1Ac reflected concentration-mortality test results, because YHD2 larval growth was mostly unaffected by the Cry1Ac concentrations tested. Growth ratios for KCBhyb and CXC indicated that they had a more moderate level of resistance to Cry1Ac than did YHD2. Relative larval growth on Vip3A was highly variable at lower concentrations, but it was more consistent on concentrations of Vip3A above 25 microg/ml. Differences in larval growth among strains on Vip3A were not as pronounced as seen in Cry1Ac experiments. Mortality and larval growth also was assessed in leaf tissue bioassays in which YDK, CxC, and KCBhyb neonates were placed onto leaf disks from non-Bt and Bt cotton, Gossypium hirsutum L., for 5 d. Three Bt lines were used in an initial bioassay and consisted of two Vip3A-containing lines, COT203 and COT102, and a Cry1Ac-producing line. Mortality of KCBhyb and CXC was lower than that of YDK larvae in the presence of leaf tissue from the Cry1Ac-producing line. Additionally, increased larval growth and leaf tissue consumption on Cry1Ac-containing leaf disks was observed for KCBhyb and CXC. Mortality and larval weights were similar among strains when larvae were fed leaf tissue of either non-Bt, COT203, or COT102. A subsequent leaf tissue bioassay was conducted that evaluated four cotton lines: non-Bt, Cry1Ab-expressing, Vip3A-expressing, and pyramided-toxin plants that produced both Cry1Ab and Vip3A. Mortality levels were similar among strains when fed non-Bt, Vip3A-expressing, or pyramided-toxin leaf tissues. Mortality was higher for YDK than for KCBhyb or CXC on Cry1Ab-expressing leaf tissues. No differences in larval weights were observed among strains for any genotype tested. Results of these experiments demonstrate that cross-resistance is nonexistent between CrylAc and Vip3A in H. virescens. Thus, the introduction of Vip3A-producing lines could delay Cry1Ac-resistance evolution in H. virescens, if these lines gain a significant share of the market.     

Effect of body size and larval experience on mate preference in Helicoverpa armigera (Hübner) (Lep., Noctuidae)

Abstract:  We studied the mating selection in the cotton bollworm, Helicoverpa armigera (Hübner), in relation to body size and larval diet in the laboratory. When provided with an artificial diet at larval stage, weight, body and forewing length did not affect the probability of a male/female moth being selected for mating, but the abdominal width of selected female moths was significantly wider than that of non-selected female moths. 30 female moths were dissected and number of eggs was counted after mating, and there was a correlation between the abdominal width and egg number. There was also significant difference of weight loss between selected and non-selected male/female moths after the mating. The effect of operational sex ratio on mating latency and copulation duration were tested, and the result indicated that mating latency of male selection was significantly longer than that of female selection, but the difference of copulation duration was not significant. Cotton, corn and peanut plants were provided to larvae to test the effect of larval host plant experience on mate choice. When cotton- and peanut-fed moth severed as potential partners, both female and male of cotton-fed moths significantly preferred cotton- to peanut-fed moths for mating. The possible reasons for mate preference based on larval host plant experience may account for host plants attributes on sex pheromone variation and sexual maturity. These findings may impact Bacillus thuringiensis resistance management.     

Influence of plant severing on movement of Ostrinia nubilalis larvae in Zea mays hybrid seed production fields

Genetically engineered corn hybrids that contain a cry gene from the bacterium Bacillus thuringiensis Berliner (Bt) are gaining popularity for controlling the corn pest Ostrinia nubilalis (Hübner). Continuous use of Bt corn, however, could select for O. nubilalis that are resistant to this corn. Monitoring for insect resistance is important, because it could help maintain the Bt technology. A possible monitoring method is to collect larval insects in commercial drying bins after harvest from Bt seed production fields. A drawback to this method is that these collections may be contaminated by insects that moved as later instars from severed non-Bt male rows into the adjacent Bt female rows. These larvae have little to no exposure to Bt toxin, resulting in possible "false positives." The objectives of this study were to first find which combination of planting and severing dates produces the least number of larvae that move from non-Bt male plants to Bt female plants and to assess O. nubilalis larval movement from severed non-Bt male rows to Bt female rows. Field studies in 2002 and 2003 were designed to simulate a hybrid seed production field. Results suggest that movement of O. nubilalis larvae from male corn is minimized when corn is planted early and male plants are severed by 2 wk post-anthesis. This reduces the likelihood of false positives by reducing the number of susceptible larvae moving between Bt and non-Bt plants. Also, larvae moved to all four female rows that were adjacent to the severed rows, but there were significantly more larvae found in the closest row compared with the other three. These results could be used to develop a monitoring program to find O. nubilalis larvae with resistance to Bt corn in field populations of O. nubilalis.     

Behavioral Response of <Emphasis Type="Italic">Helicoverpa armigera</Emphasis> (Lepidoptera: Noctuidae) to Cotton with and Without Expression of the CrylAc δ-Endotoxin Protein of <Emphasis Type="Italic">Bacillus thuringiensis</Emphasis> 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 -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.