Bt Hybrids Research Articles

Profitability and FUE of intercropping with Bt hybrid cotton in vertisols of central India

Bt hybrid cotton (Gossypium hirsutum L.) was intercropped in paired row (PR) planting at 90×135×45 cm inVertisols at Central Institute for Cotton Research, Nagpur, India (21°09’N 79° 09’E, altitude 331 MSL) during 2008, 2009 and 2010 years. Paired row planted Bt hybrid cotton was intercropped (45/135 cm) with marigold (Tagetes erecta L)/soybean (Glycine max L.)/hybrid maize (Zea mays L.)/castor (Ricinus communis L.)/field bean (Dolichus lablab L.) + fennel (Foeniculum vulgare Mill. syn. F) compared with sole Bt hybrid cotton and conventional strip cropping of Bt hybrid cotton + pigeon pea (Cajanus cajan L.) in 90×45 cm in 8:2 ratio in Randomised Block Design. Sole Bt hybrid cotton was planted at 90 × 45 cm significantly out yielded over paired row Bt hybrid cotton (135 × 45 cm) only in normal to deficit rainfall years. PR planted Bt hybrid cotton population was insufficient to exploit the resources under adverse climatic conditions and needed more closer spacing than 45 cm. Pigeon pea strip and castor intercropping reduced the biomass, yield, nutrient uptake and recovery of applied fertilizer in Bt hybrid cotton, but better performed in a year of high rainfall. N, P uptake, NPK fertilizers recovery were significantly higher in sole Bt hybrid cotton which was similar to that of PR Bt hybrid cotton based intercropping systems. NUE were not significantly improved in intercropping systems in the absence of sufficient plant populations of both cotton and intercrops under adverse climatic conditions. Paired row Bt hybrid cotton significantly improved fertilizer uptake, recovery and benefitted from N fertilizer applied to intercropped marigold/maize. Bt hybrid cotton intercropped with castor/ field bean and fennel or marigold systems produced similar net returns of 880 to 940 US $ compared to 460 and 640 US $ ha-1 in paired row and sole Bt hybrid cotton respectively. Key words: Bt hybrid cotton, cotton equivalent yield, fertilizer recovery, fertilizer use efficiency, intercropping, nutrient use efficiency, small farms.

Corn Earworm (Lepidoptera: Noctuidae) in Northeastern Field Corn: Infestation Levels and the Value of Transgenic Hybrids

Corn earworm, Helicoverpa zea (Boddie), is a polyphagous noctuid pest of agricultural crops across the United States that is gaining attention as a pest of field corn. Before the introduction of transgenic insect-resistant hybrids, this pest was largely ignored in field corn, but now many Bacillus thuringiensis (Bt) corn hybrids have activity against corn earworm. However, the value of control in the northeastern United States is unclear because the risk posed by corn earworm to field corn has not been well characterized. To understand the threat from corn earworm and the value of Bt hybrids in field corn, we assessed corn earworm injury in Bt and non-Bt hybrids at 16 sites across four maturity zones throughout Pennsylvania in 2010, and 10 sites in 2011. We also used corn earworm captures from the PestWatch pheromone trapping network to relate moth activity to larval damage in field corn. Corn earworm damage was less than one kernel per ear at 21 of 26 sites over both years, and the percentage of ears damaged was generally < 15%, much lower than in the southern United States where damage can be up to 30 kernels per ear. At sites with the highest damage levels, Bt hybrids suppressed corn earworm damage relative to non-Bt hybrids, but we found no differences among Bt traits. Cumulative moth captures through July effectively predicted damage at the end of the season. Currently, the additional benefit of corn earworm control provided by Bt hybrids is typically less than US$4.00/ha in northeastern field corn.

Comportamento de híbridos de milho transgênicos e respectivos híbridos isogênicos convencionais em relação à infestação de Spodoptera Frugiperda (j. E. Smith,1727) (Lepidoptera: Noctuidae) em duas localidades e épocas de semeadura

A lagarta-do-cartucho, Spodoptera frugiperda, é uma das mais importantes pragas do milho. Vários estudos são realizados para o seu manejo, integrando táticas de controle químico, biológico ou através de plantas resistentes. No intuito de oferecer alternativas a um manejo eficiente dessa praga com a mínima utilização de agrotóxicos, a tecnologia das plantas geneticamente modificadas resistentes a insetos tem sido objeto de muitos estudos. Neste trabalho, o objetivo foi avaliar a infestação natural de lagartas de S. frugiperda e respectivas injúrias em condições de campo, em híbridos transgênicos de milho comparados aos seus isogenes convencionais, em duas épocas de semeadura e em duas regiões. Os híbridos foram semeados na 'safrinha' de 2010 em Jaboticabal, SP, e no verão em 2010/2011, em Jaboticabal, e Pindorama, SP, em delineamento de blocos ao acaso, com sete tratamentos (híbridos) e quatro repetições. Diferentes níveis de infestação de lagartas ocorreram durante todo o desenvolvimento fenológico das plantas nos híbridos convencionais e nos híbridos geneticamente modificados, com significativas diferenças entre os dois grupos na maioria das avaliações. O híbrido 2B710HX foi o menos infestado com lagartas e o com menor área foliar danificada, o que se conclui que a toxina Cry1F foi a mais efetiva na proteção da planta em relação às demais proteínas tóxicas expressas pelos demais híbridos Bt contra a infestação e os danos promovidos por essa praga, independente da época de semeadura.

Corn pith weevil, Geraeus penicillus (Coleoptera: Curculionidae), in transgenic, insect-resistant and conventional field corn hybrids

The corn pith weevil Geraeus penicillus (Herbst) is occasionally found boring in corn stalks throughout the eastern and Midwestern United States. Injury caused by G. penicillus is not typically economical, but may be confused with that of the European corn borer Ostrinia nubilalis (Hubner), an important economic pest throughout the United States. During efforts to assess European corn borer infestations, we discovered G. penicillus in field corn in south-eastern Pennsylvania, including hybrids genetically modified (i.e. Bt hybrids) to control European corn borer among other herbivore species. Our analysis across sites indicated that tunnels of G. penicillus were significantly more abundant in transgenic Bt hybrids than non-Bt hybrids, but comparisons of Bt hybrids and their near isolines revealed mostly similar numbers of G. penicillus tunnels, suggesting other hybrid features might be affecting the distribution of G. penicillus. Tunnels of G. penicillus were equally distributed among the three transgenic trait packages represented in our study. In plants where we found G. penicillus, tunnels were more abundant in stalks free of European corn borer damage. Our report appears to be the first to note G. penicillus feeding in Bt corn hybrids. These findings are notable because they document insect damage in Bt hybrids that may be mistaken for European corn borer damage and may provide evidence of an insect herbivore proliferating following a mild winter or possibly even moving into competitor-free space.

Transgenic Corn Rootworm Protection Increases Grain Yield and Nitrogen Use of Maize

ABSTRACTMaize (Zea mays L.) hybrids expressing Bacillus thuringiensis (Bt) derived resistance to corn rootworm (Diabrotica spp.) are widely grown. Our hypothesis was that Bt hybrids exhibit increased N uptake, resulting in greater grain yield and N use efficiency (NUE) relative to their nonprotected counterparts. In 2008 and 2009, two transgenic corn rootworm resistant (Bt) hybrids with VT3 (YieldGard VT Triple) technology along with their near‐isogenic non‐Bt Roundup Ready Corn 2 (RR2) counterparts were evaluated at Champaign, IL, with supplemental N of 0, 67, 134, 201 or 268 kg N ha−1. Despite minimal corn rootworm feeding pressure on roots, the Bt hybrids produced an average of nearly 1.1 Mg ha−1 more grain than their RR2 counterparts. In the comparison of DKC61‐72 RR2 and DKC61‐69 VT3, Bt protection promoted increased grain yield at low N (+1.0 Mg ha−1; P ≤ 0.01) and a 31% greater response to fertilizer N. With adequate N, grain yields of the comparison DKC63‐45 RR2 and DKC63‐42 VT3 did not differ; however, the latter maximized its yield with an average of 38% less fertilizer N. Increases in NUE (+80%; P ≤ 0.10) and N uptake efficiency (NUpE) (+31%; P ≤ 0.10) at the N rates required to optimize grain yield of Bt hybrids were detected in 2008, but NUE and NUpE were not significantly different between isolines in 2009. We conclude that transgenic corn rootworm protection has supplemental agronomic benefits, with greater N uptake and NUE in some environments.

Asian corn borer (ACB) and non‐ACB pests in GM corn (Zea mays L.) in the Philippines

The Asian corn borer (ACB), Ostrinia furnacalis (Guenée), has become the most damaging pest in corn in south-east Asia. Corn farmers in the Philippines have incurred great yield losses in the past decades because of ACB infestation. Bacillus thuringiensis (Bt) and Bt herbicide-tolerant (BtHT) corns have been developed to reduce borer attacks worldwide. This study assessed the extent of ACB and non-ACB pest infestations in both GM and non-GM corn in Isabela Province, the Philippines. Specific aims were to reinvestigate the efficacy of Bt corn in controlling ACB, to evaluate what parts of Bt corn plants are susceptible to ACB, to monitor the potential development of ACB resistance and to evaluate whether secondary pests dominate in an ACB-free Bt corn environment. The study involved preparatory interviews with farmers, site selection, field scouting and visual inspection of 200 plants along 200 m transect lines through 198 cornfields. Bt corn can efficiently reduce the ACB pest problem and reduce borer damage by 44%, to damage levels in Bt and BtHT corn of 6.8 and 7% respectively. The leaves of Bt corn were more susceptible, while cobs of Bt corn were less affected by ACB. Non-ACB pests were common in Bt toxin-free cornfields and reduced in non-GM cornfields where ACB was abundant. No secondary pest outbreaks were found in ACB-free Bt cornfields. Bt and BtHT corn hybrids containing the Cry1Ab protein performed well in Isabela Province. Reduced cob damage by ACB on Bt fields could mean smaller economic losses even with ACB infestation. The occurrence of ACB in Bt and BtHT cornfields, although at a moderate and insignificant level, could imply the potential development of resistance to Bt toxin.

Mismatch in Source and Sink as a Reason Determining Realized Potentiality of Bt Cotton Hybrids

The onset of Bt era has lead to increase in productivity of cotton in Indian scenario. Many of the hybrids commercialized in the initial phase were those hybrids which were popular in non Bt era and some of them have failed to give same high productivity in their Bt form. A comparison of Bt cotton hybrids for productivity, plant type features has revealed that though Bt cotton hybrids revealed increased productivity some hybrids reveal early scenecense leading to a more determinate habit. The increase in yield in these hybrids mostly comes from the bolls borne on the lower half of the plant than the upper half of the plant. There is a mismatch between the source and sink especially in the upper half of the plant. These hybrids reveal visibly weakened leaves at a later stage as a result of which they show reduced photosynthetic activity. As compared to this, some of the most potential hybrids remained green and photosynthetically active enough to support the increased reproductive load. There is a fairly comparable harvest index in both upper and lower halves. Thus these hybrids reveal higher productivity in both lower and upper halves of the plant canopy. This observation has an important implication in plant breeding that the parents(at least one of them) chosen for developing Bt hybrids should have strong photosynthetic system and stay green nature so that the hybrid is capable of withstand enhanced Bt gene induced reproductive load. Keywords Bt gene; Cotton; Source and sink

Bt Pollen Dispersal and Bt Kernel Mosaics: Integrity of Non-Bt Refugia for Lepidopteran Resistance Management in Maize

Field trials were conducted at Rosemount, MN in 2009 and 2010, to measure pollen movement from Bt corn to adjacent blocks of non-Bt refuge corn. As the use of Bt corn hybrids continues to increase in the United States, and new insect resistance management (IRM) plans are implemented, it is necessary to measure the efficacy of these IRM plans. In Minnesota, the primary lepidopteran pests of corn include the European corn borer, Ostrinia nubilalis (Hübner) and corn earworm, Helicoverpa zea (Boddie). The primary IRM plan in transgenic corn is the use of hybrids expressing a high dose of insecticidal proteins and an insect refuge containing hybrids not expressing insecticidal proteins that produce susceptible insects. Wind-assisted pollen movement in corn occurs readily, and is the primary method of pollination for corn. The combination of pollen movement and viability determines the potential for cross pollination of refuge corn. In 2009 and 2010, cross pollination occurred with the highest frequency on the north and east sides of Bt corn fields, but was found at some level in all directions. Highest levels of cross pollination (75%) were found within the first four rows (3 m) of non-Bt corn adjacent to Bt corn, and in general decreasing levels of cross pollination were found the further the non-Bt corn was planted from the Bt corn. A mosaic of Bt cross-pollinated kernels was found throughout the ear, but in both years the ear tip had the highest percentage of cross-pollinated kernels; this pattern may be linked to the synchrony of pollen shed and silking between Bt and non-Bt corn hybrids. The dominant wind direction in both years was from WNW. However, in both years, there were also prevailing winds from SSW and WSW. Further studies are needed to quantify Bt levels in cross-pollinated kernels, measure the Bt dose of such kernels and associated lepidopteran pest survival, and measure the impact of Bt pollen on lepidopteran pests, particularly when considering the seed mixture refuge configuration.

Productivity and nitrogen-use efficiency yardsticks in conventional and Bt cotton hybrids on rainfed Vertisols

The yield of Bt hybrids (MCEH 184 and RCH 2) was higher than non-Bt NCS 145 and among Bt hybrids, the medium duration RCH 32 Bt was superior to short-duration MECH 184 under normal sowing date. Under late sown conditions, the performance of both RCH 2 Bt and NCS-145 were similar and Bt hybrids are not likely to be more advantageous late sown conditions where a decline in temperature induces cut out. N UtiE and not N Upt E was primarily responsible for a higher NUE in Bt hybrids RCH 2 and MECH 184 than the non-Bt hybrid NCS-145. A higher HI enabled Bt hybrids to utilize N more efficiently.

Leaf δ- endotoxin content and performance of Bt cotton (Gossypium hirsutum) genotypes as influenced by fertility levels for target yields

Two Bollgard I (cvs.MRC 6322 and MRC 6918) and two Bollgard II (cvs.MRC 7351 and MRC 7201) hybrids were supplied with three fertilizer levels (145:39:99, 181:49:124 and 217:59:148 NPK kg/ha, respectively1) targeted for 2.0, 2.5 and 3.0 tonnes/ha of seed cotton yield. d- Endotoxin content was higher in Bollgard II hybrids and MRC 6322 was comparable and the toxin content increased with increase in fertilizer level. However, the number of bad opened bolls differed among genotypes and it followed the total number of bolls rather than d- endotoxin content. Thus, the study brings two important points. First one is that in the selection of Bollgard I or Bollgard II hybrids, the intensity of population of pest in question matters most. And, secondly targeted yields set in the study were easily achieved and often exceeded. Hence, it is possible to achieve target yield of 3.0 tonnes/ha using desired level of fertilizer in most of the Bt hybrids, and this point needs emphasis as cotton is a commercial crop and for farmer monetary returns matter most.

Phenotypic variations between non-transgenic and transgenic maize hybrids

The use of transgenic maize (Zea mays L.) hybrids (Bt, RR, Bt-RR) has simplified crop husbandry, mainly due to a more effective control of pests and weeds. The effects of transgenes insertion on phenotypic traits of maize hybrids are not fully documented, especially without the incidence of pests and weeds. The objectives of this work were (i) to establish phenotypic differences in terms of phenology, growth and yield among a non-transgenic maize hybrid and their transgenic versions (Bt hybrid, RR hybrid and Bt-RR hybrid) and among the transgenic versions of other genetic background and (ii) to analyze the impact of crowding stress on inter-plant variability of the different traits under study. Field experiments were conducted in Buenos Aires (34°36′S, 58°26′W), Argentina during 2008–2009 (Exp 1), 2009–2010 (Exp 2) and 2010–2011 (Exp 3). Genotypes were cultivated at contrasting plant densities (6 and 12plm−2 in Exp 1 and Exp 2) and with contrasting inter-plant spaces within the row (Exp 3), irrigated, without nutrient limitations, and with chemical and mechanical controls of weed and animal pests. At both plant densities, phenotypic variability of several traits among versions within each group was recorded. For example, the RR version of DK747 group had the shortest thermal time to flowering. The stacked transgenic hybrids DK747MGRR and DK190MGRR, exhibited the highest ear growth rate (EGRCP) during the critical period for kernel set. However, both genotypes had the lowest reproductive efficiency (i.e. kernel number per unit of EGRCP), that counterbalanced their higher EGRCP. Only the Bt-RR version of DK747 showed a higher inter-plant variability (CV) of EGRCP at low mean EGRCP values a symptom of the intolerance of this genotype to crowing stress. Consequently, the different versions of each group of hybrids had a similar kernel number per plant and grain yield. We conclude that the introduction of transgenes may alter in some way, the behavior of plants in different eco-physiological aspects in the absence of pests and weeds.

Impact of Bt transgenic cotton on population dynamics of aphids and natural enemies

The dynamic aphids and its predator in RCH 2 Bt and non-Bt cotton hybrids was studied to know the effect of Cry toxin. The mean incidence of aphids was 23.82 and 21.37/ leaf in Bt and non-Bt cotton respectively indicating no significant variation. Mean population of coccinellids, Chrysoperla and syrphids was 0.89, 0.78 and 1.0/plant in RCH 2 Bt which was almost similar to the incidence on RCH 2 non Bt. There was strong and positive correlation between incidence of predators and aphid on both Bt and non- Bt cotton. The ‘r’ value for syrphids v/s aphids was 0.94 in RCH 2 Bt and 0.96 in non- Bt. The feeding assay with Bt intoxicated cotton leaves could not affect the biology and feeding potential of Chrysoperla.

Reaction of Bt Cotton to Rotylenchulus reniformis Infection

Experiments were conducted twice to evaluate the influence of Bt cotton on Rotylenchulus reniformis under laboratory (28 o + 2 o C) and glasshouse conditions. Three Bt hybrids viz., Bunny Bt, RasiBt and Jagannath fusion Bt were compared with their respective non Bt (Refugia) for their impact on egg hatching, juveniles attraction, root penetration and development of R. reniformis. Jagannath fusion Bt and Rasi Bt were effective in inhibiting/delaying the egg hatching as compared to their non Bt lines whileBunny Bt had no significant effect in inhibiting the egg hatching over its non Bt. But Bt hybrids had no significant influence in causing mortality of juveniles, attraction of infective pre adult (J4 ) and penetration of R. reniformis. In all Bt hybrids, on 7 th day after inoculation reduction in number of females with egg mass was observed over its non Bt. The penetrated juveniles of R. reniformis were not able to become adults in Bt cotton.

Histopathology of Bt Cotton Roots Infectedwith Rotylenchulus reniformis

Histopathological studies on Rotylenchulus reniformis infected three Bt hybrids viz., Bunny Bt, Rasi Bt, and Jagannath fusion Bt in comparison with their respective non Bt (Refugia) were carried out. R. reniformis caused heavy damage to cortical, endodermis, pericycle, phloem cells in addition to extensive hypertrophy, dissolution and breakdown of cell wall in non Bt cotton. Whereas in Bt the cell damage caused due to was not severe compared to non Bt cotton. The compact arrangement of cortical and phloem cells and reduced area of phloem vessels in Bt cotton might have reduced severity of R. reniformis damage in Bt cotton.

Studies on Seed Vigour in Bt Cotton Hybrids

Studies on seed vigour tests in Bt and non Bt cotton hybrids were taken up during 2007 at Seed Research and Technology Centre, Rajendranagar, Hyderabad. Irrespective of Bt and Non Bt cotton hybrids, TP (87%), S methods (85%) recorded higher germination than BP (83%) and soil method (82%). Among the vigour tests, first count (78%), final count (86%), brick gravel test (81%) and cold test (79%) recorded higher germination in all the Bt cotton hybrids than Non Bt hybrids which recorded superior values for speed of germination (74%), field emergence (76%), paper exhaustion test (68%) and tetrazolium test for viability. Bt cotton hybrids especially TCH 9 and PRCH 31 recorded higher germination in all germination testing methods as well as vigour tests. Germination and seedling vigour gradually declined with an increase in period of accelerated ageing from 0 to 30 days. Such decline was more in Non Bt cotton hybrids when compared to Bt cotton hybrids. Among different Bt cotton hybrids, PRCH 31 and TCH 9 have shown higher values in all vigour tests as against the other hybrids. There was a rapid decline in germination and vigour in NCS 145, NCS 207, Rudra and Sandeep.

Interaction between maize phenology and transgenic maize hybrids on stalk rots following European corn borer infestation

Field studies were carried out at Hoytville and Wooster, Ohio, USA from 2006 to 2008 to determine the influence of planting date and transgenic Bt maize on incidence and severity of stalk rots of maize. Transgenic and non-transgenic Bt maize hybrids with different maturity ratings were planted on late April/early May, mid-May and early June each year. The incidence and severity of the maize stalk rot complex were compared among planting dates and between Bt and non-Bt genotypes of different relative maturities. Significant linear correlations between European corn borer (Ostrinia nubilalis Hub.) injury to stalk and stalk rots were observed in 2006 and 2007, but not in 2008. We observed significant reduction in stalk discoloration and incidence of maize stalk rot on Bt hybrids compared with non-Bt genotypes. Short season hybrids had significantly lower stalk rots than full season hybrids. The benefit of using Bt hybrids to reduce stalk rots was more evident as planting was delayed than early planting.

PERFORMANCE OF BT AND NON BT COTTON HYBRIDS AGAINST AMERICAN BOLLWORM, HELICOVERPA ARMIGERA (HUBNER)

ABSTRACT Field trials were conducted in farmers’ fields at Warangal district, Andhra Pradesh, India during 2005-06 and 2006-07 to monitor the population and fruiting body damaged caused by H. armigera in commercially released Bt cotton hybrids under unprotected conditions. In the present study it was found that there was no variability in egg laying among Bt, non Bt hybrids and Narasimha. However, the larval population and fruiting body damage was significantly lower in Bt hybrids than their corresponding non Bt hybrids. Seed cotton yield in Bt cotton hybrids were more than that of non Bt cotton hybrids.

Cross-Pollination of Nontransgenic Corn Ears With Transgenic Bt Corn: Efficacy Against Lepidopteran Pests and Implications for Resistance Management

The efficacy of nontransgenic sweet corn, Zea mays L., hybrids cross-pollinated by Bacillus thuringiensis (Bt) sweet corn hybrids expressing Cry1Ab toxin was evaluated in both field and laboratory studies in Minnesota in 2000. Non-Bt and Bt hybrids (maternal plants) were cross-pollinated with pollen from both non-Bt and Bt hybrids (paternal plants) to create four crosses. Subsequent crosses were evaluated for efficacy in the field against European corn borer, Ostrinia nubilalis (Hübner), and corn earworm, Helicoverpa zea (Boddie), and in laboratory bioassays against O. nubilalis. Field studies indicated that crosses with maternal Bt plants led to low levels of survival for both O. nubilalis and H. zea compared with the non-Bt x non-Bt cross. However, the cross between non-Bt ears and Bt pollen led to survival rates of 43 and 63% for O. nubilalis and H. zea larvae, respectively. This intermediate level of survival also was reflected in the number of kernels damaged. Laboratory bioassays for O. nubilalis, further confirmed field results with larval survival on kernels from the cross between non-Bt ears and Bt pollen reaching 60% compared with non-Bt crossed with non-Bt. These results suggest that non-Bt refuge plants, when planted in proximity to Bt plants, and cross-pollinated, can result in sublethal exposure of O. nubilalis and H. zea larvae to Bt and may undermine the high-dose/refuge resistance management strategy for corn hybrids expressing Cry1Ab.

Assessment of the arbuscular mycorrhizal fungal community in roots and rhizosphere soils of Bt corn and their non-Bt isolines

Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) in combination with sequencing of amplified partial 18S ribosomal DNA was used to assess the effects of transgenic Bt corn 5422Bt1 (event Bt 11) and 5422CBCL (event MON810) on the community structure of a non-target microorganism, namely the arbuscular mycorrhizal fungi (AMF) Glomus in corn roots and rhizosphere soils, relative to their non-Bt isolines 5422 (conventional parent) and 5422wx (conventional hybrid). AMF colonization in roots of different corn genotypes was also assessed using microscopic visualization. No adverse effect was detected on the indigenous AMF colonization of the roots of Bt hybrids 5422Bt1 and 5422CBCL. Two-way indicator species analysis (TWINSPAN ®) and detrended correspondence analysis (DCA) of the DGGE data from corn roots presented differences between Bt and non-Bt corn isolines (5422Bt1 vs. 5422wx, and 5422CBCL vs. 5422wx). However, differences were also recognized between the two non-Bt corn cultivars (5422 vs. 5422wx), and between the two Bt corn lines (5422Bt1 vs. 5422CBCL) in roots. Our results suggest that corn genotypes may have a greater influence on the AMF community structure of plant roots and rhizosphere soils than other factors, such as the age of the growing plants.

INTERAÇÃO ENTRE TRANSGÊNICOS (BT) E INSETICIDAS NO CONTROLE DE PRAGAS-CHAVE EM HÍBRIDOS DE MILHO-SAFRINHA

RESUMO A cultura do milho é atacada por diversos lepidópteros-praga. Recentemente, foi liberada no Brasil a comercialização de híbridos de milho Bt, integrando em seu genoma o gene Cry1Ab, proveniente de Bacillus thuringiensis subsp. kurstaki, que codifica a proteína com efeito tóxico sobre insetos da ordem Lepidooptera. Dessa forma, o objetivo deste trabalho foi avaliar, em condições de campo, a eficácia de híbridos de milho convencionais e transgênicos para o controle de pragas, submetidos ou não à pulverização com inseticidas. O ensaio foi realizado em área experimental do Polo Apta Centro Norte, em Pindorama, Estado de São Paulo. Foram avaliados cinco híbridos de milho convencionais em comparação com os respectivos híbridos isogênicos Bt. A avaliação dos danos visuais de Spodoptera frugiperda (Smith) foi realizada a cada 10 dias, em 20 plantas das duas linhas centrais da parcela, e para Helicoverpa zea (Bod.) e Diatraea saccharalis (Fab.) na pré-colheita. As interações entre os híbridos e a transgenia ocorreram em todas as avaliações a partir dos 40 dia após emergência (DAE). Os híbridos transgênicos (Bt) avaliados apresentaram as menores notas de danos visuais de S. frugiperda em todas as avaliações. A transgenia também proporcionou menor percentagem de espigas danificadas por H. zea e S. frugiperda e de colmos atacados por D. saccharalis. Conclui-se que a transgenia proporciona redução nos danos visuais ocasionados pelas pragas, embora apresente desempenho diferente entre os híbridos.