Larval development of Spodoptera eridania and Spodoptera frugiperda fed on fresh ear of field corn expressing the Bt proteins (Cry1F and Cry1F + Cry1A.105 + Cry2Ab2)

Desenvolvimento larval de Spodoptera eridania e Spodoptera frugiperda alimentadas com espigas de milho verde expressando proteínas Bt (Cry1F e Cry1F + Cry1A.105 + Cry2Ab2)

ABSTRACT:

The objective of this study was to evaluate extent of larval period, larval survival (%), food consumption, and pupal biomass ofSpodoptera eridania and Spodoptera frugiperda (Lepidoptera:Noctuidae ) fed on fresh ears of field corn expressing Bt proteins (Cry1F and Cry1F+Cry1A.105+Cry2Ab2). Larvae ofSpodoptera spp. survived less than two days when they consumed Bt corncobs and showed 100% mortality.Spodoptera eridania reared on non-Bt corn cobs showed higher larval development (21.6 days) thanS. frugiperda (18.4 days) and lower viability (56.4% and 80.2% forS. eridania and S. frugiperda , respectively). A higher amount of corn grains was consumed byS. eridania (5.4g) than byS. frugiperda (3.9g). In summary, this study demonstrated that the toxins Cry1F and Cry1F + Cry1A.105 + Cry2Ab2 expressed in fresh corn cobs contributed to protect ears of corn against S. frugiperda and the non-target pestS. eridania . However, itis important to monitor non-Bt cornfields because of the potential of both species to cause damage to ear sof corn.

Key words:
biology; integrated pest management; fall armyworm; Southern armyworm; varietal resistance

RESUMO:

Este estudo teve como objetivo avaliar o período de desenvolvimento larval, sobrevivência larval (%), consumo de alimento e biomassa pupaldeSpodoptera eridania eSpodoptera frugiperda (Lepidoptera: Noctuidae ), alimentadas com grãos verdes de milho Bt que expressam as proteínas Cry1F e Cry1F + Cry1A.105 + Cry2Ab2). As larvas deSpodoptera spp. alimentadas com milho Bt sobreviveram por até dois dias, registrando-se 100% de mortalidade. A espécie S. eridania alimentada com grãos de milho não Bt apresentou um maior desenvolvimento larval (21,6 dias) do queS. frugiperda (18,4 dias), e menor viabilidade larval (56,4% e 80,2% paraS. eridania eS. frugiperda respectivamente). Uma maior taxa de consumo de grãos foi observada para S. eridania (5,4g) do que paraS. frugiperda (3,9g). Em síntese, este estudo comprova que as toxinas Cry1F e Cry1F + Cry1A.105 Cry2Ab2 expressas em grãos imaturos de milho contribuem na proteção das espigas de milho contraS. frugiperda e a praga não-alvoS. eridania , entretanto, é importante monitorar as lavouras cultivadas com milho não Bt, devido o potencial de ambas as espécies em ocasionar danos nas espigas da planta.

Palavras-chave:
biologia; manejo itegrado de pragas; lagarta-do-cartucho; lagarta-das-vagens; resistência varietal

For integrated pest management (IPM), the use ofresistant genetically modified organisms (GMOs) contributes in reducing the insecticides applications and environ mental impact. Cultivation of improved plants that express the insecticidal gene cry via the bacterium Bacillus thuringiensis Berliner (Bt) is performed almost worldwide (JAMES, 2013). Efficiency of the Cry1F protein for controlling Spodoptera frugiperda (Lepidoptera: Noctuidae ) has been reported (BUNTIN, 2008; SIEBERT et al., 2008), but it was observed when only maize leaves were offered. Recently, BORTOLOTTO et al. (2015) reported the slightly poor development of Spodoptera eridania (Lepidoptera: Noctuidae ) when fed with maize leaves that expressed single Cry1F and pyramided Cry1F + Cry1A.105 + Cry2Ab2 insecticidal proteins; however, the impactof these Bt proteins remains unclearbecause of high larval mortality even when non-Bt maize leaves were offered.

Spodoptera spp. can damage the reproductive structures of plants (KING & SAUNDERS, 1984), and there is little information on the development of the Spodoptera complex in corncobs; studies are required to formulate implementation strategies for IPM. In this study, we aimed to assess the impact of the Bt protein Cry1F and pyramided protein Cry1F + Cry1A.105 + Cry2Ab2 expressed in fresh corncobs on the larval development of S. eridania and S. frugiperda .

The study was performed in the entomology laboratory of Embrapa Soybean, and the experiments were performed using BOD type climatic chambers under the following controlled environmental conditions: temperature, 25±2°C; relative humidity, 60±10%; and photoperiod, 14/10h (L/D). Larvae of S. eridania and S. frugiperda were obtained by mass insect rearing at Embrapa Soybean, where the insects were reared for about 20 generations.

The two Bt corn isolines used in the study were as follows: Herculex^(r) I (event TC1507), which contained the gene for the insecticidal protein Cry1F, and genotype PowerCore^(r) (event MON89034 x MON00603 x DAS01507), which contained five genes with stacking traits that code for three different Bt proteins (Cry1F + Cry1A.105 + Cry2Ab2) with insecticidal effects and two genes (PAT + EPSPS) that code for tolerance to the herbicides glyphosate and glufosinate. The non-Bt isoline used was genotype 2B688 DOW. Sowing of corn genotypes was performed in a greenhouse. After seedling emergence, irrigation was performed daily. Chemical fertilization was performed according to the regional recommendation (FANCELLI & DOURADO-NETO, 2000). Corncobs were harvested in the dough stage (paste-like) and immediately peeled to feed S. eridania and S. frugiperda larvae.

Comparison of the biological aspects of S. eridania and S. frugiperda larvae was carried out using a completely randomized experimental design, with three treatments (two isogenic Bt corn genotypes and one non-Bt corn genotype) and 11 replications. Each replicate was composed of eight larvae. Newly hatched larvae (until 24 hours of age) were individually addedusing a thin-tip brush (0.6mm) into paraffin cups (capacity, 50ml) for each treatment, resulting in a total of 88 larvae for each treatment. Larvae for each replicate were fed with pieces of fresh corncobs prepared previously (treatments). Fresh corncobs were husked and sliced into pieces for the larvae. The food was replaced daily at the same time, and larval instar and larvae mortality rate were assessed daily until the larvae reached the pupal stage.

When the larvae reached the fifth instar stage, consumption of corn grains was assessed by the weight difference of corncobs sliced before feeding and the weight of the same corncobs 24h after feeding. Pieces of corncobs were weighed before they were offered to the fifth instar larvae to verify the consumption rate, and the weight of food consumed was corrected with the value obtained for the weight of corn grains used in the control treatment (without larva) to assess the weight lost by the grains due to dehydration. Larvae were fedwith fresh corn grains ad libitum until they completed the larval phase, i.e., until the pre-pupal phase. Results obtained for S. eridania larvae were then comparedwith those obtained for S. frugiperda larvae. Within 24h after the larvae reached thepupal stage, the individuals were weighedusing an analytical balance with 0.001g of precision. Parameters assessed for both species were as follows: extent of the larval period (days); larval survival (%); grain consumption (g); and pupal biomass(g).

Results of the different bioassays were subjected to exploratory analysis to assess the assumptions of normality of residuals, homogeneity of variance of treatment, and additivity of the model to allow for application of analysis of variance. Mean values were compared using Tukey's test. To compare biological parameters and grain consumption, Student' st-test was used. For both tests, the difference was considered significant only when the significance level was equal to P≤0.05.

This study demonstrated 100% mortality for S. eridania and S. frugiperda fed on fresh grains to both Bt events with expression of a single (Cry1F) and pyramided event (Cry1F + Cry1A.105 + Cry2Ab2) (Table 1). Thus, although S. eridania is a non-target pest, these proteins act againstits development. When larvae were fed with fresh grains of the non-Bt corn genotype, a large percentage of the larvae were able to complete larval stage; this indicates that the grains are nutritionally suitable for the development of S. eridania larvae. This is an important observation because S. eridania larvae has a poor development when fed on maize leaves (BORTOLOTTO et al., 2015). Higher corn cob consumption by S. eridania rather than by S. frugiperda was observed, but lower pupal biomass was reported for S. eridania . Nevertheless, the pupal biomass of S. eridania collected in the present study is similar to that of S. eridania observed on suitable hosts, such as cotton bolls and morning glory (SANTOS et al., 2005). So, is possible that the higher consumption of corncobs might be associated with toxins or nutritional imbalance present in the food (LARA, 1991), suggesting a poor quality to larval development of S. eridania . In this sense, the lower pupal biomass of S. frugiperda could be attributed to the biological differences between the species and adaptation to consume ears of field corn.

Table 1
Biological aspects of Spodoptera eridania and Spodoptera frugiperda (Lepidoptera: Noctuidae ) fed with Bt and non-Bt fresh corncobs.

• ¹Means ± SEM followed by the same letter in the same stage of development of the insect column do not differ by Tukey test (P<0.05). ²Means ± SEM followed by the same letter in the column do not differ by t-test (P<0.05).
• ³Parameter non existent.

Efficiency of Cry1F protein in controlling S. frugiperda has been reported in other studies (BUNTIN, 2008; SIEBERT et al., 2008), but it was observed when to S. frugiperda were offered corn leaves only. There fore, this is the first study in which insecticide proteins (single Cry1F and pyramided Cry1F + Cry1A.105 + Cry2Ab2) expressed in fresh corncobs helped to kill S. frugiperda like thenon-target pest S. eridania . There wasgenetic variability in different populations that can show resistance to maize Bt. For example, FARIAS et al. (2014) recently reported S. frugiperda populations that were resistant against Cry1F protein expressed in corn leaves. It is crucial to adopt integrated resistance management IRM, monitoring pest infestation in crop fields, and adoptinga refuge area to less risks of increase resistance.

Overall, although both Bt events showed similar effects in Spodoptera spp. larvae, it is usually recommended to prefer pyramided events because of the low risk of resistant development. This is reinforced by a previous study that showed reduced damage of corncobs by S. frugiperda in Bt pyramided (expressing Cry1A.105 e Cry2Ab2) cornfields when compared with single Bt corn Cry1A(b) (WAQUIL et al., 2013). Thus, Bt pyramided corn allowed to reduce the refuge area in which the number of pests that survive is lower than that in a field which a single Bt corn has been cultivated (SIEBERT, 2012). Finally, this study indicated that Cry1F and Cry1F + Cry1A.105 + Cry2Ab2 expressed in corncobs helped to control S. eridania and S. frugiperda larvae . However, non-Bt corn fields should be constantly monitored, since these larvae have the potential to damage corn plants.

ACKNOWLEDGEMENTS

Authors wish to thank Embrapa Soybean, the sponsor agencies Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support.

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Publication Dates

• Publication in this collection
  01 Aug 2016
• Date of issue
  Nov 2016

History

• Received
  26 Oct 2015
• Accepted
  25 Apr 2016
• Reviewed
  30 June 2016