Population fluctuation of Spodoptera frugiperda eggs and natural parasitism by Trichogramma in maize

Flutuação populacional de ovos de Spodoptera frugiperda e parasitismo natural por Trichogramma em milho

CROP PROTECTION

Population fluctuation of Spodoptera frugiperda eggs and natural parasitism by Trichogramma in maize

Flutuação populacional de ovos de Spodoptera frugiperda e parasitismo natural por Trichogramma em milho

Sônia Thereza Bastos Dequech^I,*; Cátia Camera^I; Vinícius Soares Sturza^I; Leandro do Prado Ribeiro^II; Ranyse Barbosa Querino^III; Sônia Poncio^I

^IDepartamento de Defesa Fitossanitária, Universidade Federal de Santa Maria, Av. Roraima, 1000, 97105-900, Santa Maria, Rio Grande do Sul, Brazil

^IIDepartamento de Entomologia e Acarologia, Escola Superior de Agricultura "Luiz de Queiroz", Universidade de São Paulo, Piracicaba, São Paulo, Brazil

^IIIEmpresa Brasileira de Pesquisa Agropecuária, Teresina, Piauí, Brazil

ABSTRACT

This study aimed to evaluate the fluctuation of S. frugiperda eggs and natural parasitism by Trichogramma and identify and quantify parasitoid species in maize fields in the state of Rio Grande do Sul, Brazil. Surveys were carried out in two growing seasons: in 2006/2007 we assessed one growing season (late sowing) in Santa Maria, and in 2007/2008 we assessed two growing seasons (early and late sowing) in Santa Bárbara do Sul. Daily air temperature and rainfall were recorded at both locations. In each evaluation, entire plants were examined for the presence or absence of S. frugiperda egg masses, which were analyzed for larvae hatching or parasitoid emergence. The number of S. frugiperda eggs is higher between 8 and 30 days after plant emergence, decreasing in the later stages of maize growth; the occurrence of parasitoids begins about two days after pest egg occurrence; S. frugiperda egg parasitism is low; and eggs are parasitized by Trichogramma pretiosum and Trichogramma atopovirilia, with marked predominance of the first and possible occurrence of both parasitoid species in the same egg mass.

Keywords: biological control, fall armyworm, Noctuidae, Trichogrammatidae, Zea mays L.

RESUMO

O presente trabalho objetivou estudar a flutuação de ovos de S. frugiperda e do parasitismo natural por Trichogramma, além de identificar e quantificar as espécies de parasitoides encontradas em cultivos de milho no Rio Grande do Sul. Para isso, foram realizados levantamentos em dois anos agrícolas: 2006/2007 numa época de semeadura (do tarde) em Santa Maria, e 2007/2008 em duas épocas de semeadura (do cedo e do tarde) em Santa Bárbara do Sul. Diariamente, a temperatura do ar e a precipitação pluviométrica foram registradas em ambos os locais. Em cada avaliação, toda a parte aérea das plantas foi examinada quanto à presença ou ausência de posturas de S. frugiperda, as quais foram analisadas quanto à eclosão de lagartas ou à emergência de parasitoides. O número de ovos de S. frugiperda foi maior entre 8 e 30 dias após a emergência das plantas, diminuindo nas fases mais adiantadas da cultura; a ocorrência de parasitoides iniciou cerca de dois dias após a ocorrência de ovos da praga; o parasitismo de ovos de S. frugiperda é baixo; e os ovos são parasitados por Trichogramma pretiosum e Trichogramma atopovirilia, com acentuado predomínio do primeiro e com a possibilidade de ocorrência de ambas as espécies de parasitoides em uma mesma postura.

Palavras-chave: controle biológico, lagarta-do-cartucho, Noctuidae, Trichogrammatidae, Zea mays L.

Introduction

Spodoptera frugiperda (J. E. Smith, 1797) (Lepidoptera, Noctuidae), also known as fall armyworm, is the main insect pest of maize in Brazil. It occurs in all producing regions during early and late sowings and causes plant damage from plant emergency to reproductive phases (BESERRA et al., 2002). S. frugiperda lays its egg masses at night in layers (three or more) (LEIDERMAN; SAUER, 1953), preferring the lower regions of the plant and the abaxial leaf surface during the early stages of maize growth (V4-V6 stages) (BESERRA et al., 2002). In later stages (V8-V10), the eggs are found in the middle and upper plant regions and on adaxial leaf surfaces.

Because they are poliphagous and resistant to the control methods used (DIEZ-RODRÍGUEZ; OMOTO, 2001), problems with S. frugiperda have intensified, necessitating the use of alternative tools for integrated pest management (IPM), such as natural or applied biological controls. In the presence of natural enemies, fall armyworms cause less damage and may make insecticide spraying unnecessary (FIGUEIREDO et al., 2006b).

Although the literature describes a great number of S. frugiperda parasitoids (ANDREWS, 1988; FIGUEIREDO et al., 2006a), few have been effectively researched in Brazil, especially for maize crops (SÁ et al., 1993; FIGUEIREDO et al., 1999).

Among these, microhymenopteran egg parasitoid Trichogramma (Hymenoptera, Trichogrammatidae) species have been widely used in biological control programs in several regions of Brazil; inundative release might provide great results because this parasitoid reduces insect pest populations in the field before they enter the larval phase (NAVA; NACHTIGAL, 2010; ZUCCHI; PARRA, 2004). In Rio Grande do Sul (RS) State, such extension programs are developed by local technical assistance and rural extension services (EMATER), especially with small producers. However, the biological material used comes from other Brazilian regions, which reduces the chances of success because it disregards species and races that occur naturally under subtropical conditions that are adapted to maize fields in southern Brazil.

So far, records of natural parasitism by Trichogramma spp. in S. frugiperda eggs in Brazil include: Trichogramma atopovirilia Oatman & Platner, 1983, Trichogramma pretiosum Riley, 1879 and Trichogramma rojasi Nagaraja & Nagarkatti, 1973 (CAMERA et al., 2010; ZUCCHI et al., 2010). Thus, we aimed to identify and quantify parasitoid species in S. frugiperda eggs, as well as evaluate parasitoid fluctuation and its relationship with the host (S. frugiperda) egg fluctuation in maize crops in Rio Grande do Sul State.

Material and methods

The study was undertaken in Rio Grande do Sul State during two growing seasons: 2006/2007 in Santa Maria city during one season (late sowing), and in 2007/2008 in Santa Bárbara do Sul city during two seasons (early and late sowing) (Table 1). The climate in both areas according to the Köppen classification, is 'Cfa', which is characterized by rain throughout the year and a mean temperature greater than 22ºC in the hottest month (MORENO, 1961).

Both regions cultivated the hybrid maize Pioneer 3069 in an area of 30 × 30 m (900 m²). All the local technical recommendations for this culture were adopted except those regarding insect pest control. The experimental design included randomized blocks with samples in plots. Each crop season was considered a repetition. All areas were divided into 16 plots, and six plants in each plot were assessed for a total of 96 plants per area. In each evaluation, all aerial parts of the plant were examined to determine whether S. frugiperda egg masses were present. Initially, the assessments were made on a daily basis, but this frequency was reduced over time as the number egg masses declined. The S. frugiperda egg masses and part of the attached leaf were collected, stored in individual plastic bags, numbered, and transferred to the laboratory.

In the laboratory, the egg masses were labeled and individually packed in jelly capsules (0.8 cm diameter × 2.0 cm length). Afterwards, we determined the total number of eggs in each egg mass by counting hatched larvae, emerged parasitoids, no hatched eggs and the number of eggs with a parasitoid exit hole. The emerged parasitoids were identified according to Querino and Zucchi (2011).

For Santa Maria, data about air temperature were collected from the 8^th District of Meteorology (DISME) station located at the Federal University of Santa Maria. It is nearly 10 kilometers from the experimental area, and the precipitation data were collected in loco through a pluviometer (rain gauge). In Santa Bárbara do Sul, the air temperature and precipitation data were collected with a digital thermometer and a pluviometer, respectively.

The data analysis was carried out with an 'SOC - NTIA' statistics package (EMBRAPA, 1997). The data obtained were tested for normality (Shapiro-Wilk test) and homogeneity of variances (Bartlett's test), and t-tests were performed to compare means. Differences were considered significant at p < 0.05.

The fluctuation of S. frugiperda egg masses (parasitized and total) was graphed along with temperature and precipitation data for the different counties. For each date, we compared the egg numbers and the meteorological data using Pearson's correlation tests.

Results and discussion

Number of S. frugiperda egg masses, eggs and parasitized eggs

The mean number of S. frugiperda egg masses per plant ranged from 0.58 in Santa Maria to 1.93 in Santa Bárbara do Sul, Rio Grande do Sul State, early sowing (ES). The number of eggs per plant ranged from 61.80 in Santa Maria to 270.00 in Santa Bárbara do Sul ES. In spite of the large variation in egg number, the mean number of parasitized eggs did not show a wide range and were between 1.46 and 1.97 per plant (Table 2).

The large number of eggs in Santa Bárbara do Sul ES and late sowings (LS) (Table 2) might be related to the fact that these two areas are located next to large fields of soybean crops. Maize is the favorite host for oviposition of S. frugiperda moths (PITRE et al., 1983), which are also found in soybean areas (CASMUZ et al., 2010).

The percentage of S. frugiperda parasitized eggs reflects the occurrence of natural parasitism in both places, corresponding to 2.36, 0.72 and 1.01% of eggs found in Santa Maria, Santa Bárbara do Sul ES and Santa Bárbara do Sul LS, respectively. These results were similar to parasitism rates of approximately 2.21% found by Beserra et al. (2002) in Brazil in the São Paulo State and by Toonders and Sánchez (1987), who observed values from 0 to 10% in Mexico.

Most of the collected egg masses had layers of eggs that were covered by scales. These variables might have influenced the low number of parasitized eggs, in accordance with Beserra et al. (2002) and Beserra and Parra (2004), who highlighted that S. frugiperda egg masses have three or more egg layers covered by scales. Such behavior is a strategy that females employ to avoid egg parasitism that prevents their progeny's development. Another aspect that might have led to low parasitism rates was noted by Noldus (1989), ho verified that S. frugiperda eggs are rarely attacked by T. pretiosum because this parasitoid does not respond to its semiochemicals.

Population fluctuation of S. frugiperda eggs and parasitism by Trichogramma spp.

Spodoptera frugiperda appeared in all areas, with egg masses from the fourth day after emergence (DAE) of maize plants (Figures 1, 2 and 3). This appearance was earlier than observed by Sá and Parra (1994), who recorded egg masses from the 11^th DAE.

The peak of S. frugiperda egg population (26.15 eggs plant^-1) occurred in Santa Bárbara do Sul ES at 13 DAE (Figure 2). In each location, the moth oviposition apparently followed a pattern in which the number of eggs was higher during the initial maize plant stages compared to the later stages, when a decrease in the egg population was observed. The population peaks occurred from 8 to 30 DAE (Figures 1, 2 and 3).

These values differed from those reported by Pitre et al. (1983), who found that S. frugiperda moths preferred maize plants between 54 and 64 days compared to younger ones (22 to 42 days).

This difference is likely due to the different meteorological conditions in the regions (e.g., temperature) because the period observed in the current paper corresponded to stage V4, which is the preferred stage for S. frugiperda oviposition (BESERRA et al., 2002).

In contrast, parasitism occurred from 6 to 26 DAE in Santa Maria (Figure 1), from 20 to 33 DAE in Santa Bárbara do Sul ES (Figure 2) and from 19 to 45 DAE in Santa Bárbara do Sul LS (Figure 3). Sá and Parra (1994) observed parasitism from 12 to 53 DAE, a period that is similar to what we observed in the present study considering all three areas. Natural parasitism was low during crop development and was not observed in some periods, such as the initial vegetative phase, which has density-dependent synchronization with the host, and after 45 DAE. The maximum number of parasitized eggs was 0.35 per plant, and this occurred from 21 to 23 and 26 to 30 DAE in Santa Maria and in Santa Bárbara do Sul ES, respectively (Figures 1 and 2). The results suggest that in Rio Grande do Sul State, applied biological control programs using egg parasitoids such as Trichogramma should release natural enemies in maize crop areas beginning in the period within the first and the fourth week after crop emergence. This is when the population peaks occur and natural parasitism is low.

Apparently, the largest S. frugiperda egg populations were found when the pluviometric precipitation was lower (Figures 1, 2 and 3). However, the correlation analysis between the total number of eggs and the precipitation indicated that there was no linear relationship between these factors (Table 3). Conversely, the mean air temperature during the crop cycles did not seem to influence the S. frugiperda population. However, the data collected from Santa Bárbara do Sul during LS suggest a negative correlation of temperature and the total number of eggs of S. frugiperda (Table 3).

S. frugiperda egg parasitoids

Among 500 S. frugiperda parasitized eggs, including those that showed evidence of emergence and those that only had vestigial exit holes, we were able to specifically identify 141 parasitoids.

All emerged parasitoids belong to Trichogrammatidae family; 117 were Trichogramma pretiosum Riley (1879), and 24 were Trichogramma atopovirilia Oatman & Platner (1983), corresponding to 82.73 and 17.27%, respectively. A total of 45 egg masses which there was emergency of parasitoids (13 in Santa Maria, 20 in Santa Bárbara do Sul ES and 12 in Santa Bárbara do Sul LS) and six egg masses (13.33%) were parasitized simultaneously by both species.

The predominance of T. pretiosum (93.79% of parasitized eggs) was also reported by Beserra et al. (2002), in Piracicaba, São Paulo State, followed by T. atopovirilia (2.07%). Davies and Zalucki (2008) found that it accounted for 97% of collected parasitized sample in north Australia. In Venezuela, there is a reference of an occurrence of T. atopovirilia parasiting eggs of S. frugiperda (MORALES et al., 2007; RÍOS; TERÁN, 2003), but they did not quantify the level.

Despite being found in a smaller proportion, T. atopovirilia is more aggressive to S. frugiperda and presents a higher specificity than T. pretiosum (BEZERRA; PARRA, 2004) because it has a greater capability of parasitism in egg masses with different physical barriers (egg layers and scale densities over egg masses). Thus, programs of applied biological control in Rio Grande do Sul State must consider the usage of this parasitoid species for S. frugiperda control in maize crops once its adaptation is ensured based on its natural occurrence.

Conclusion

Based on these results, it is possible to conclude the following:

- the number S. frugiperda eggs is highest from 8 to 30 days after crop emergence and decreases in the advanced stages of maize development;

- the occurrence of parasitoids begins around two days after the insect pest eggs are laid;

- the natural parasitism of S. frugiperda eggs is low (between 0.72 and 2.36%);

- S. frugiperda eggs are parasitized by Trichogramma pretiosum and by Trichogramma atopovirilia; the first species is highly predominant over the second, and both can occur in the same egg mass;

- the occurrence of T. atopovirilia in Rio Grande do Sul State areas suggest that this parasitoid species must be considered in applied biological control programs of S. frugiperda.

Acknowledgements

The authors thank Dr. Sidinei José Lopes for supervising the statistical analysis.

Received on April 11, 2012.

Accepted on June 19, 2012.

License information: This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Publication Dates

• Publication in this collection
  25 Sept 2013
• Date of issue
  Sept 2013

History

• Received
  11 Apr 2012
• Accepted
  19 June 2012