JADER BRAGA MAIA¹, GERALDO ANDRADE CARVALHO², MARIA ISABELLA SANTOS LEITE³, RODRIGO LOPES DE OLIVEIRA⁴ and LETÍCIA MAKYAMA⁵

¹ Ph.D. candidate in Entomology at the Department of Entomology. Federal University of Lavras (UFLA). Cx. Postal: 3037, CEP: 37200-000, Lavras. Minas Gerais, Brazil. maiajader@yahoo.com.br Corresponding author.

² Dr. Prof. gacarval@den.ufla.br.

³ M.Sc. maryisabella@yahoo.com.br.

⁴ Undergraduate student of Agronomy. rodrigo_lopes_oliveira@yahoo.com.br.

⁵ Undergraduate student of Zootechny. lemakiyama@gmail.com.

Recibido: 21-nov-2009 • Aceptado: 1-sep-2010

Fall armyworm Spodoptera frugiperda (J.E. Smith, 1797) (Lepidoptera: Noctuidae) is the most common pest to affect corn crops, being present every crop year and causing increasingly extensive damage to plantations, potentially affecting as much as 37% of the crop (Mendes et al. 2008). That and other pests of corn crops are mainly controlled by chemical products, which in turn have been causing emergence of secondary pests, resurgence and selection of resistant populations, in addition to causing a negative impact on the environment (Economic and Social Research Council 2009). Bearing that in mind, the use of alternative pest control methods to protect corn crops is a matter of the greatest importance as an effort to minimize these damaging effects, including biological control using parasitoids and predators.

Among natural enemies that help control fall armyworm in corn crops are parasitoids of the genus Trichogramma, drawing worldwide attention for being egg parasites and killing their hosts before emergence and attack to the plant (Lundgren et al. 2002).

Some authors noted the presence of Trichogrammaatopovirilia Oatman & Platner,1983 (Hymenoptera: Trichogrammatidae) parasitizing eggs of S. frugiperda in field conditions, demonstrating their potential as pest control agents in corn crops (Alvarez and Roa 1995; Zucchi and Monteiro 1997). Beserra and Parra (2004) evaluated the parasitic capacity and number of adults emerging per egg of S. frugiperda in a laboratory and observed that mean values were higher for T. atopovirilia than for Trichogramma pretiosum Riley, 1879 (Hymenoptera: Trichogrammatidae). They reported that T. atopovirilia has stronger chances of increasing its population in a shorter time than T. pretiosum, thus being apparently more suitable for biological control of S. frugiperda.

The effectiveness of these parasitoids in integrated pest control management programs, however, is conditional on the use of chemical products that will not affect the parasitism and development of parasite populations, in other words, it is conditional to the application of selective compounds (Carvalho et al. 2002; Degrande et al. 2002; Foerster 2002).

In studying the effect of 40 agrochemicals in various commercial formulations on adult individuals of T. atopovirilia, Manzoni et al. (2007) verified that 45% (18) were found to be harmless, 15% (6) were slightly harmful, 12.5% (5) were moderately harmful and 27.50% (11) were harmful. The variations in how this parasitoid species responded to pesticides demonstrate the need for studies in search of new molecules to find compatibility between chemical and biological methods for this particular parasitoid species.

Given the above, the objective of this work is to evaluate the effect of new insecticides recommended for corn crops on T. atopovirilia in order to gather information that will help decision making as to which pesticides to use in pest management programs involving corn crops, ultimately seeking to preserve this parasitoid species.

The following insecticides were used in this study (technical and commercial name, formulation, dosage and chemical group are listed for each one): imidacloprid/β-cyfluthrin (Connect 100/12.5 SC - 0.33/0.04 g a.i. L^-1, Neonicotinoid/ Pyrethroid), chlorfenapyr (Pirate 240 SC - 0.6 g a.i. L^-1, Pyrazole Derivative), chlorpyrifos (Astro 450 EW - 0.75 g a.i. L^-1, Organophosphate), novaluron (Rimon 100 CE - 0.05 g a.i. L^-1, Benzoylurea), spinosad (Tracer 480 SC - 0.16 g a.i. L^-1, Spinosyn) and triflumuron (Certero 480 SC - 0.048 g a.i. L^-1, Benzoylurea). Water was used as control treatment.

Twenty females up to 24 hours old per treatment were collected from a laboratory nursery and were placed separately in 8 cm x 2.5 cm glass tubes, fed with honey droplets smeared on the inside of the tubes which were then sealed with PVC film. Approximately 125 eggs of Anagasta kuehniella (Zeller, 1879) (Lepidoptera: Pyralidae) up to 24 hours old were glued to 5 cm x 0.5 cm strips of blue cardstock and placed under a germicidal lamp to kill embryos according to Parra (1997), then pulverized with chemical compounds using a Potter spray tower regulated at a pressure of 15 lb/in² and applied volume of 1.5 ± 0.5 μL/cm².

The treated eggs were placed in an environmental chamber at 24±2oC, RH 70±10%, with 14 hours of photophase, being then offered to female individuals of T. atopovirilia 24, 48, and 96 hours after application of the insecticides, for 24 hours. The female individuals were then kept in the same tubes in order to assess their mortality throughout, while the cards containing the supposedly parasitized eggs were transferred into new tubes and stored in an environmental chamber until emergence of F₁ generation parasitoids. Items being assessed included daily mortality over eight days, parasitic capacity, and percentage of insect emergence.

To evaluate the effects of pesticides on the newly emerged F₁ generation adults originating from the eggs of A. kuehniella previously treated and then exposed to parasitism 24, 48, and 96 hours after application of the products, twenty female individuals of T. atopovirilia per treatment were placed in separate glass tubes, each receiving 125 untreated eggs of A. kuehniella up to 24 hours old, glued to card strips and placed under a germicidal lamp to kill embryos, as mentioned previously. The parasitic period was 24 hours, at the end of which the females were discarded and the card strips containing the supposedly parasitized eggs were kept in an environmental chamber under the above mentioned conditions, until full development and emergence of F₂ generation parasitoids was reached. Items assessed included parasitic rate of F₁ generation females and emergence percentage of F₂ generation specimens.

Each treatment consisted of five replicates and each experimental portion comprised four card strips with eggs previously offered to the wasps for parazitation. The bioassays used a completely randomized design in a three exposure times x seven compounds factorial arrangement, totaling 21 treatments.

The data were submitted to analysis of variance and the mean values were compared using the Scott-Knott cluster analysis at the 5% significance level (Scott and Knott 1974). The evaluated pesticides were further grouped into the following toxicity categories as a function of the reduction in parasitoid survival rate according to IOBC recommendations: class 1 = harmless (<30% reduction), class 2 = slightly harmful (30% to 79% reduction), class 3 = moderately harmful (80% to 99% reduction) and class 4 = harmful (>99% reduction) (Sterk et al. 1999, Van de Veire et al. 2002). The control treatment was used as reference parameter. The mean percentage of reduction in parasitoid survival was obtained using the following equation: % of reduction = 100 – [(% general mean of treatment with pesticide / % general mean of control treatment) x 100].

The parasitic capacity of female individuals of T. atopovirilia that had come into contact with host eggs 24, 48, and 96 hours after their contamination was reduced by virtually every compound, although triflumuron and novaluron allowed 100% parasitism and thus these were classified as harmless (class 1); the remaining products were considered moderately harmful (class 3) (Table 1). These results agree with Carvalho et al. (2001) and Parreira (2007), who failed to identify negative effects of triflumuron and novaluron respectively on the parasitic capacity of T. pretiosum when exposed to eggs of A. kuehniella 24 and 48 hours after contamination with the product.

A substantial decrease in the number of eggs parasitized by parent generation females of T. atopovirilia when exposed to eggs treated with chlorpyrifos 24, 48 and 96 hours after contamination was also verified by Moscardini et al. (2008). These authors evaluated fenitrothion and methidathion, organophosphate products, and concluded that the reduced parasitism probably resulted from the mortality inflicted on female individuals, whether by direct contact with residues or by ingestion of such products during the parasitic process.

As for emergence of F₁ generation parasitoids, chlorfenapyr was noted to be slightly harmful to females exposed to host eggs 24, 48 and 96 hours after application of the compound. Insecticide novaluron was slightly harmful, noting that 96 hours after application it presented one of the lowest mean values of emergence, approximately 31.3%. The remaining compounds were rated as harmless (class 1) (Table 2).

Parreira (2007) verified that novaluron caused a reduction in the emergence percentage of F₁ generation individuals of T. pretiosum when exposed to treated eggs of alternative host A. kuehniella, similarly to the result found in this work, where novaluron was rated as slightly harmful (class 2) (Table 2).

The parasitic capacity of F₁ generation females of T. atopovirilia could not be evaluated in chlorfenapyr and chlorpyrifos treatments when the parent generation females were exposed to contaminated eggs 24, 48 and 96 hours after contamination due the high mortality inflicted by these products soon after insect emergence (Table 3).

None of the insecticides were noted to affect the number of eggs parasitized by the F₁ generation of T. atopovirilia 24 hours after application of the compounds, with novaluron causing a decreasing reduction in the number of parasitized eggs throughout the evaluations. When parasitoids came into contact with host eggs 48 and 96 hours after contamination with triflumuron and novaluron, no reduction occurred in the parasitic rate, with mean values 46,9 and 43,9, and 21,1 and 22,4 for parasitized eggs / female respectively (Table 3).

Similar results for triflumuron and novaluron were found by Carvalho et al. (2001) and Stefanello Jr et al. (2008), who, in evaluating the effects of these products on the parasitic capacity of T. pretiosum on treated eggs of A. kuehniella, observed that they were slightly harmful.

As for the emergence of F₂ generation of T. atopovirilia originating from F₁ generation females that had come into contact with eggs of A. kuehniella 24 hours after their contamination, it was noted that parasitoid emergence was not affected, with mean values ranging from 96.8% to 99.2% (Table 4).

Chlorfenapyr and chlorpyrifos caused 100% of mortality in F₁ generation insects immediately after their emergence (Tables 3 and 4), preventing evaluation of the emergence percentage of F₂ generation specimens. Similar results were found by Moscardini et al. (2008), who, in evaluating the effect of fenitrothion and methidathion on T. pretiosum, failed to obtain the emergence percentage of F₂ generation specimens due to 100% of mortality in F₁ generation parasitoids.

Spinosad, triflumuron and novaluron did not affect the emergence of F₂ generation females throughout the evaluation period, being rated as harmless (class 1). The evaluation at 96 hours revealed that spinosad and imidacloprid reduced the percentage of parasitoid emergence, with mean values 76.7% and 58.9% respectively (Table 4).

Parreira (2007) evaluated the action of triflumuron, imidacloprid and novaluron on the emergence of F₂ generation specimens of T. pretiosum originating from F₁ generation females that had come into contact with eggs of A. kuehniella 24 and 48 hours after contamination and noted that these compounds did not cause reduction in the emergence percentage, being thus rated as harmless. Carvalho et al. (2003) studied the effect of triflumuron on T. pretiosum and also observed innocuousness when this parasitoid was exposed to eggs of alternative host A. kuehniella 24 and 48 hours after application of the product, causing no reduction in the emergence of F₂ generation insects.

As a function of the reduction in emergence percentage of F₂ generation females as caused by spinosad, triflumuron, imidacloprid/β-cyfluthrin and novaluron, these products were rated as class 1 = harmless (Table 4).

Spinosad and chlorpyrifos caused 95% and 100% of mortality in the fourth evaluation (4 days after application) and on the first evaluation (24 hours after application) respectively, while triflumuron caused 65.0% of mortality eight days after application. Imidacloprid/β-cyfluthrin only caused 10.0% of mortality 24 hours after application, increasing throughout the evaluation period to 95.0% of mortality on day eight. Novaluron revealed a similar pattern to the control treatment, presenting a peak mortality of 70,0% (Fig. 1A).

Chlorfenapyr and chlorpyrifos caused 100% of mortality in insects right in the first evaluation, while spinosad caused 100% of mortality in the second evaluation. Triflumuron and novaluron revealed a similar pattern to the control treatment, with mean values of mortality at around 95.0% and 80.0% on the last evaluation day respectively (Fig. 1B).

Chlorpyrifos caused 100% of mortality 24 hours after being applied, while chlorfenapyr caused 60.0% of mortality in the first evaluation, reaching 100% on day three. Spinosad presented 90.0% of mortality right on day one and 100% on day four. Triflumuron, imidacloprid/β-cyfluthrin and novaluron caused 40.0%, 20.0% and 55.0% of mortality respectively in the first evaluation, increasing to around 95.0% on the last day. The control treatment presented 5.0% of mortality in the first evaluation, increasing to around 85.0% on the last day (Fig. 1C).

The results found in this work can contribute to a better understanding of the potentialities of combined use of parasitoids T. atopovirilia and selective compounds, ultimately seeking to control fall armyworm infestation in corn crops. However, additional studies are required using natural hosts, preferably under field conditions, to finally validate or refute the toxicity of compounds to this beneficial species.

Many thanks to the Research Aid Foundation of Minas Gerais State (FAPEMIG) and CNPq for providing the necessary financial support to accomplish this work.

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