Tetrastichus howardi Olliff (Hymenoptera: Eulophidae) parasitizes the larvae, pupae and adults of Diatraea saccharalis, and therefore seems to be a suitable candidate for the biological control of D. saccharalis in commercial sugarcane in Brazil and other industries where this stem borer is an important pest. The aim of our study was to analyze the biological characteristics of this natural enemy on sugarcane borer. The research was conducted in the Laboratory of Entomology/Biological Control (LECOBIOL) at the “Faculdade de Ciências Agrárias” of the “Universidade da Grande Dourados (UFGD)” in Dourados, Mato Grosso do Sul State, Brazil. Ten of each of the following life stages: 24-h old egg masses, fifth instar larvae, pupae and adults of D. saccharalis were isolated and exposed to parasitism by seven T. howardi parasitoids females. Parasitism rates by T. howardi of the adult, fifth instar larva and pupal stages of D. saccharalis were 2%, 56% to 68%, respectively. Emergence rates of 14%, 100% and 100% were recorded for adult, fifth instar larvae, and adults, respectively. The duration of each life cycle (egg to adult) of T. howardi on the pupae, fifth instar larvae and adults of D. saccharalis were 20 ± 0.03, 27.00 ± 0.01, 33 ± 0.00 days, respectively. Fecundity and the female-based sex ratio of T. howardi were greatest in the pupae of D. saccharalis, at 70.44 ± 5.22 and 0.85 ± 0.41, respectively. Parasitism and the emergence of T. howardi from the fifth instar larvae, pupae and adults of D. saccharalis revealed the ability of this natural enemy to establish itself in culture, even in the absence of host pupae.
Diatraea saccharalis (Fabricius) (Lepidoptera: Crambidae), a major pest of sugarcane in the Americas (White & Wilson 2008; Dinardo-Miranda et al. 2012; Svedese et al. 2013) is capable of causing loss in biomass, death of the apical meristem and reduction in sugar and alcohol production (Simões et al. 2012; Rossato et al. 2013).
Diatraea saccharalis caterpillars develop inside the sugarcane stalks, which diminishes the efficacy of the insecticides used to control them (Pinto et al. 2009); hence the interest in the use of biological control to suppress D. saccharalis infestations (Cruz et al. 2011; Rodrigues et al. 2013). Hymenopteran parasitoids are frequently used as natural enemies in numerous applied biological control programs targeting D. saccharalis (Zanuncio et al. 2008; Mafi & Ohbayashi 2010; Tavares et al. 2011, 2013).
The parasitoid Tetrastichus howardi Olliff (Hymenoptera: Eulophidae) has been recorded from the pupae of several lepidopteran families, including: Crambidae, Noctuidae and Plutellidae (Kfir et al. 1993; Moore & Kfir 1995; Kfir 1997; Baitha et al. 2004; Hayat & Shahi 2004; Prutz et al. 2004; Silva-Torres et al. 2010; Duong et al. 2011; Costa et al. 2014), and it has been used to effectively control several lepidopteran pests. Tetrastichus howardi was also found parasitizing D. saccharalis larvae (Vargas et al. 2011), which motivated this study on the parasitism of T. howardi on various life stages of this important sugarcane pest. The aim of this study was to evaluate the biological characteristics of T. howardi parasitizing D. saccharalis eggs, larvae, pupae and adults stages.
Experiments were performed in the Laboratory of Entomology/ Biological Control (LECOBIOL) of the “Faculdade de Ciências Agrárias” of the “Universidade Federal da Grande Dourados (UFGD)” in Dourados, Mato Grosso do Sul State, Brazil. Life stages of D. saccharalis (eggs, larvae, pupae and adults) were obtained from D. saccharalis rearing of LECOBIOL (Parra 2007). Tetrastichus howardi females were reared on D. saccharalis caterpillars (Vargas et al. 2011).
Each experimental parcel contained 10 of the following stages of D. saccharalis: egg masses, larvae, pupae or adults, which were individualized with 7 T. howardi females. Each experiment was replicated 5 times (50 D. saccharalis individuals in total were used per treatment).
The parasitism intervals were as follows: 24 h for D. saccharalis eggs and pupae and 96 h for fifth instar caterpillar larvae and adults. Following parasitism, the T. howardi females were removed, D. saccharalis stages were placed in glass tubes (13-cm height and 8.5-cm diam), and transferred to climatic chambers set at 25 ± 2 °C, 70 ± 10% RH and 14:10 h L:D.
Percent parasitism adjusting for natural host mortality (Abbott 1925), percentage of emergence, life cycle duration (from egg to adult), the number of parasitoids that emerged (progeny) from the different stages of D. saccharalis, and the sex ratio of T. howardi were evaluated. The sex of T. howardi was determined based on morphological characteristics (La Salle & Polaszek 2007) under a stereoscopic microscope.
Tetrastichus howardi did not parasitize D. saccharalis eggs. Parasitism by T. howardi was evident in fifth instar larvae (Figs. 1A, C and D), pupae (Fig. 1B, Figs. 2A, B and C) and adults of D. saccharalis (Fig. 2D) showing rates of parasitism of 2%, 56% and 68%, respectively. The emergence of T. howardi was 14%, 100% and 100% for fifth instar larvae, pupae, and adults of D. saccharalis, respectively.
The life cycle (egg-adult) durations of T. howardi in pupae, fifth instar larvae and adult stages of D. saccharalis were 20.00 ± 0.03, 27.00 ± 0.01 and, 33.00 ± 0.00 days, respectively. Tetrastichus howardi also showed a higher fecundity and sex ratio (females) with the D. saccharalis pupae at 70.44 ± 5.22 and 0.85 ± 0.41 individuals, respectively.
Tetrastichus howardi parasitized and developed within the larvae, pupae and adults of D. saccharalis. The greatest number of offspring and greatest sex ratio (female based) for this parasitoid was observed in the pupal stage of the sugarcane borer, which suggests that this host stage is the most suitable for parasitism and development by this natural enemy. The low parasitism rates of D. saccharalis larvae and adult stages by T. howardi females suggests that these life stages may possess a stronger immune response against this parasitoid, as reported for Cotesia flavipes (Cameron) (Hymenoptera: Braconidae) parasitizing the sugarcane borer larvae (Mahmoud et al. 2012). Cellular defenses, encapsulation and melanization of the endoparasitoid eggs (Pennacchio & Strand 2006; Andrade et al. 2010) can be some of the mechanisms involved in this phenomenon.
Tetrastichus howardi parasitized the fifth instar D. saccharalis larvae and emerged in the pupal stage. Also T. howardi parasitized young D. saccharalis pupae and emerged from the adults. This is the first time that this pattern of behavior and development of T. howardi has been reported.
Parasitism of the different biological stages of D. saccharalis by T. howardi revealed the ability of this natural enemy to regulate the development of various sugarcane borer life stages, and this can be attributed the longer life span of the adult stage of T. howardi (Vargas et al. 2011) compared with that of the egg parasitoids Trichogramma spp. (Hymenoptera: Trichogrammatidae) (Pereira-Barros et al. 2005; Oliveira et al. 2013) and the larval parasitoid C. flavipes (Simões et al. 2012). Thus, in sugarcane fields, this eulophid has more time than other studied parasitoid species to locate and parasitize its host.
Tetrastichus howardi displays high plasticity by parasitizing larvae, pupae and adults of D. saccharalis. However, further studies on the cell biology, physiology and ecology of this parasitoid are necessary to better understand the parasitoid-host relationships and to increase the chances of success of this important natural enemy in biological control programs.
Thanks to “Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)” and “Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)” for financial support. Global Edico Services ( www.globaledico.com) edited and proofread this manuscript.