Introduction
Tomato crops (Solanum licopercum L.) have a great economic and social importance worldwide (Jordão and Nakano 2002). The small tomato borer Neoleucinodes elegantalis (Guenée) (Lepidoptera: Crambidae) is a pest of economic importance due to the damage caused in the fruits and thus reducing the commercial value of the crop (Leiderman and Sauer 1953; Picanço et al. 2007). The distribution of N. elegantalis is strictly Neotropical including both Central and South America (Capps 1948).
The damage caused by N. elegantalis is direct, a single larva is sufficient to depreciate and render the fruit unsuitable for consumption (Toledo 1948). Yield losses are estimated at as much as 50% (Gallo et al. 2002), 79% (Miranda et al. 2005) and 90% (Carneiro et al. 1998). According to Blackmer et al. (2001) under field conditions the females lay their eggs preferably beneath the sepals of small (23.0 mm diameter) green fruits and soon after hatching the larva penetrates the fruit and causes the irreversible damage.
The knowledge on the biology of the small tomato borer is scarce; Marcano (1991a) evaluated the development time and reproduction of N. elegantalis on tomato fruits of the cultivar Rio Grande. Other studies include the ecology and behavior of N. elegantalis (Salas et al. 1991) and the role of pheromones (Badji et al. 2003). Blackmer et al. (2001) and Barbosa et al. (2010) evaluated the preference for oviposition of N. elegantalis on tomato and Jaffe et al. (2007) investigated the role of chemical stimulus on the sex attraction of N. elegantalis. Benvenga et al. (2010) conducted field sam- plings of the small tomato borer for decision making on the control in staked tomato plants.
Apart from tomato, N. elegantalis is able to develop and reproduce in other cultivated Solanaceae, green peppers, eggplant and scarlet eggplant (Toledo 1948, Zucchi et al. 1993). On scarlet eggplant Picanço et al. (1997) found all stages of N. elegantalis and described the tunneling caused by the larvae in the fruits. Uncultivated solanaceous plants also host the pest (Toledo 1948, Zucchi et al. 1993).
Due to the limited efficiency of chemical control measures against the damage of boring insects such as N. elegantalis, research on cultivar performance is a sound alternative to reduce the larval damage on tomato yield. This work was conducted to evaluate the effect of three tomato cultivars on the growth, reproduction and survival of N. elegantalis.
Materials and methods
A colony of N. elegantalis was established in laboratory from larvae collected on tomato hybrid Alambra crops in Southern Paraná state (25°19'S 49°18'W). Larvae were reared on tomato fruits of the group Salad. Field collected larvae were introduced into the colony every week in order to maintain the genetic diversity of the population. One cultivar (Santa Clara) and two hybrids (Giuliana and Paronset) were employed to compare the development cycle, reproduction and longevity of N. elegantalis. These materials were chosen for being commonly employed by local tomato growers and their high commercial value. Tomato plants were grown in a greenhouse in 5 L pots. Green tomato fruits, 23 to 40 mm in diameter from each cultivar were used in the experiment. For surface disinfection, the fruits were immersed in water + 1% sodium hypochlorite for five minutes, rinsed under running water and dried with paper towels. Three neonate larvae were incubated in each fruit totaling 99 larvae per tomato cultivar. The experiment was conducted in a climatic chamber at a constant temperature of 20 ± 1 °C, relative humidity of 60 ± 10% and photoperiod of 12:12 h (light: dark). At the end of the larval period, the insects left the tomatoes to pupate outside the fruits. Pupae were kept in 7x4 cm polyethylene vials and after 72 hours were weighed and checked for the presence of deformities. Sex determination was made after adult emergence according to Muñoz et al. (1991) and Carneiro et al. (1998).
The biological parameters evaluated were development time, larval and pupal viability, pupal weight and sex-ratio (females / males + females). To evaluate the influence of the cultivar consumed in the larval stage on adult performance, twenty pairs were formed from each treatment. Each pair was kept in polyethylene tubes 20 cm high x 10 cm diameter closed at the bottom and on the top with a polyethylene cover. Tomato leaves of each cultivar were placed inside the cages as stimulants for oviposition. The adults were fed with 10% honey diluted in water, provided in a cotton ball soaked in the solution. Food renewal and egg collection were carried out daily. The eggs were separated according to the female and kept in moistured paper towel to assess the incubation period and viability.
The effect of the cultivars consumed by larvae on the adult stage of N. elegantalis was evaluated through longevity, length of pre-oviposition and oviposition periods, number of eggs laid and their viability.
The experimental design was completely randomized, the condition of normality of the data was submitted to Shapiro-Wilks test. Results were subjected to analysis of variance and the means were compared by Tukey's test or nonparametric Kruskal-Wallis test (P<0.05). Data were analyzed with Statistica 7.0 software (Statsoft 2004).
Results and discussion
The incubation period of N. elegantalis at 20 ± 1 °C lasted from 7.3 to 7.4 days, did not differ significantly among the cultivars (Table 1). There are no other references on the development of N. elegantalis other than the work of Marcano (1991a) which describes an incubation period of 7.1 days at 20 °C.
Larval survival was not significantly different among the treatments (P = 0.162) (Table 1). The larval stage was significantly longer when the larvae were reared on Giuliana fruits (29.5 days) in comparison to Paronset (26.1 days) and Santa Clara (25.8 days) (Table 1). There are no data in the literature comparing the influence of different tomato cultivars on the biological traits of N. elegantalis. Marcano (1991a, 1991b) estimated the development of N. elegantalis on tomato and eggplant; when feeding on tomato, the larval period lasted 22.7 days compared to 31.4 days for larvae reared on eggplant fruits. According to Muñoz (1991) newly hatched larvae move by means of a silk thread until a point chosen for penetration through the epicarp and then feed on the mesocarp and later reach the endocarp where the rate of feeding is higher. In the endocarp the larvae tunnel galleries and feed on the placenta and seeds provoking most of the damage seen on the fruits.
Between the end of the larval stage and the beginning of the pupal phase N. elegantalis remained over five days in the pre-pupal stage (Table 1). No deformed pupae were recorded among the three cultivars. Likewise the larval development, adults took longer to emerge from the pupae proceeding from Giuliana in comparison to the other cultivars (Table 2). These values are similar to the ones reported by Marcano (1991a, 1991b) for eggplant and tomato fruits for the pupal stage of N. elegantalis (13.9 days). Paredes et al. (2010) also recorded a pupal period of 13 days for the species.
The sex-ratio of N. elegantalis was 0.4 in all cultivars. A predominance of males was also recorded by Salas (1992), Carneiro et al. (1998) and Jaffe et al. (2007). Fernández and Salas (1985) found in 688 sexed pupae 370 females and 318 males.
During the reproductive period, no statistical differences were observed among the treatments in relation to the preoviposition (P = 0.510) and oviposition (P = 0.299) periods, oviposition rate (P = 0.495) and egg viability (P = 0.813). Mating success of N. elegantalis is low in laboratory; only 10 out of 20 pairs evaluated laid eggs on Giuliana, 11 on Santa Clara and 13 on Paronset (Table 2). According to Jaffe et al. (2007) N. elegantalis copulates monogamously, more vigorous males have a faster answer to the sexual pheromone released by females, thus functioning as a process of natural selection. This behavior explains the low number of females ovipositing, since the couples were randomly formed and no option were given for the choice of partners by the females.
The pre-oviposition period (Table 2) was more than twice longer than the values reported by Marcano (1991a) and Muñoz et al. (1991). These authors used tomato leaves as stimulants for oviposition instead of fruits as we did and the longer pre-oviposition period may be due to the less stimulant effect of tomato fruits in comparison to leaves. The oviposition period in N. elegantalis lasted less than a week for females emerged from the three cultivars (Table 2), similar to the value found by Carneiro et al. (1998), but longer than the two days of oviposition described by Marcano (1991a).
The mean number of eggs laid by each female was more than twice as high in Giuliana and Paronset than in Santa Clara (Table 2). However due to the wide amplitude in the number of eggs laid per female, no statistical difference was detected among the treatments, indicating that the cultivar had no effect on the fertility of N. elegantalis. Marcano (1991a) obtained a mean of 52.3 eggs per female, also with a large variation in the number of eggs per female among the replicates (19 to 77 eggs/female).
No differences were recorded on egg viability among treatments, which ranged from 67.6% in females reared in the larval stage on Paronset to 79.0% in Santa Clara (Table 2). Similar values were described by Fernández and Salas (1985) in tomatoes, while Muñoz et al. (1991) recorded 98.0% of egg viability when the larvae were reared on nightshade fruits, a perennial solanaceaus species, Solanum quitoense grown in Northwestern South America.
The longevity of males and females was longer than 15 days, and no differences were found both among cultivars and between the sexes (Table 3). These values are more than twice the longevity recorded by Marcano (1991a, 1991b) for N. elegantalis kept at the same temperature and reared during the larval stage on eggplant and tomato fruit.
Development from egg hatching to adult emergence resulted significantly longer when larvae were reared on the cultivar Giuliana (56.2 days) in comparison to Paronset and Santa Clara (50.2 and 50.7 days, respectively). This difference may be due to the effect of antibiosis caused by cultivar, because according to Smith (2005) antibiotic effects may be expressed in a moderate way, increasing the duration of development and reducing weight or decreasing the adult life span. These results differ from those found by Marcano (1991a), which reported the development cycle in 43.7 days using cultivar Rio Grande. In the field the cultivar Santa Clara is considered as susceptible to the major insect pests and diseases of tomato (Bettiol et al. 2004) and thus is employed in laboratory tests as a pattern for susceptibility. According to the companies responsible for the hybrids (Sakata Seeds for Giuliana and Syngenta Seeds for Paronset), these hybrids possess genetic resistance to numerous diseases, but little is known about their tolerance to insect attack.
Previous results showed significant differences in the performance of N. elegantalis developing on different tomato cultivars (Lara et al. 1980, Moreira et al. 1985, Lyra Netto and Lima 1998). According to Viáfara et al. (1999) increased susceptibility of N. elegantalis to industrial tomato cultivars, is assigned to the higher content of soluble solids, which promote the feeding habits of the larvae.
Conclusion
The increased development time and the lower pupal weight of the small tomato borer reared on the hybrid Giuliana may reduce larval fitness, and thus varietal control can function as a strategy in programs of integrated pest management of N. elegantalis in tomato.
