Introduction

The recent increase in international trade, mainly of vegetal products, has generated concern due to the possibility of threats by introducing exotic pests (Dias 2000). In this context, eucalyptus forests have been seriously threatened by exotic invasive pests in the last two decades (Wingfield et al. 2001; 2008).

When attacked by insects, forest plantations can suffer a significant reduction in the quantity and quality of wood produced (Candy et al. 1992; Ohmart 1990; Shepherd 1994; Zobel et al. 1987). In recent years some exotic species have compromised Brazilian plantations, such as the redgum lerp psyllid. In 2008, the eucalyptus bronze bug, Thaumastocoris peregrinus Carpintero and Dellapé 2006 (Hemiptera: Thaumastocoridae), was detected in Brazil (Wilcken et al. 2010).

The damage caused by T. peregrinus to eucalyptus trees includes silvering chlorosis followed by the bronzing and drying of leaves. These symptoms occur due to the bug’s feeding habit, which punctures the leaves and twigs to suck the sap, leaving them chlorotic (Button 2007; Wilcken et al. 2008; 2010).

The hemipterans from the family Thaumastocoridae correspond to small phytophagous bugs (Carpintero and Dellapé 2006; Jacobs and Neser 2005). Biological data on insects in the Thaumastocoridae family are scarce (Cassis et al. 1999). Most of the studies have been of species of the subfamily Xylastodorinae, which occurs in South America feeding on palms (Noack and Rose 2007). Recently Noack et al. (2011) published a systematic revision of Thaumastocoris and described nine new species.

Some biological parameters of species of the Thaumastocorinae and Xylastodorinae subfamilies were studied over the years (Couturier et al. 2002; Drake and Slater 1957; Hill 1988; Kumar 1963; Slater 1973). The first study of the biology of the genus Thaumastocoris was conducted by Noack and Rose (2007), but it was performed at variable temperatures on the species Eucalyptus scoparia, for use in urban forestry and is less representative than the species used in large forest plantations of South America.

Knowledge of the biological aspects of T. peregrinus constitutes a basic and essential tool for developing strategies for monitoring and control. In this context the present study aimed to determine the biological development of T. peregrinus in different eucalyptus species and hybrids planted in Brazil.

Materials and methods

The study was conducted under laboratory conditions. The species and hybrids of eucalyptus used in the experiment were provided by the forest species arboretum, situated in an experimental area of the School of Agronomic Sciences—UNESP—Campus of Botucatu. The collected leaves were transported to the laboratory, cleaned in running water, dried, and cut in discs of 3.2 cm diam with a hole-puncher.

The eucalyptus species and hybrids used were E. camaldulensis, E. urophylla, E. grandis, and the following clones: ‘1277’ (Hybrid E. grandis x camaldulensis—HGC), ‘VM-1’ (Hybrid E. urophylla x camaldulensis—HUC) and ‘H-13’ (Hybrid E. urophylla x grandis—HUG).

The study was initiated utilizing eggs collected in the field, and methodology adapted from Firmino-Winckler et al. (2009), but with several modifications due to peculiarities of T. peregrinus. The leaves, containing the egg masses, were cut and placed in petri dishes 15 cm in diam, on a leaf of the eucalyptus species to be tested. Through daily observations the newly hatched nymphs were transferred to the assay, for a total of 100 nymphs per treatment. The experiment was carried out in a climate-controlled chamber with a temperature of 26 ± 1°C, 12-h photophase and 60 ± 10% r.h.

Nymph phase

Newly hatched nymphs were placed singly on a eucalyptus leaf disc 3.2 cm in diam, placed in a petri dish 3.5 cm in diam containing a slide of distilled water used to maintain leaf turgor. The dishes were placed in plastic trays 26 cm long x 17 cm wide x 13 cm high, covered with a perforated lid and lined with voile tissue for aeration. Every 2 days the insects were transferred to a new leaf and observed daily under a stereoscopic microscope. The duration and viability of instars were evaluated.

Adult phase

The newly emerged adults were sexed and paired to obtain clutches. The couples were maintained on petri dishes 6.2 cm in diam with perforated lids, re-covered with a 50% plastic screen containing eucalyptus leaf discs (3.2 cm in diam) on water-retaining gel diluted at 1 g of gel per 400 ml of distilled water. The leaf discs were changed when the females oviposited or when they were resected. Based on the behavior of the species in laying eggs on irregular surfaces in a natural environment and on the facility in evaluations, a double-faced tape containing filter paper on its exterior face was utilized for counting eggs laid. This procedure enabled oviposition by the females onto the center of the leaf to facilitate the evaluation and reduce the losses of eggs that had been laid on the edge of the leaf disc in contact with water. The evaluations were performed daily throughout the lifespan of the adults to determine: pre-oviposition period, daily and total oviposition, and longevity of the males and females.

Egg phase

During the daily observation of the pairs, the egg masses obtained were separated and maintained on a leaf disc in a petri dish 3.5 cm in diam (similar to those utilized in the nymph phase) to determine the incubation period and egg viability.

Statistical analysis

The experiment was conducted utilizing a completely randomized delineation, with the treatments represented by six eucalyptus genotypes, with 100 repetitions (nymph phase). The data were submitted to analysis of variance and the means compared by the Kruskal-Wallis non-parametric test (P ≤ 0.05).

Results

Nymph phase

The bronze bug nymphs have a body flattened dorsoventrally and a hind wing visible from the fourth instar, with significant growth during the fifth instar.

The mean duration of first instar presented a difference between the extreme values observed, varying from 2.96 days for E. grandis to 3.53 days for clone ‘1277’ (HGC). For the other genotypes no significant differences were observed (Table 1). In general, the minimum duration was 2 days, registered in E. urophylla, E. camaldulensis and E. grandis, whereas the maximum was 5 days—in E. urophylla and clone ‘H-13’ (HUG).

Table 1 Average duration (D, in days) and viability (V, in%), of Thaumastocoris peregrinus instars and nymph stage reared in different eucalyptus genotypes (temperature 26 ± 1°C; 60 ± 10% r.h.; 12-h photophase)
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The shorter mean duration of the second instar was verified in clone ‘1277’, which was statistically distinct from clone ‘H-13’, with the results obtained in the other genotypes evaluated being considered intermediate. In the third and fourth nymph stages, only clone ‘1277’ differed from the other treatments, tending toward a longer and shorter duration, respectively (Table 1).

When compared with the mean duration of earlier instars, the fifth instar was relatively prolonged in all treatments. The longer mean durations were obtained in E. grandis and E. camaldulensis, and were significantly distinct from the other treatments (Table 1).

The duration of nymph period of T. peregrinus, from the hatching of the egg until the emergence of the adult, varied from 14 to 20 days for the different eucalyptus genotypes. The mean duration in the E. camaldulensis nymph stage was different from all other treatments. On the other hand, clone ‘1277’ had the shortest mean duration (Table 1).

The mean nymph period among males was longer in E. camaldulensis, and differed from the other treatments with the exception of clone ‘H-13’. Among the females, the longest nymph period was also in E. camaldulensis, and differed significantly from the others. The females obtained in the clones ‘VM-1’, ‘1277’ and E. urophylla had the fastest nymph development, of 15.27, 15.41 and 15.56 days, respectively (Table 1).

The lowest nymph viability, after the five instars, was observed among the nymphs maintained in E. urophylla (76%) and the highest among nymphs of clone ‘H-13’ (96%); the other treatments exhibited viabilities between 87% and 93%. The nymph viability of all treatments was greater than 75%.

Adult phase

The adults have brownish coloration, with the females and males able to be sexed through ventral observation of the lower part of the abdomen, where the males possess a genital capsule that opens to the right from a ventral perspective. The females are typically larger than the males, with a round abdomen, in contrast to the males—that possess a narrow abdomen. Martinez and Bianchi (2010) and Noack et al. (2011) describe and provide details of sexual dimorphism.

The sexual proportion was approximately 1:1 (male:female), as observed in E. urophylla and in the clone ‘H-13’. Only in the clone ‘1277’ was the number of males higher than that of females, whereas in the other treatments the number of females was slightly greater (Table 2).

Table 2 Number of males and females and M:F ratio of Thaumastocoris peregrinus maintained on leaves of different eucalyptus species and hybrids (temperature 26 ± 1°C; 60 ± 10% r.h.; 12-h photophase; Botucatu, SP, 2010)
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The pre-oviposition period was determined from the emergence date of the females until the laying of the first egg. The shortest period was verified in E. grandis, and there were no significant differences among E. urophylla, ‘H-13’ and ‘1277’. Nevertheless, females maintained in E. camaldulensis had the longest period for oviposition initiation, followed by ‘VM-1’ (HUC) (Table 3).

Table 3 Mean duration of pre-oviposition and longevity (D, in days), and number of eggs (Q) of Thaumastocoris peregrinus raised on different eucalyptus genotypes (temperature 26 ± 1°C; 60 ± 10% r.h.; 12-h photophase; Botucatu, SP, 2010)
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The greatest egg production per female was observed in E. grandis and E. urophylla (Table 3). The mean number of viable eggs per female indicates the true reproductive capacity, which is allied with the number of descendants generated. In this manner, E. urophylla and E. grandis differed significantly from the other materials tested (Table 3).

The mean reproduction values under all the treatments reveal that the number of eggs per clutch was relatively greater at the beginning of the evaluations and subsequently decreased. The highest number of eggs per clutch was nine for E. urophylla followed by eight in E. grandis, ‘H-13’ and ‘1277’, and seven in E. camaldulensis and ‘VM-1’.

Regarding the number of surviving females (n), egg production for the clone ‘VM-1’ peaked between the first and fifth clutches (n = 27) and at the 22nd clutch (n = 2); in clone ‘1277’ it peaked at the sixth and ninth clutches (n = 14); in ‘H-13’ at the 3rd clutch (n = 42); in E. grandis from the second until the 17th clutch (n = 41) and after the 34th clutch (n = 2); E urophylla from the 16th to 22nd clutches (n = 24); and E. camaldulensis only at the beginning, around the third clutch (n = 45) (Fig. 1).

Fig. 1
figure 1

Mean number of eggs per clutch of Thaumastocoris peregrinus maintained on leaves from different eucalyptus species and hybrids. [Temperature 26 ± 1°C, 60 ± 10% r.h., 12-h photophase; Botucatu, SP, Brazil, 2010]

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The average male longevity was extremely reduced in the clone ‘1277’, and the males fed E. urophylla had the longest period, with no difference among the other treatments (Table 3). The mean female longevity was greater in eucalyptus species (E. urophylla, E. camaldulensis and E. grandis) than in the clones (‘H-13’, ‘1277’ and ‘VM-1’) (Table 3). The longevity of males and females showed no significant difference except for clone ‘1277’. On average, all the males had a longer lifespan than the females (Table 3).

The mean longevity of adults varied from 4 to 78 days among the eucalyptus genotypes tested. For mean total longevity of males and females, the clone ‘1277’ had the shortest and E. urophylla had the longest duration (Table 3).

Total lifespan

The lifespan of T. peregrinus was determined starting from the hatching of the nymph until the adult death. Under the different treatments the total lifespan of males varied from 19 to 99 days, and was longer than that of females. The total cycle of males was longer in E. urophylla than in the others, particularly the clone ‘1277’ (Table 4). The total cycle of females, in all treatments, varied from 19 to 77 days.

Table 4 Total cycle duration of males, females and total (days) of Thaumastocoris peregrinus) maintained on leaves of different eucalyptus species and hybrids (temperature 26 ± 1°C; 60 ± 10% r.h.; 12-h photophase; Botucatu, SP, 2010)
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Egg phase

The eggs possess a round flat form, and when recently laid they have a bright black color, with a slight depression in the center, shifting to opaque black with an elevated depression in the center after the hatching of the nymph. A nymph hatches from the egg by the operculum, which opens like a lid. In some eggs, even after the nymphs have hatched, the operculum is not prominent and apparently the egg was closed; but it is easily removed with a small brush or air jet.

The incubation period was analyzed from the laying of the egg clutch until the nymph hatching. This period varied from 4 to 9 days for the different eucalyptus genotypes but in clones ‘1277’ and ‘H-13’ no egg hatched in fewer than 4 days or more than 9 days, respectively. The average period of incubation was more rapid in ‘VM-1’, and slower in ‘1277’ (Table 5).

Table 5 Incubation period and viability of Thaumastocoris peregrinus eggs maintained on leaves of different eucalyptus species and hybrids (temperature 26 ± 1°C; 60 ± 10% r.h.; 12-h photophase; Botucatu, SP, 2010)
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The mean egg viability was obtained by calculating the difference between the total number of eggs laid and the total number of nymphs hatched. E. urophylla and clone ‘H-13’ provided the highest viability and E. camaldulensis the lowest (Table 5).

The females maintained in E. grandis laid the most eggs, and E. camaldulensis the least (Table 5).

Discussion

Nymph phase

The nymphs possessed red eyes, a characteristic also observed in Discocoris drakei (Couturier et al. 2002), antennae with final antennomers darker than the rest of the antenna, similar to Martinez and Bianchi’s (2010) observation, and a round red spot at the center of the primary uromeres of the abdomen.

The nymph phase of T. peregrinus had five instars, similar to the findings of Noack and Rose (2007), and the same number as observed in other Thaumastocoridae such as Thaumastocoris petilus Drake and Slater, Baclozygum brachypterum Slater (Slater 1973) and Discocoris drakei (Couturier et al. 2002), in addition to other hemipterans of forestry importance, including Leptopharsa heveae (Tingidae), a pest in rubber trees (Cividanes et al. 2004).

Noack and Rose (2007) obtained the highest average of 4.6 days for the duration of the first instar at a temperature varying from 17° to 20°C in Eucalyptus scoparia. The mean durations of the first and final instars were longer than those of the intermediate instars, results similar to those obtained by Noack and Rose (2007) in E. scoparia at variable temperatures.

The mean duration of the nymph period for all the materials tested was 15.8 days, superior to that reported by Firmino-Winckler et al. (2009) for Glycaspis brimblecombei (Hemiptera: Psyllidae) of 15.1 days but less than the 20 days obtained for T. peregrinus by Noack and Rose (2007).

The sexes differed significantly in mean duration of nymph period only among the males and females fed E. camaldulensis (Table 1).

In E. urophylla the phase most susceptible to mortality was the second instar, but in E. camaldulensis the critical period occurred in the fifth instar (Table 1). In the other genotypes utilized in the assay the mortality was distributed in all nymph stages, contrary to Noack and Rose (2007) who observed the first nymph stage as the most sensitive to mortality. This difference is probably related to the methodology employed.

Adult phase

The mean pre-oviposition period obtained by Noack and Rose (2007) was 8.5 days, similar only to the ‘VM-1’ treatment.

The number of eggs produced by the female indicates an elevated capacity for reproduction, but in the experimental conditions it was verified that the true reproductive capacity was inferior since a portion of the eggs were infertile. The elevated number of eggs produced by Thaumastocoridae, according to Kumar (1963), is due to the physiological capacity of the females, since they possess two ovaries with three ovarioles each.

Hill and Schaefer (2000) reported that Baclozygum depressum females laid an average of 1.25 eggs per day whereas Crosa (2008) emphasizes that each T. peregrinus female lays a daily mean of 2 eggs, but in the species studied by us this value was much higher (Fig. 1).

The total egg production per female obtained by Noack and Rose (2007) was 45.5, higher than the levels observed in the clones ‘1277’, ‘VM-1’ and ‘H-13’ and in E. camaldulensis, but females of the species B. depressum can lay up to 80 eggs (Hill and Schaefer 2000).

The longevity of males reported by Noack and Rose (2007) ranged from 2 to 38 days, with a mean of 16 days, a finding similar to that of clone ‘1277’ (Table 3). For Xylastodoris luteolus, also belonging to the subfamily Thaumastocorinae, the longevity of adults varies from 23 to 37 days (Hill and Schaefer 2000).

Noack and Rose (2007) found a mean longevity of 15 days in E. scoparia, similar to the average values observed in the hybrids of the present study (Table 3). In general, all the males had a longer life cycle than the females (Table 3), which may be an inherent characteristic of the species or, according to de Queiroz and Milward-de-Azevedo (1991), may be due to reproductive stress.

Lifespan duration

In the eucalyptus species studied, the duration of the adult phase lasted longer than in the clones. Crosa (2008) verified that the longevity of females was approximately 30 days, slightly greater than the duration of approximately 20 days reported by Noack and Rose (2007), but in the latter work the temperature varied from 17o to 20°C.

The data obtained suggest the possibility that T. peregrinus males maintained principally on three species, E. grandis, E. urophylla and E. camaldulensis, could mate with females emerging from the second generation, given that the egg incubation and nymph periods were approximately 6 and 15 days, respectively.

Egg phase

The eggs of T. peregrinus are black (Jacobs and Neser 2005) and relatively large in relation to the size of the female, as observed in other species of the subfamily Thaumastocorinae such as Discocoris drakei Slater & Ashlock (Couturier et al. 2002) and Xylastodorus luteolus (Hill 1988; Hill and Schaefer 2000).

In the field, females of T. peregrinus lay eggs together, forming great egg masses (Jacobs and Neser 2005; Wilcken et al. 2010); and in Brazil the clutch was verified principally on irregular surfaces, suggesting thigmotropism. These irregular surfaces can be present on eucalyptus fruit, branches, stems or leaves. In the leaves the clutches are found near the main vein, on leaf limb deformations or at locations close to the egg clutches of other insects such as the redgum lerp psyllid G. brimblecombei, and associated with eggs laid by other T. peregrinus females. In the present article, this thigmotropism hypothesis was proven by using the oviposition director placed on the leaf offered in the adult phase.

The eggs viability in all treatments was higher than 80%, in contrast to the data of Noack and Rose (2007), who obtained only 19% viability in E. scoparia. This difference may be due to the experimental conditions, mainly by maintaining the eggs in plates with high relative humidity. However, other causes could be the use of different eucalyptus species than other authors.

In conclusion, species and hybrid clones of eucalyptus affect the longevity and reproductive capacity of T. peregrinus; E. urophylla and E. grandis are the species most suitable for the development and reproduction of T. peregrinus; and the bronze bug reproduces and generates fertile descendants in the genetic base of the predominant eucalyptus plantations in Brazil.