Introduction
Trichospilus diatraeae Cherian and Margabandhu, 1942 (Hymenoptera: Eulophidae) is a gregarious endoparasitoid that preferentially attacks Lepidoptera pupae. It was first reported on Diatraea venosata Walker, 1863 (Lepidoptera: Crambidae) and dates from 1942 (Bouček 1976). In Brazil, the first occurrence was on pupae of Arctiidae (Paron and Berti-Filho 2000), but recently it was also reported attacking species of Oecophoridae (Oliveira et al. 2001), Geometridae (Pereira et al. 2008; Zaché et al. 2010) and Pyralidae (Melo et al. 2011).
One the most important issues in a program of biological control of insect is to find an adequate alternative host to rear the natural enemy. This host should be reared with low production costs and without affecting the efficiency of natural enemy in the field (Zanuncio et al. 2008). Tenebrio molitor Linnaeus, 1758 (Coleoptera: Tenebrionidae) is easy and inexpensive to rear in mass (Otuka et al. 2006) and shows very good results to rear pupae parasitoids (Zanuncio et al. 2008). For this reason, the parasitism of T. diatraeae on T. molitor pupae was studied.
The experiment was carried out in an acclimatized room, under the following conditions: mean maximum temperature of 25.88 ± 0.18 °C and minimum of 24.12 ± 0.16 °C, maximum relative humidity of 62.33% ± 1.15 and a minimum of 55.63% ± 1.00 and photophase of 12 hours. Forty pupae of T. molitor 24 hours old and weighing between 0.11 g and 0.14 g were individualized in glass tubes (2.5 x 8.5 cm) and exposed to parasitism by 20 T. diatraeae females with age between 48 and 72 hours. The experiment was set up in a completely randomized design. There were evaluated the duration of the life cycle (egg to adult), the percentage of parasitism discounting host natural mortality (Abbott 1925), the percentage of parasitoid emergence, the number of parasitoids per pupa of T. molitor, the longevity of males and females and sex ratio (rs= number of females/number of females + males).
The life cycle (egg to adult) of T. diatraeae in pupae of T. molitor took 26.14 ± 0.99 days, being longer than on Diatraea saccharalis (Fabricius, 1794) (Lepidoptera: Crambidae) 19.8 ± 0.7 days, at 25 °C, RH of 70 ± 10% and 14 hours photophase (Paron 1999). This may be due to a better adaptation of this parasitoid to pupae of the host. Although important, it is not the only factor responsible for variations on the development of this parasitoid. Also, it is affected by temperature, humidity and photoperiod (Zago et al. 2006).
The percentages of parasitism and emergence of T. diatraeae were 80 and 50%, respectively from pupae of T. molitor. The total number of individuals emerged per pupa of this host was 97.06 ± 17.50 and with 4.62 ± 0.87 females produced per female of this parasitoid with a sex ratio of 0.94 ± 0.02. The longevity (days) of females and males of this natural enemy was 18.15 ± 0.99 and 11.67± 0.62, respectively. Paron and Berti Filho (2000) also reported high parasitism of T. diatraeae on pupae of D. saccharalis and some Noctuidae (Lepidoptera) such as Spodoptera frugiperda Smith, 1797, Anticarsia gemmatalis Hübner, 1818, Heliothis virescens (Fabricius, 1781). These authors used one and several females, without mentioning clearly the number of parasitoids used. Trichospilus diatraeae is a gregarious parasitoid, which makes necessary to determine the ideal density of its females per pupa to increase the efficiency of production techniques in the laboratory. The progeny of T. diatraeae per pupa of T. molitor was higher than that reported for Palmistichus elaeisis Delvare and LaSalle, 1993 (Hymenoptera: Eulophidae) (70.07 ± 2.50) with this host (Zanuncio et al. 2008), but this latter parasitoid has greater body size than the first. However, the number of individuals of this parasitoid emerged per pupa of S. frugiperda (208.3 ± 4.5), D. saccharalis (194.7 ± 6.8), A. gemmatalis (186.7 ± 2.8) and H. virescens (170.5 ± 1.3) (Paron and Berti-Filho 2000) was higher than that of this work. These variations can be attributed to host size (bigger pupae can support the development of more immature), hardness of pupae integument (Godfray 1994) and the methodologies used.
The sex ratio of T. diatraeae produced per pupae of T. molitor was similar to that of P. elaeisis on this host (0.94 ± 0.01) (Zanuncio et al. 2008) what shows the high reproductive capacity and great potential of both parasitoids. The higher number of females is an important issue, especially in a system of mass rearing and in the selection of individuals to release in the field, because the predominance of females can increase the number of individuals produced in the following generation (Pereira et al. 2009). The high sex ratio of T. diatraeae from T. molitor pupae, also, indicates that this host is adequate for this parasitoid. The survival of females and males of T. diatraeae from T. molitor pupae in the laboratory may be sufficient for this parasitoid to copulate, find and parasitize hosts in the field. The longevity of T. diatraeae was lower in pupae of T. molitor than that of P. elaeisis with this host (Zanuncio et al. 2008). This may be due to food competition between immature because T. diatraeae produced more offspring per pupa of T. molitor than P. elaeisis. The temperature also affects the longevity but its impact differs between parasitoids species. The longevity of insects is also related to factors such as diet, environment conditions and energy spent during copulation and oviposition (Kapranas and Luck 2008).
The high reproductive performance of T. diatraeae shows the adequacy of T. molitor pupae for this parasitoid. This is the first report of T. molitor pupae as an alternative host to rearing T. diatraeae. As this natural enemy preferentially attacks Lepidoptera pupae; the qualitative and quantitative biological characteristics of this parasitoid may increase with the number of generations due to a better conditioning in this Coleoptera pupa. This can be explained by the fact that the experiment was developed with T. diatraeae reared with pupae of T. molitor between the second and third generation in the laboratory. The low cost of producing T. molitor allows the use of this host to mass rearing T. diatraeae for biological control programs of lepidopterous.
