Fertility life table of Trichogramma pretiosum and Trichogramma acacioi on eggs of Anagasta kuehniella at different temperatures

Species of the Trichogramma genus are among the most important ones for biological control. The objective of this research was to evaluate parasitism potential of two species of Trichogramma on eggs of Anagasta kuheniella through life fertility table, at temperatures between 15oC and 35oC. These species were collected in the State of Espírito Santo parasitising eggs of the avocado defoliator Nipteria panacea. Trichogramma pretiosum and T. acacioi showed adequate reproductive potential between 15oC and 35oC which indicates possibilities of using them in biological control programs in avocado plantations.

Plants of avocado can be damaged by the defoliator Nipteria panacea (Thierry-Mieg) (Lep.: Geometridae) in the Espírito Santo State, Brazil, and the aggressiveness of this pest has been preoccupying producers and authorities (Pratissoli & Fornazier, 1999). This pest could be controlled by ecological methods considering association of cultural, biological and chemical procedures (Pratissoli & Parra, 2000a) as two species of the genus Trichogramma were found parasitizing eggs of N. panacea (Pratissoli & Fornazier, 1999). Trichogramma species have been used against avocado pests in the United States and Israel and against other key-pests in more than 30 countries (Wajnberg & Hassan, 1994;Parra & Zucchi, 1997).
The success of Trichogramma species in biological control depends on their identification in the field, collections, maintenance and behaviour in laboratory, selection of adequate species and/or lineages, besides thermal requirements, liberation techniques and evaluation of efficiency and dynamic models for parasitoids and hosts in the field (Parra et al., 1987). Adequate alternative hosts such as eggs of Sitotroga cerealella (Olivier) (Lep.: Gelechiidae) and Anagasta kuehniella (Zeller) (Lep.: Pyralidae) are important to multiply Trichogramma (Flanders, 1930;Lewis et al., 1976) because they can affect their biological traits (Oliveira et al., 2000).
Fertility life table is used to evaluate biological performance and to compare development of insects such as lineage and/or species of Trichogramma (Maceda et al., 1994;Pratissoli & Parra, 2000b Recently emerged T. acacioi and T. pretiosum females were individualised in glass tubes (3.5x0.5 cm) sealed with plastic PVC film and fed with droplets of pure honey in their internal wall. Twenty tubes with 40 eggs each of the alternative host A. kuehniella glued in a cardboard (3.5x0.5 cm) were used per Trichogramma species and temperature. These eggs were turned unfeasible by exposing them to a germicidal lamp during 45 minutes (Parra & Zucchi, 1997). Parasitism was allowed during 24 hours in a chamber with relative humidity of 70+10% and photoperiod of 14 hours at the temperatures mentioned. After this period females of these parasitoids were removed under a stereoscopic microscope and the tubes were maintained in these chambers at the respective temperature.
Twenty-hour old females of T. pretiosum and T. acacioi were used to evaluate parasitism capacity of these species. Each female of these species was fed by honey droplets inside a glass tube (13x1 cm) sealed with PVC plastic film. Each parasitoid female received daily a blue cardboard (3.5x0.5 cm) with 40 eggs of A. kuehniella glued with Arabic glue. These cardboards were removed after 24 hours and individualised in plastic bags (23x4 cm) at the respective temperatures until emergence of the parasitoids. Number of parasitized eggs, accumulated percentage of parasitism, total number of eggs parasitized per female and longevity of parasitoid females were evaluated. Data obtained were used to calculate duration of a generation and to elaborate fertility life tables for T. acacioi and T. pretiosum by calculating their liquid reproductive rate (Ro), and infinitesimal (rm) and finite (λ) rates of increase (Silveira Neto et al., 1976).
Fertility life table of T. pretiosum showed an inverse duration of a generation (T) as temperature increased from 15°C to 35°C with values of 49.76 and 7.54 days, respectively (Table 1). This was similar to that found for T. acacioi with 50.66 and 7.55 days for one generation at these temperatures. These Trichogramma species showed an inverse relationship between temperature and duration of a generation what was similar to that of other Trichogramma species (Pratissoli & Parra, 2000b).
Liquid reproductive rate (Ro) of T. pretiosum varied from 13.98 to 54.97 times as function of temperature with maximum population increase at 30°C while these values varied from 9.36 to 62.89 times for T. acacioi with maximum capacity of increase at 20°C (Table 1). These Trichogramma species showed lower liquid reproductive rate at extreme temperatures (15°C and 35°C). Population increase of T. pretiosum and T. acacioi was higher between 20°C and 30°C and 20°C and 25°C, respectively, with values 1.6 times higher than the first one. The infinitesimal rate of increase (rm) of T. pretiosum increased between 15°C and 30°C and decreased at 35°C. Trichogramma pretiosum and T. acacioi showed maximum capacity of population increase at 25°C and 30°C, respectively. The second species had a direct relationship with temperature increase between 15°C and 25°C. The finite rate of increase of T. pretiosum showed a direct relationship with its liquid reproductive and infinitesimal rate of increase between 15°C and 30°C and a decrease at 35°C. However, the finite rate of increase of T. acacioi presented a direct relationship with temperature between 15°C and 25°C (Table 1). Trichogramma pretiosum presented higher finite rate of increase (number of females added to the population per female) at 30°C while this was registered between 25°C and 30°C for T. acacioi.
Life fertility tables can be used to compare biological traits of insects submitted to variations on biotic and abiotic factors (Silveira Neto et al., 1976) such as temperature, adaptability to host and reproductive potential of species or lineages of Trichogramma (Maceda et al., 1994;Pratissoli & Parra, 2000b). Trichogramma cordubensis (Vargas & Cabello) (Hym.: Trichogrammatidae) showed better biological performance at higher temperatures while the reproductive potential of T. pretiosum in natural and alternative hosts was evaluated through parameters of life fertility table. Trichogramma pretiosum showed better adaptation to the host Phthorimaea operculella (Zeller) (Lep.: Gelechiidae) than to Tuta absoluta (Zeller) (Lep.: Gelechiidae) (Pratissoli & Parra, 2000b).
Trichogramma pretiosum and T. acacioi had lower liquid reproductive rate at temperatures below 20°C and above 30°C (Table 1). This was similar to that reported for T. pretiosum with the host P. operculella (Pratissoli & Parra, 2000b) but it differs from results for this parasitoid with Alabama argillacea (Hübner) (Lep.: Noctuidae). Maximum population increase (Ro) of T. pretiosum and T. acacioi occurred at different temperatures (30°C and 20°C, respectively). This fact can be related to adaptation of these species to conditions of their habitat where they were collected and also to the host used to rear them (Parra et al., 1987;Fernandes et al., 1999). The infinitesimal rate of increase showed maximum population increase of T. pretiosum and T. acacioi at different temperatures and lower values at extreme ones. This is important because temperature is one of the main factors affecting the adaptability of egg parasitoids to climatic conditions of their habitat.
Fertility life table represents an important biological tool to evaluate the behaviour of T. pretiosum and T. acacioi at different temperatures. These parasitoids have potential to control the Lepidoptera defoliator of avocado, N. panacea due to their high reproductive potential at different temperatures. This behaviour increases the possibilities of using these species in integrated management programs of N. panacea in avocado plantations.