There is an urgent need for minimizing the dependence of chemical pesticides in the management of insect pests. Therefore, in view of the problems based on the chemical, alternative environmentally friendly methods are essentially needed. Most of the researches focused on different Trichogramma species against different Spodoptera species and none of them studied T. bactrae and its association with S. littoralis. It is important to bear in mind that understanding and knowing the factors affecting the efficacy of T. bactrae wasps against S. littoralis eggs with different physical characteristics is crucial to improve the control program against this pest in the field. Host preference is an important parameter for biological control programs as more than one pest species of different physical characteristics may occur in the field (Goulart et al. 2011). Meanwhile, host recognition and acceptance are mainly driven by physical and chemical cues, which are mostly detected by the sensilla on the antennae and the ovipositor; these assess the suitability of a host for parasitization (Schmidt 1994). Parasitism capacity is an important indicator for the parasitoid efficiency, reproduction, and flourishing as a bio-control agent (Nurindah and Cribb 1997). Based on the present results, the higher parasitism rates of T. bactrae were observed on the single-layer eggs, maybe due to the parasitoid was able to access and examine all the eggs deposited in the mass, followed by the two-layers. However, three-layer eggs had the least rate in both no-choice and choice tests, because the parasitoid was only able to arrive and parasitize the outer, the edge, and/or the exposed eggs, but unable to parasitize the lower layer. Based on our observations, scales with low and mid thickness degree covering the eggs seemed to be as a chemical stimulus and attracted the female parasitoid of T. bactrae to parasitize, but with different rates related to the overlapping layers. The findings are mostly in accordance with those reported by other researchers on different Trichogramma species as mentioned by (Beserra and Parra 2005) that the parasitism percentage on S. frugiperda egg masses with one, two, and three layers were (66.24, 45.20, and 40.10%), respectively. Besides, Beserra et al. (2005) recorded 63.2% parasitism by T. atopovirilia on single-layer eggs of the same pest. As a result of releasing T. pretiosum parasitoid against this pest on maize fields, the rates increased according to the number of releases (1–3) (69.8, 79.2, and 68.75%), respectively (Figueiredo et al. 2015). On the other hand, T. pretiosum and T. minutum wasps were parasitized on (44.8 and 51.6%) of S. exigua eggs, respectively (Greenberg et al. 1998). On S. litura eggs, the parasitism rate by T. chilonis reached 80.31% (Puneeth and Vijayan 2013). Obtained results on the high-scaly eggs seemed to be less attractive or unpreferable host and had a negative impact on overall behavior. This may be because of different females of Trichogramma species reabsorbed their oocytes during unsuitable conditions or hosts’ deprivation, causing a reduction in parasitism percentage and the period of fertility (Hougardy et al. 2005). The parasitism rates on S. frugiperda egg layers with thick scales were very low (≤ 10%) as reported by Toonders and Sánchez (1987) and Beserra et al. (2002, 2005) in Brazil and Mexico. This may be due to the females avoided parasitism on high-scaly eggs that prevents their progeny, searching for new preferable ones. Another aspect as reported by Noldus (1989), S. frugiperda eggs are rarely attacked by T. pretiosum because this parasitoid does not respond to its semiochemicals and thus led to low parasitism.
Occasionally, T. bactrae parasitoid tried to remove the scales by moving on the egg mass and passing the front legs over the antennae, also rubbing the hind legs with each other’s, cleaning the scales adhered to the body. Similar behavior was observed by the egg parasitoid, Telenomus remus Nixon females on S. frugiperda egg masses (Carneiro and Fernandes 2012).
Ultimately, the variability in the number of parasitized eggs noticed on the previous and the present studies may be related to the different Spodoptera species, also to the geographical origin, and the parasitoid strain. Therefore, an adaptation of a specific parasitoid strain from a specific region against the target pest must be taken into consideration for the success of the biological control program.
Afterwards, high adult emergence rates were recorded in all examined egg masses, with exception of high-scaly three-layer eggs. This finding was in similar line with that of Bueno et al. (2010) who recorded the emergence rate of T. pretiosum from S. frugiperda eggs was higher than 88% in all tested temperatures (18–32 °C). Another important parameter in the biological control programs is sex ratio. Higher rates of females in their progeny are vital in mass production and responsible for the direct suppression of the target pest when released in the field (Bueno et al. 2009). With the following mating, the female wasps store their sperms in the spermatheca, and through parasitization, it can decide the sex ratio of their progeny by controlling the entry of sperm to the egg (Suzuki et al. 1984). At the present experimental conditions, the overall sex ratio of emerged adults among all examined egg masses was female-biased. Contrary to this prediction, on high-scaly eggs with a single layer, the percentage of females decreased significantly (< 45%) in both no-choice and choice tests. This may be due to the presence of densely scales covered the eggs, making them as unsuitable or unattractive hosts for the decision to deposit fertilized eggs and thus decreased the number of produced females in the progeny. In addition, as reported by Houseweart et al. (1983), the sperm depletion or decrease in the females’ spermatheca led to a decrease in the number of fertilized eggs, so it forced to parasitize but produced unfertilized eggs (males).
Certainly, this laboratory study is necessary to improve the biocontrol program against this pest. Moreover, further field studies of the efficacy of this parasitoid toward different Spodoptera species are needed.