Effect of gamma irradiation on the susceptibility of the cotton leaf worm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) to the infection with nucleopolyhedrosis virus

Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) is an economically important polyphagous pest in Egypt. It was reported to attack a wide range of food plants (112 cultivated plants belonging to 44 families world wide and 60 plants in Egypt) causing serious economic losses in many crops (Abd El-Razik and Mostafa 2013). A multifaceted approach is required because of the many records of resistance developing in this insect to several groups of pesticides (Ramakrishnan et al. 1984 ; Armes et al. 1997). Microbial insecticides such as baculoviruses are environmentally safe and selective bio-insecticides and can be used as alternatives to chemical pesticides (Armenta et al. 2003). Baculovirus products are commercially available with trade names for use in certain parts of the world (Black et al. 1997); however, the use of one or a combination of techniques to increase the efficacy of baculovirus production is still needed. The efficacy of NPV may be improved if any factor goes in line with its mode of action, such as effect on the peritrophic membrane, epithelial cells of the midgut, or depression of the insect immune system. However, many studies have been attempted to increase the efficiency of virus production, including the use of artificial diets (Chen et al. 2000; Gupta et al. 2007; Elvira et al. 2010), specific inoculum dose and stage of inoculation (Narayanan and Jayaraj 2002), and the optimization of rearing temperature and harvesting time of infected insects (Cherry et al. 1997; Subramanian et al. 2006). The reduction in production time or the use of alternative hosts or vectors may also enhance the efficiency of baculovirus production (Monobrullah et al. 2007; Beek and Davis 2007).

It was noticed that gamma radiations can cause deleterious effect on reproductive potential and is mainly used in insect control programs. There are two effects of gamma irradiation on the cells; the first is the direct effects such as clustered DNA damage and DNA double-strand breaks, and the second is the indirect effect causing DNA damage by the induction of reactive oxygen and free radicals. In the case of low doses, the direct effect is lower than the indirect one; therefore, the effects are stochastic and depend mainly on the efficiency of the stress response’s protective mechanisms and are known to induce hormesis (Moskalev et al. 2011). Low doses of gamma radiation affect the processes of cell proliferation and differentiation, causing DNA damage, apoptosis, proteolytic degradation, autophagy, and oxidative stress. Also, it has an impact on the immune response and the development of the organism; changes the metabolism of proteins, lipids, fatty acids, amino acids, and hormones; and alters energy metabolism leading to changes in the cell cycle (Feinendegen 2005; Seong et al. 2011; Zhikrevetskaya et al. 2015).

Irradiated Ceratits capitata and Anastrepha ludens exhibited signs of damage to midgut tissue, cellular organelles, and peritrophic membrane formation. In addition, bacterial growth appeared diminished in the midguts of irradiated flies compared to un-irradiated ones (Lauzon and Potter 2012).

Therefore, this study aimed to evaluate the use of gamma irradiation against the cotton leaf worm, S. littoralis, as a synergistic factor for increasing NPV yield.

The mortality of third-instar larvae treated with the SpliNPV concentration of 1 × 10² PIBs/ml increased significantly to 8.7, 10.73, 10.46, and 10.53% at the doses of 40, 60, 80, and 100 Gy, respectively, as compared to 0.4% for the un-irradiated larvae (Fig. 1). For the 1 × 10³ PIBs/ml concentration, the percentages of mortality increased significantly to 25.14, 46.53, 93.2, and 91.3% at the same doses, respectively, as compared to 3.9% for the un-irradiated treatment (Fig. 1). In the case of 1 × 10⁴ PIBs/ml concentration, the mortality percentages of F₁ larvae drastically increased to 98.7, 99.0, 100.0, and 100.0% at the doses 40, 60, 80, and 100 Gy, respectively, as compared to (7.0%) for the un-irradiated treatment (Fig. 1).

These results indicated that there was no clear difference in SpliNPV pathogenicity to F₁ neonate larvae from irradiated pupae with different doses. These findings may be due to high sensitivity of neonate larvae to the NPV (Narayanan and Jayaraj 2002; Beek and Davis 2007). This high sensitivity made the morality of neonate larvae by NPV high in both the control (treated with virus only) and the irradiation and virus treatments. In contrast, the third-instar larvae were found to have low sensitivity to NPV, especially at the lower concentrations (1 × 10², 1 × 10^3, and 1 × 10⁴ PIBs/ml).

The NPV mass production was influenced by several factors such as virus concentration, larval age at virus treatment, temperature, and harvesting time (Cherry et al. 1997; Subramanian et al. 2006; Rios-Velasco et al. 2012). Furthermore, numerous techniques have been proposed to improve the production of baculoviruses such as juvenile hormones used for increasing the production of Spodoptera exigua multicapsid NPV (SeMNPV) (Lasa et al. 2007) and for the production of Spodoptera litura NPV (SpltNPV) (Liao et al. 2016). In addition, Elvira et al. (2010) developed a low-cost diet for the large scale in vivo production of SeMNPV.

The present study suggested that the two doses of 40 and 60 Gy were the effective doses for irradiating S. littoralis male pupae to produce very sensitive F₁ larvae to SpliNPV. Furthermore, the two doses of gamma radiation slightly affected the reproductive biology of S. littoralis, while the first generation larvae resulted from irradiated full-grown pupae were more susceptible to SpliNPV than un-irradiated ones. These finding may help in the mass production of baculoviruses.